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Top 100 Java Design Patterns Interview Questions and Answers

Top 100 Java Design Patterns Interview Questions and Answers
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1. What is the Singleton Design Pattern?

Answer:

The Singleton pattern ensures a class has only one instance and provides a global point of access to that instance.

Example:

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

2. Explain the Factory Method Design Pattern.

Answer:

The Factory Method pattern defines an interface for creating objects, allowing subclasses to alter the type of objects that will be created.

Example:

interface Product {
    void create();
}

class ConcreteProductA implements Product {
    public void create() {
        System.out.println("Product A created.");
    }
}

// ... ConcreteProductB and other product implementations ...

abstract class Creator {
    public abstract Product factoryMethod();
}

class ConcreteCreatorA extends Creator {
    public Product factoryMethod() {
        return new ConcreteProductA();
    }
}

// ... ConcreteCreatorB and other creator implementations ...

3. What is the Observer Design Pattern?

Answer:

The Observer pattern defines a one-to-many dependency between objects. When one object changes state, all its dependents are notified and updated automatically.

Example:

import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update(String message);
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();
    private String message;

    // ... addObserver, removeObserver, setMessage methods ...
}

4. Explain the Builder Design Pattern.

Answer:

The Builder pattern separates the construction of a complex object from its representation, allowing the same construction process to create different representations.

Example:

class Product {
    private String partA;
    private String partB;

    // ... setters for partA and partB ...
}

interface Builder {
    void buildPartA();
    void buildPartB();
    Product getResult();
}

class ConcreteBuilder implements Builder {
    private Product product = new Product();

    // ... buildPartA, buildPartB, and getResult methods ...
}

5. Describe the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

6. What is the Decorator Design Pattern?

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

7. Can you explain the Prototype Design Pattern?

Answer:

The Prototype pattern creates new objects by copying an existing object, known as the prototype. This can be more efficient than creating a new instance from scratch.

Example:

class Prototype implements Cloneable {
    @Override
    public Prototype clone() throws CloneNotSupportedException {
        return (Prototype) super.clone();
    }
}

8. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

9. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

10. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

11. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

12. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

13. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

14. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

15. What is the Composite Design Pattern?

Answer:

The Composite pattern lets clients treat individual objects and compositions of objects uniformly. It composes objects into tree structures to represent part-whole hierarchies.

Example:

interface Component {
    void operation();
}

class Leaf implements Component {
    public void operation() {
        System.out.println("Leaf operation");
    }
}

class Composite implements Component {
    private List<Component> components = new ArrayList<>();

    public void addComponent(Component component) {
        components.add(component);
    }

    public void removeComponent(Component component) {
        components.remove(component);
    }

    public void operation() {
        System.out.println("Composite operation");
        for (Component component : components) {
            component.operation();
        }
    }
}

16. Explain the Flyweight Design Pattern.

Answer:

The Flyweight pattern minimizes memory usage or computational expenses by sharing as much as possible with related objects. It’s used for performance optimization.

Example:

class Flyweight {
    private String data;

    public Flyweight(String data) {
        this.data = data;
    }

    public String getData() {
        return data;
    }
}

17. What is the Proxy Design Pattern?

Answer:

The Proxy pattern provides a surrogate or placeholder for another object to control access to it. It’s used to add a level of control over accessing a resource.

Example:

interface Image {
    void display();
}

class RealImage implements Image {
    private String fileName;

    public RealImage(String fileName) {
        this.fileName = fileName;
        loadFromDisk(fileName);
    }

    public void display() {
        System.out.println("Displaying " + fileName);
    }

    private void loadFromDisk(String fileName) {
        System.out.println("Loading " + fileName);
    }
}

class ProxyImage implements Image {
    private RealImage realImage;
    private String fileName;

    public ProxyImage(String fileName){
        this.fileName = fileName;
    }

    public void display() {
        if(realImage == null){
            realImage = new RealImage(fileName);
        }
        realImage.display();
    }
}

18. Explain the Adapter Design Pattern.

Answer:

The Adapter pattern allows the interface of an existing class to be used as another interface. It’s often used to make existing classes work with others without modifying their source code.

