Java的设计模式基本概念及使用场景

设计模式是软件开发中常用的解决问题的可重用模式。它提供了一套经过验证的解决方案，可以帮助开发人员解决常见问题，并提高代码的可维护性和可扩展性。Java中有23种常见的设计模式，可以分为三个类别：创建型模式、结构型模式和行为型模式。

创建型模式：用于创建对象的模式，隐藏了对象的创建过程。

单例模式（Singleton）：确保一个类只有一个实例，并提供全局访问点。

java 复制代码

public class Singleton {
    private static Singleton instance;

    private Singleton() {
    }

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

工厂模式（Factory）：通过一个工厂类创建对象，将对象的实例化过程封装起来。

java 复制代码

public interface Shape {
    void draw();
}

public class Circle implements Shape {
    @Override
    public void draw() {
        System.out.println("Draw a circle");
    }
}

public class Rectangle implements Shape {
    @Override
    public void draw() {
        System.out.println("Draw a rectangle");
    }
}

public class ShapeFactory {
    public Shape createShape(String shapeType) {
        if (shapeType.equalsIgnoreCase("circle")) {
            return new Circle();
        } else if (shapeType.equalsIgnoreCase("rectangle")) {
            return new Rectangle();
        }
        return null;
    }
}

结构型模式：用于描述如何将对象和类组合成更大的结构。

适配器模式（Adapter）：将一个类的接口转换成客户端所期望的接口。

java 复制代码

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

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

public class VlcPlayer implements AdvancedMediaPlayer {
    @Override
    public void playVlc(String fileName) {
        System.out.println("Playing vlc file: " + fileName);
    }

    @Override
    public void playMp4(String fileName) {
        // do nothing
    }
}

public class Mp4Player implements AdvancedMediaPlayer {
    @Override
    public void playVlc(String fileName) {
        // do nothing
    }

    @Override
    public void playMp4(String fileName) {
        System.out.println("Playing mp4 file: " + fileName);
    }
}

public class MediaAdapter implements MediaPlayer {
    private AdvancedMediaPlayer advancedMediaPlayer;

    public MediaAdapter(String audioType) {
        if (audioType.equalsIgnoreCase("vlc")) {
            advancedMediaPlayer = new VlcPlayer();
        } else if (audioType.equalsIgnoreCase("mp4")) {
            advancedMediaPlayer = new Mp4Player();
        }
    }

    @Override
    public void play(String audioType, String fileName) {
        if (audioType.equalsIgnoreCase("vlc")) {
            advancedMediaPlayer.playVlc(fileName);
        } else if (audioType.equalsIgnoreCase("mp4")) {
            advancedMediaPlayer.playMp4(fileName);
        }
    }
}

public class AudioPlayer implements MediaPlayer {
    private MediaAdapter mediaAdapter;

    @Override
    public void play(String audioType, String fileName) {
        if (audioType.equalsIgnoreCase("mp3")) {
            System.out.println("Playing mp3 file: " + fileName);
        } else if (audioType.equalsIgnoreCase("vlc") || audioType.equalsIgnoreCase("mp4")) {
            mediaAdapter = new MediaAdapter(audioType);
            mediaAdapter.play(audioType, fileName);
        } else {
            System.out.println("Format not supported");
        }
    }
}

行为型模式：用于描述对象之间的通信方式和职责分配。

观察者模式（Observer）：定义对象之间的一对多依赖关系，当一个对象的状态发生变化时，所有依赖它的对象都会收到通知。

java 复制代码

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

public class Subject {
    private List<Observer> observers = new ArrayList<>();
    private int state;

    public int getState() {
        return state;
    }

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

    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void notifyAllObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

public abstract class Observer {
    protected Subject subject;

    public abstract void update();
}

public class BinaryObserver extends Observer {
    public BinaryObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("Binary: " + Integer.toBinaryString(subject.getState()));
    }
}

public class OctalObserver extends Observer {
    public OctalObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("Octal: " + Integer.toOctalString(subject.getState()));
    }
}

public class HexObserver extends Observer {
    public HexObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("Hex: " + Integer.toHexString(subject.getState()));
    }
}

public class Main {
    public static void main(String[] args) {
        Subject subject = new Subject();

        new BinaryObserver(subject);
        new OctalObserver(subject);
        new HexObserver(subject);

        subject.setState(10);
        subject.setState(15);
    }
}