Interface注意事项

Interface

成员声明

• 声明属性，默认static & final
• 声明方法，默认public

interface Instrument {
    int VALUE = 5; // static & final
    // Cannot have method definitions:
    void play(Note n); // Automatically public
    void adjust();
}

接口多重继承的重名问题

• 尽量避免接口方法重名

interface I1 { void f();}
interface I2 { int f(int i);}
interface I3 { int f();}

class C {
    public int f() {
        return 1;
    }
}

// Methods differ only by return type:
class C1 extends C implements I1 {}
interface I4 extends I1, I3 {}

类中嵌套接口

• 在类中嵌套接口，可以声明接口为private
• A.DImp2只能被其自身使用，无法向上转型
• 要想使用getD()的返回值，只能通过A的其他对象对这个返回值进行操作，如receiveD()所展示的那样

class A {
    private interface D {
        void f();
    }

    public class DImp2 implements D {
        @Override
        public void f() {
        }
    }

    public D getD() {
        return new DImp2();
    }

    private D dRef;

    public void receiveD(D d) {
        dRef = d;
        dRef.f();
    }
}

class NestingInterfaces {
    public static void main(String[] args) {
        A a = new A();

//// Doesn’t return anything but A.D:
//        A.DImp2  di2 = a.getD();
//// Cannot access a member of the interface:
//        a.getD().f();
// Only another A can do anything with getD():
        A a2 = new A();
        a2.receiveD(a.getD());
    }
}

• 正常的向上转型如下：

interface BB {
    void f();
}

class DD implements BB {

    @Override
    public void f() {
        System.out.println("DD");
    }
}
class UU {
    public static void main(String[] args) {
        BB dd = new DD();
        dd.f();
    }
}