Example:

interface MediaPlayer {
    void play(String audioType, String fileName);
}

class AudioPlayer implements MediaPlayer {
    public void play(String audioType, String fileName) {
        // Implement playback logic here
    }
}

interface AdvancedMediaPlayer {
    void playVlc(String fileName);
    void playMp4(String fileName);
}

class VlcPlayer implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Implement Vlc playback logic here
    }

    public void playMp4(String fileName) {
        // Do nothing
    }
}

class Mp4Player implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Do nothing
    }

    public void playMp4(String fileName) {
        // Implement Mp4 playback logic here
    }
}

19. What is the Bridge Design Pattern?

Answer:

The Bridge pattern separates abstraction from implementation so that they can vary independently. It’s useful when both need to be extended independently.

Example:

interface DrawAPI {
    void drawCircle(int radius, int x, int y);
}

class RedCircle implements DrawAPI {
    public void drawCircle(int radius, int x, int y) {
        System.out.println("Drawing Circle[ color: red, radius: " + radius + ", x: " + x + ", y: " + y + "]");
    }
}

class GreenCircle implements DrawAPI {
    public void drawCircle(int radius, int x, int y) {
        System.out.println("Drawing Circle[ color: green, radius: " + radius + ", x: " + x + ", y: " + y + "]");
    }
}

20. Explain the Composite Design Pattern.

Answer:

The Composite pattern lets clients treat individual objects and compositions of objects uniformly. It composes objects into tree structures to represent part-whole hierarchies.

Example:

interface Component {
    void operation();
}

class Leaf implements Component {
    public void operation() {
        System.out.println("Leaf operation");
    }
}

class Composite implements Component {
    private List<Component> components = new ArrayList<>();

    public void addComponent(Component component) {
        components.add(component);
    }

    public void removeComponent(Component component) {
        components.remove(component);
    }

    public void operation() {
        System.out.println("Composite operation");
        for (Component component : components) {
            component.operation();
        }
    }
}

21. What is the Facade Design Pattern?

Answer:

The Facade pattern provides a simplified interface to a set of interfaces in a subsystem, making it easier to use.

Example:

class SubsystemA {
    public void operationA() {
        System.out.println("Subsystem A - Operation A");
    }
}

class SubsystemB {
    public void operationB() {
        System.out.println("Subsystem B - Operation B");
    }
}

class SubsystemC {
    public void operationC() {
        System.out.println("Subsystem C - Operation C");
    }
}

class Facade {
    private SubsystemA subsystemA;
    private SubsystemB subsystemB;
    private SubsystemC subsystemC;

    public Facade() {
        subsystemA = new SubsystemA();
        subsystemB = new SubsystemB();
        subsystemC = new SubsystemC();
    }

    public void operation() {
        subsystemA.operationA();
        subsystemB.operationB();
        subsystemC.operationC();
    }
}

22. Explain the Proxy Design Pattern.

Answer:

The Proxy pattern provides a surrogate or placeholder for another object to control access to it. It’s used to add a level of control over accessing a resource.

Example:

interface Image {
    void display();
}

class RealImage implements Image {
    private String fileName;

    public RealImage(String fileName) {
        this.fileName = fileName;
        loadFromDisk(fileName);
    }

    public void display() {
        System.out.println("Displaying " + fileName);
    }

    private void loadFromDisk(String fileName) {
        System.out.println("Loading " + fileName);
    }
}

class ProxyImage implements Image {
    private RealImage realImage;
    private String fileName;

    public ProxyImage(String fileName){
        this.fileName = fileName;
    }

    public void display() {
        if(realImage == null){
            realImage = new RealImage(fileName);
        }
        realImage.display();
    }
}

23. What is the Adapter Design Pattern?

Answer:

The Adapter pattern allows the interface of an existing class to be used as another interface. It’s often used to make existing classes work with others without modifying their source code.

