java 设计模式有哪些，不同的设计模式应用是什么，提供一下相关的代码示例

Java 设计模式是基于面向对象编程思想的一套经验总结，主要用于解决软件设计中的一些常见问题，提高代码的可复用性、可维护性和可扩展性。常见的 Java 设计模式包括但不限于以下几种：

1. 工厂模式（Factory Pattern）：用于创建对象，将对象的创建过程和客户端代码分离，使得客户端代码无需知道具体的创建过程，只需要知道创建的对象的类型即可。

示例代码：

interface Shape {
    void draw();
}

class Circle implements Shape {
    @Override
    public void draw() {
        System.out.println("画了一个圆形");
    }
}

class Rectangle implements Shape {
    @Override
    public void draw() {
        System.out.println("画了一个矩形");
    }
}

class ShapeFactory {
    public Shape getShape(String shapeType) {
        if(shapeType == null) {
            return null;
        }
        if(shapeType.equalsIgnoreCase("CIRCLE")) {
            return new Circle();
        } else if(shapeType.equalsIgnoreCase("RECTANGLE")) {
            return new Rectangle();
        }
        return null;
    }
}

public class Demo {
    public static void main(String[] args) {
        ShapeFactory shapeFactory = new ShapeFactory();
        Shape circle = shapeFactory.getShape("CIRCLE");
        circle.draw();
        Shape rectangle = shapeFactory.getShape("RECTANGLE");
        rectangle.draw();
    }
}

输出：
画了一个圆形
画了一个矩形

2. 单例模式（Singleton Pattern）：确保一个类只有一个实例，并提供全局访问点。

示例代码：

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

public class Demo {
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1 == s2); // true
    }
}

3. 适配器模式（Adapter Pattern）：用于将一个类的接口转换成另一个接口，使得原本不兼容的类可以一起工作。

示例代码：

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

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

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

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

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

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

class MediaAdapter implements MediaPlayer {
    AdvancedMediaPlayer advancedMusicPlayer;

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

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

class AudioPlayer implements MediaPlayer {
    MediaAdapter mediaAdapter;

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

public class Demo {
    public static void main(String[] args) {
        AudioPlayer audioPlayer = new AudioPlayer();
        audioPlayer.play("mp3", "beyond the horizon.mp3");
        audioPlayer.play("mp4", "alone.mp4");
        audioPlayer.play("vlc", "far far away.vlc");
        audioPlayer.play("avi", "mind me.avi");
    }
}

输出：
Playing mp3 file. Name: beyond the horizon.mp3
Playing mp4 file. Name: alone.mp4
Playing vlc file. Name: far far away.vlc
Invalid media. avi format not supported

4. 装饰器模式（Decorator Pattern）：动态地给一个对象添加一些额外的职责，而不需要通过继承来实现。

示例代码：

interface Shape {
    void draw();
}

class Circle implements Shape {
    @Override
    public void draw() {
        System.out.println("画了一个圆形");
    }
}

abstract class ShapeDecorator implements Shape {
    protected Shape decoratedShape;
    public ShapeDecorator(Shape decoratedShape) {
        this.decoratedShape = decoratedShape;
    }
    public void draw() {
        decoratedShape.draw();
    }
}

class RedShapeDecorator extends ShapeDecorator {
    public RedShapeDecorator(Shape decoratedShape) {
        super(decoratedShape);
    }
    public void draw() {
        decoratedShape.draw();
        setRedBorder(decoratedShape);
    }
    private void setRedBorder(Shape decoratedShape){
        System.out.println("Border Color: Red");
    }
}

public class Demo {
    public static void main(String[] args) {
        Shape circle = new Circle();
        Shape redCircle = new RedShapeDecorator(new Circle());
        Shape redRectangle = new RedShapeDecorator(new Rectangle());
        circle.draw();
        redCircle.draw();
        redRectangle.draw();
    }
}

输出：
画了一个圆形
画了一个圆形
Border Color: Red
画了一个矩形
Border Color: Red

5. 观察者模式（Observer Pattern）：定义了对象之间的一对多依赖关系，当一个对象的状态发生改变时，所有依赖它的对象都会得到通知并自动更新。

示例代码：

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

interface Subject {
    void attach(Observer observer);
    void detach(Observer observer);
    void notifyObservers();
}

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

    public int getState() {
        return state;
    }

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

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

    @Override
    public void detach(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

interface Observer {
    void update();
}

class ConcreteObserver implements Observer {
    private ConcreteSubject subject;

    public ConcreteObserver(ConcreteSubject subject) {
        this.subject = subject;
        subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println("观察者收到了新的通知，状态改变为：" + subject.getState());
    }
}

public class Demo {
    public static void main(String[] args) {
        ConcreteSubject subject = new ConcreteSubject();
        new ConcreteObserver(subject);
        new ConcreteObserver(subject);
        new ConcreteObserver(subject);

        System.out.println("第一次改变状态为：10");
        subject.setState(10);
        System.out.println("第二次改变状态为：20");
        subject.setState(20);
    }
}

输出：
第一次改变状态为：10
观察者收到了新的通知，状态改变为：10
观察者收到了新的通知，状态改变为：10
观察者收到了新的通知，状态改变为：10
第二次改变状态为：20
观察者收到了新的通知，状态改变为：20
观察者收到了新的通知，状态改变为：20
观察者收到了新的通知，状态改变为：20