依賴倒置原則（Dependence Inversion Principle）

1，高層模塊不應該依賴底層模塊
2，依賴於抽象，依賴於接口。不要依賴實現細節。
3，細節依賴抽象。

案例：

模仿用戶通過鑰匙開門，門的鎖是電子鎖

public class ElectronicLock {

    private String style;

    public String getStyle() {
        return style;
    }

    public void setStyle(String style) {
        this.style = style;
    }

    public void open() {
        System.out.println("開鎖功能");
    }

    public void backLocking() {
        System.out.println("反鎖功能");
    }
}

public interface ILock {
    public void installLock(ElectronicLock elLock);
}

public interface IDoor {

    public void open();

    public void close();

}

public class Door implements ILock, IDoor {

    ElectronicLock elLock;

    @Override
    public void installLock(ElectronicLock elLock) {
        this.elLock = elLock;
    }

    @Override
    public void open() {
        elLock.open();
    }

    @Override
    public void close() {
        elLock.close();
    }
}

上面的情況若果鎖壞了，或者要換安全性比較高鎖，換成電控鎖，需要修改鎖的依賴的實現。
public class ElectricControlLock {

    private String style;

    public String getStyle() {
        return style;
    }

    public void setStyle(String style) {
        this.style = style;
    }

    public void open() {
        System.out.println("開鎖功能");
    }

    public void close(){
        System.out.println("啓動所鎖功能");
    }

    public void backLocking() {
        System.out.println("反鎖功能");
    }

    public void remoteControl(){
        System.out.println("通過手機開鎖");
    }
}
public class Door implements ILock, IDoor {

    ElectricControlLock elLock;

    @Override
    public void installLock(ElectricControlLock elLock) {
        this.elLock = elLock;
    }

    @Override
    public void open() {
        elLock.open();
    }

    @Override
    public void close() {
        elLock.close();
    }
}
如果換成依賴抽象。
public interface IKey {
    public void close();

    public void open();
}

public class Door implements ILock, IDoor {

    IKey elLock;

    @Override
    public void installLock(IKey elLock) {
        this.elLock = elLock;
    }

    @Override
    public void open() {
        elLock.open();
    }

    @Override
    public void close() {
        elLock.close();
    }
}
//電子鎖
public class ElectronicLock implements IKey {

    private String style;

    public String getStyle() {
        return style;
    }

    public void setStyle(String style) {
        this.style = style;
    }

    public void open() {
        System.out.println("打開鎖功能");
    }

    public void close(){
        System.out.println("啓動鎖功能");
    };
}

//電控鎖
public class ElectricControlLock implements IKey {

    private String style;

    public String getStyle() {
        return style;
    }

    public void setStyle(String style) {
        this.style = style;
    }

    public void open() {
        System.out.println("通過手機開鎖功能");
    }

    public void close() {
        System.out.println("通過手機啓動鎖功能");
    }
}
新增之後實現不用去修改依賴的代碼