Example:

interface MediaPlayer {
    void play(String audioType, String fileName);
}

class AudioPlayer implements MediaPlayer {
    public void play(String audioType, String fileName) {
        // Implement playback logic here
    }
}

interface AdvancedMediaPlayer {
    void playVlc(String fileName);
    void playMp4(String fileName);
}

class VlcPlayer implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Implement Vlc playback logic here
    }

    public void playMp4(String fileName) {
        // Do nothing
    }
}

class Mp4Player implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Do nothing
    }

    public void playMp4(String fileName) {
        // Implement Mp4 playback logic here
    }
}

24. Explain the Chain of Responsibility Design Pattern.

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

25. Explain the Observer Design Pattern.

Answer:

The Observer pattern defines a one-to-many dependency between objects. When one object changes state, all its dependents are notified and updated automatically.

Example:

import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update(String message);
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();
    private String message;

    // ... addObserver, removeObserver, setMessage methods ...
}

26. What is the Strategy Design Pattern?

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

27. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

28. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

29. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

30. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

31. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

32. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

33. Explain the Composite Design Pattern.

Answer:

The Composite pattern lets clients treat individual objects and compositions of objects uniformly. It composes objects into tree structures to represent part-whole hierarchies.

Example:

interface Component {
    void operation();
}

class Leaf implements Component {
    public void operation() {
        System.out.println("Leaf operation");
    }
}

class Composite implements Component {
    private List<Component> components = new ArrayList<>();

    public void addComponent(Component component) {
        components.add(component);
    }

    public void removeComponent(Component component) {
        components.remove(component);
    }

    public void operation() {
        System.out.println("Composite operation");
        for (Component component : components) {
            component.operation();
        }
    }
}

34. What is the Flyweight Design Pattern?

Answer:

The Flyweight pattern minimizes memory usage or computational expenses by sharing as much as possible with related objects. It’s used for performance optimization.

Example:

class Flyweight {
    private String data;

    public Flyweight(String data) {
        this.data = data;
    }

    public String getData() {
        return data;
    }
}

35. Explain the Proxy Design Pattern.

Answer:

The Proxy pattern provides a surrogate or placeholder for another object to control access to it. It’s used to add a level of control over accessing a resource.

Example:

interface Image {
    void display();
}

class RealImage implements Image {
    private String fileName;

    public RealImage(String fileName) {
        this.fileName = fileName;
        loadFromDisk(fileName);
    }

    public void display() {
        System.out.println("Displaying " + fileName);
    }

    private void loadFromDisk(String fileName) {
        System.out.println("Loading " + fileName);
    }
}

class ProxyImage implements Image {
    private RealImage realImage;
    private String fileName;

    public ProxyImage(String fileName){
        this.fileName = fileName;
    }

    public void display() {
        if(realImage == null){
            realImage = new RealImage(fileName);
        }
        realImage.display();
    }
}

36. What is the Adapter Design Pattern?

Answer:

The Adapter pattern allows the interface of an existing class to be used as another interface. It’s often used to make existing classes work with others without modifying their source code.

Example:

interface MediaPlayer {
    void play(String audioType, String fileName);
}

class AudioPlayer implements MediaPlayer {
    public void play(String audioType, String fileName) {
        // Implement playback logic here
    }
}

interface AdvancedMediaPlayer {
    void playVlc(String fileName);
    void playMp4(String fileName);
}

class VlcPlayer implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Implement Vlc playback logic here
    }

    public void playMp4(String fileName) {
        // Do nothing
    }
}

class Mp4Player implements AdvancedMediaPlayer {
    public void playVlc(String fileName) {
        // Do nothing
    }

    public void playMp4(String fileName) {
        // Implement Mp4 playback logic here
    }
}

37. Explain the Bridge Design Pattern.

Answer:

The Bridge pattern separates abstraction from implementation so that they can vary independently. It’s useful when both need to be extended independently.

