请教大家interface问题------thinking in java中的一个例子

[测试一下]

//: interfaces/nesting/NestingInterfaces.java
//package interfaces.nesting;

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();
  }
  // Redundant "public":
  public interface H {
    void f();
  }
  void g();
  // Cannot be private within an interface:
  //! private interface I {}
}	

public class NestingInterfaces {
  public class BImp implements A.B {
    public void f() {}
  }
  class CImp implements A.C {
    public void f() {}
  }
  // Cannot implement a private interface except
  // within that interface's defining class:
  // ! 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();
    // Can't access A.D:
    //! A.D ad = a.getD();
    // 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());
  }
} ///:~

上面例子为什么这句会不可以了?
//! A.DImp2 di2 = a.getD();
前面不是有 public D getD() { return new DImp2(); } 吗？ 这里面返回的就是new Dimp2( ).
另外interface嵌套用的多吗?一般什么时候用?麻烦大家了。谢谢～～～

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