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# 适配器模式

将类的接口转换为客户期望的另一个接口，适配器可以让不兼容的两个类一起协同工作。实现适配器有两种基本形式：

* 类适配器，使用继承关系实现
* 对象适配器，使用组合关系实现（常用）

![图 1](/files/zwjll6OiadbIdcjZjUCa)\
![图 2](/files/JZWUKxNLSCefKpE5vAf6)

使用场景：

* 统一多个类的接口设计时，某个功能的实现依赖多个外部系统（或者说类）。通过适配器模式，将它们的接口适配为统一的接口定义
* 需要依赖外部系统时，当我们把项目中依赖的一个外部系统替换为另一个外部系统的时候，利用适配器模式，可以减少对代码的改动
* 原有接口无法修改时或者原有接口功能太老旧但又需要兼容；比如JDK1.0 Enumeration 到 Iterator 的替换,适用适配器模式保留 Enumeration 类，并将其实现替换为直接调用 Itertor.
* 适配不同数据格式时；Slf4j 日志框架,定义打印日志的统一接口,提供针对不同日志框架的适配器

## Java代码实现 SDK适配器

类适配器，如下图：

![图 4](/files/BqxCqzdYMgQiTLwKjc84)

```java
/**
 * SD卡接口
 **/
public interface SDCard {

    //读取SD卡方法
    String readSD();
    //写入SD卡功能
    void writeSD(String msg);
}

/**
 * SD卡实现类
 **/
public class SDCardImpl implements SDCard {
    @Override
    public String readSD() {
        String msg = "sd card reading data";
        return msg;
    }

    @Override
    public void writeSD(String msg) {
        System.out.println("sd card write data : " + msg);
    }
}

/**
 * TF卡接口
 **/
public interface TFCard {

    //读取TF卡方法
    String readTF();
    //写入TF卡功能
    void writeTF(String msg);
}

/**
 * TF卡实现类
 **/
public class TFCardImpl implements TFCard {

    @Override
    public String readTF() {

        String msg = "tf card reading data";
        return msg;
    }

    @Override
    public void writeTF(String msg) {
        System.out.println("tf card write data : " + msg);
    }
}

/**
 * 定义适配器类(SD兼容TF)
 **/
public class SDAdapterTF extends  TFCardImpl implements SDCard{

    @Override
    public String readSD() {
        System.out.println("adapter read tf card ");
        return readTF();
    }

    @Override
    public void writeSD(String msg) {
        System.out.println("adapter write tf card");
        writeTF(msg);
    }
}

public class Client {

    public static void main(String[] args) {

        Computer computer = new Computer();
        SDCard sdCard = new SDCardImpl();
        System.out.println(computer.read(sdCard));

        SDAdapterTF adapterTF = new SDAdapterTF();
        System.out.println(computer.read(adapterTF));
    }
}
```

如果是对象适配器：

```java
public class SDAdapterTF implements SDCard{

    private TFCard tfCard;

    public SDAdapterTF(TFCard tfCard) {
        this.tfCard = tfCard;
    }

    @Override
    public String readSD() {
        System.out.println("adapter read tf card ");
        return tfCard.readTF();
    }

    @Override
    public void writeSD(String msg) {
        System.out.println("adapter write tf card");
        tfCard.writeTF(msg);
    }
}


public class Client {

    public static void main(String[] args) {

        Computer computer = new Computer();
        SDCard sdCard = new SDCardImpl();
        System.out.println(computer.read(sdCard));

        TFCard tfCard = new TFCardImpl();
        SDAdapterTF adapterTF = new SDAdapterTF(tfCard);
        System.out.println(computer.read(adapterTF));
    }
}
```