Example:

interface DrawAPI {
    void drawCircle(int radius, int x, int y);
}

class RedCircle implements DrawAPI {
    public void drawCircle(int radius, int x, int y) {
        System.out.println("Drawing Circle[ color: red, radius: " + radius + ", x: " + x + ", y: " + y + "]");
    }
}

class GreenCircle implements DrawAPI {
    public void drawCircle(int radius, int x, int y) {
        System.out.println("Drawing Circle[ color: green, radius: " + radius + ", x: " + x + ", y: " + y + "]");
    }
}

38. What is the Composite Design Pattern?

Answer:

The Composite pattern lets clients treat individual objects and compositions of objects uniformly. It composes objects into tree structures to represent part-whole hierarchies.

Example:

interface Component {
    void operation();
}

class Leaf implements Component {
    public void operation() {
        System.out.println("Leaf operation");
    }
}

class Composite implements Component {
    private List<Component> components = new ArrayList<>();

    public void addComponent(Component component) {
        components.add(component);
    }

    public void removeComponent(Component component) {
        components.remove(component);
    }

    public void operation() {
        System.out.println("Composite operation");
        for (Component component : components) {
            component.operation();
        }
    }
}

39. What is the Facade Design Pattern?

Answer:

The Facade pattern provides a simplified interface to a set of interfaces in a subsystem, making it easier to use.

Example:

class SubsystemA {
    public void operationA() {
        System.out.println("Subsystem A - Operation A");
    }
}

class SubsystemB {
    public void operationB() {
        System.out.println("Subsystem B - Operation B");
    }
}

class SubsystemC {
    public void operationC() {
        System.out.println("Subsystem C - Operation C");
    }
}

class Facade {
    private SubsystemA subsystemA;
    private SubsystemB subsystemB;
    private SubsystemC subsystemC;

    public Facade() {
        subsystemA = new SubsystemA();
        subsystemB = new SubsystemB();
        subsystemC = new SubsystemC();
    }

    public void operation() {
        subsystemA.operationA();
        subsystemB.operationB();
        subsystemC.operationC();
    }
}

40. Explain the Observer Design Pattern.

Answer:

The Observer pattern defines a one-to-many dependency between objects. When one object changes state, all its dependents are notified and updated automatically.

Example:

import java.util.ArrayList;
import java.util.List;

interface Observer {
    void update(String message);
}

class ConcreteObserver implements Observer {
    private String name;

    public ConcreteObserver(String name) {
        this.name = name;
    }

    public void update(String message) {
        System.out.println(name + " received message: " + message);
    }
}

class Subject {
    private List<Observer> observers = new ArrayList<>();
    private String message;

    // ... addObserver, removeObserver, setMessage methods ...
}

41. What is the Strategy Design Pattern?

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

42. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

43. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

44. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

45. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

46. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

47. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

48. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

49. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

50. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

51. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

52. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

53. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

54. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

55. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

56. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

57. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

58. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

59. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

60. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

61. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

62. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

63. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

64. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

65. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

66. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

67. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

68. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

69. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

70. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

71. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

72. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

73. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

74. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

75. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

76. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

77. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

78. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

79. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

80. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

81. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

82. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

83. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

84. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

85. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

86. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

87. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

88. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

89. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

90. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

91. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}

92. What is the Interpreter Design Pattern?

Answer:

The Interpreter pattern defines a grammar for the language and provides an interpreter to interpret sentences of the language.

Example:

interface Expression {
    boolean interpret(String context);
}

class TerminalExpression implements Expression {
    private String data;

    public TerminalExpression(String data) {
        this.data = data;
    }

    public boolean interpret(String context) {
        return context.contains(data);
    }
}

class OrExpression implements Expression {
    private Expression expr1;
    private Expression expr2;

    public OrExpression(Expression expr1, Expression expr2) {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }

    public boolean interpret(String context) {
        return expr1.interpret(context) || expr2.interpret(context);
    }
}

93. Explain the Memento Design Pattern.

Answer:

The Memento pattern provides the ability to restore an object to its previous state. It is used to implement an undo mechanism.

