一个关于嵌套interface的问题

问题：
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 NestingIntegerfaces
{
public class BImp implements A.B
{
public void f() {};
}

class CImp implements A.C
{
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();
//A.D ad=a.getD();
A a2=new A();
a2.receiveD(a.getD());
}
}
在main()中为什么//A.D ad=a.getD()要注释掉，不注释的话，为什么编译通过不了？