《thinking in java》 接口与内部类

书本上的例子

改编后的。

package test2;

class A{
    interface B{void f();}
    public class BImp implements B{public void f(){}}
    private class BImp2 implements B{public void f(){}}
    
    public interface C{void f();}
    class CImp implements C{public void f(){}}
    private class CImp2 implements C{public void f(){}}
    
    private interface D{void f();}
    private class DImp implements D{public void f(){}}
    public class DImp2 implements D{public void f(){}}
    
    public D getD(){return new DImp2();}
    private D dRef;
    public void receiveD(D d){dRef=d;dRef.f();}    
    
}
interface E{
    interface G{void f();}
    
    public interface H{void f();}
    void g();
}
public class NestingInterfaces {

    public class BImp implements A.B{public void f(){}}
    class CImp implements A.C{public void f(){}}
    
//    The type A.D is not visible
//    class DImp implements A.D{public void f(){}}
    
    class EImp implements E{public void g(){}}
    class EGimp implements E.G{public void f(){}}
    
    class EImp2 implements E{
        public void g(){}
        class EG implements E.G{public void f(){}}
    }
    
    public static void main(String[] args) {
        A a=new A();
        
        //编译器在A.D处报错：The type A.D is not visible
//        A.D ad=a.getD();
        
        //编译器在 a.getD() 处报错：Type mismatch: cannot convert from A.D to A.DImp2
//        A.DImp2 di2=a.getD();  
        
        A.DImp2 di2=(A.DImp2)a.getD();
        System.out.println(di2.getClass().getName());//test2.A$DImp2
         
        //The type A.D is not visible
//        a.getD().f();
        
        ((A.DImp2)a.getD()).f();
        
        A a2=new A();
        a2.receiveD(a.getD());
    }
    
}

可以顺利运行。

PS：编译器识别代码总是从左到右，从上到下，一步步进行的。至于代码的运行逻辑，那是识别之后的事情。