Example:

class Memento {
    private String state;

    public Memento(String state) {
        this.state = state;
    }

    public String getState() {
        return state;
    }
}

class Originator {
    private String state;

    public void setState(String state) {
        this.state = state;
    }

    public Memento saveStateToMemento() {
        return new Memento(state);
    }

    public void getStateFromMemento(Memento memento) {
        state = memento.getState();
    }
}

94. What is the Visitor Design Pattern?

Answer:

The Visitor pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

Example:

interface Visitor {
    void visit(ElementA elementA);
    void visit(ElementB elementB);
}

interface Element {
    void accept(Visitor visitor);
}

class ElementA implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ElementB implements Element {
    public void accept(Visitor visitor) {
        visitor.visit(this);
    }
}

class ConcreteVisitor implements Visitor {
    public void visit(ElementA elementA) {
        System.out.println("Visitor visited ElementA");
    }

    public void visit(ElementB elementB) {
        System.out.println("Visitor visited ElementB");
    }
}

95. Explain the Strategy Design Pattern.

Answer:

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Example:

interface PaymentStrategy {
    void pay(int amount);
}

class CreditCardPayment implements PaymentStrategy {
    private String cardNumber;

    // ... constructor, getters, and setters ...
}

class PayPalPayment implements PaymentStrategy {
    private String email;

    // ... constructor, getters, and setters ...
}

class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    // ... setPaymentStrategy and checkout methods ...
}

96. Explain the Decorator Design Pattern.

Answer:

The Decorator pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Example:

interface Coffee {
    String getDescription();
    double getCost();
}

class SimpleCoffee implements Coffee {
    public String getDescription() {
        return "Simple Coffee";
    }

    public double getCost() {
        return 2.0;
    }
}

// ... CoffeeDecorator, MilkDecorator, etc. ...

97. What is the Command Design Pattern?

Answer:

The Command pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.

Example:

interface Command {
    void execute();
}

class ConcreteCommand implements Command {
    private Receiver receiver;

    public ConcreteCommand(Receiver receiver) {
        this.receiver = receiver;
    }

    public void execute() {
        receiver.action();
    }
}

class Receiver {
    public void action() {
        System.out.println("Receiver action executed.");
    }
}

98. Explain the State Design Pattern.

Answer:

The State pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Example:

interface State {
    void doAction(Context context);
}

class StartState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in start state");
        context.setState(this);
    }

    public String toString() {
        return "Start State";
    }
}

class StopState implements State {
    public void doAction(Context context) {
        System.out.println("Player is in stop state");
        context.setState(this);
    }

    public String toString() {
        return "Stop State";
    }
}

class Context {
    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public State getState() {
        return state;
    }
}

99. What is the Chain of Responsibility Design Pattern?

Answer:

The Chain of Responsibility pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along.

Example:

abstract class Logger {
    public static int INFO = 1;
    public static int DEBUG = 2;
    public static int ERROR = 3;

    protected int level;

    protected Logger nextLogger;

    public void setNextLogger(Logger nextLogger){
        this.nextLogger = nextLogger;
    }

    abstract protected void write(String message);
}

class ConsoleLogger extends Logger {
    public ConsoleLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("Console Logger: " + message);
    }
}

class FileLogger extends Logger {
    public FileLogger(int level){
        this.level = level;
    }

    protected void write(String message) {
        System.out.println("File Logger: " + message);
    }
}

100. Explain the Template Method Design Pattern.

Answer:

The Template Method pattern defines the structure of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

Example:

abstract class Game {
    abstract void initialize();
    abstract void startPlay();
    abstract void endPlay();

    public final void play(){
        initialize();
        startPlay();
        endPlay();
    }
}

class Cricket extends Game {
    void initialize() {
        System.out.println("Cricket Game Initialized! Start playing.");
    }

    void startPlay() {
        System.out.println("Cricket Game Started. Enjoy the game!");
    }

    void endPlay() {
        System.out.println("Cricket Game Finished!");
    }
}