知識點--（private interface)

剛開始在論壇裏提到private interface時候，很多人都很迷惑：interface
不是讓別人來實現的嗎，爲什麼會有private的interface啊！應該說這不全對，
的確，同class一樣，一般的interface只能是public或是friendly的（當然還
可以是class還可以是final的，如果你不想別人改寫你的class），但是對於
nesting inteface 是可以被聲明爲private的：先看看《TIJ》中是一個簡單例子：
+--------------------------------------------------------------+
//: c08:NestingInterfaces.java
// From 'Thinking in Java, 2nd ed.' by Bruce Eckel
// www.BruceEckel.com. See copyright notice in CopyRight.txt.

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 {//---private
    void f();
}
private class DImp implements D { //was implemented by private class
    public void f() {}
}
public class DImp2 implements D {//Notice:was implements by public class
    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:
    //because D is private
    //! A.D ad = a.getD();
    // Doesn't return anything but A.D:
    //!A.DImp2 di2 = a.getD();
    /**Error message---------------+
     ---------- Compile ----------
      NestingInterfaces.java:83: incompatible types
      found   : A.D
      required: A.DImp2
      A.DImp2 di2 = a.getD();                       ^
      1 error
    ----------------------------------*/
    // Cannot access a member of the interface:
    //! a.getD().f();
    /**--Error Messae-------------------
    ---------- Compile ----------
    NestingInterfaces.java:85: f() in A.D is not defined
    in a public class or interface; cannot be accessed from
    outside package
    a.getD().f();       ^
    1 error
    -------------------------------------*/
    // Only another A can do anything with getD():
    A a2 = new A();
    a2.receiveD(a.getD());
}
} ///:~
==================================================================
剛開始我認爲privte interface只能被private 實現，但是很明顯， public class DImp2
也實現了private interface，不過雖然DImp2是public的，但是他卻不能對外界說他實現了一個
private interface，也就是說在inteface D所在class外產生的DImp2的對象不能向上轉型，
+--動手實驗一下-----------------------+
class PrivateInterface
{
private interface InterfaceTest {//private
void f();
}
public class ImplementsTest implements InterfaceTest
{
public void f(){//override
    System.out.println("Test_1");
}
}
public InterfaceTest Getme(){
    return new ImplementsTest();
}
public void Test(InterfaceTest ab){
    ab.f();
    System.out.println(ab);
}
public static void main(String arg[]){
PrivateInterface a = new PrivateInterface();
//PrivateInterface.InterfaceTest pi=a.Getme();//Upcasting:可以向上轉型，在同一個class中
   pi.f(); //NO problem
   a.Test(a.Getme());//NO problem
}
}
class Mytest
{
   public static void main(String args[]){
   PrivateInterface m= new PrivateInterface();
   // m.Test(m.Getme());//可以通過這個來調用f()
   /**--------------------Reason-----------------------------+
   因爲Getme()和Test()都是public的，所以對外是可被
   調用的,且Test()方法中的對象擁有對Getme()返回值(x)的
   使用權，所以可以通過Test()來調用(x),並通過x來調用
   ImplementsTest中的f(),[實際上是：x先向上轉型爲
   InterfaceTest，然後再調用對應的覆寫方法，這裏因該是
   用到了多態裏面的後期綁定吧（late-binding）]
   +----------------------------------------------------------------*/
   //由於InterfaceTest是private的，所以對外不可見。
   //更談不上向上轉型了（見下）：
   PrivateInterface.InterfaceTest pi=m.Getme();//Errior
   pi.f();//Error
}
}
---------- Compile ----------
PrivateInterface.java:30: PrivateInterface.InterfaceTest has private
access in PrivateInterface
    PrivateInterface.InterfaceTest pi=m.Getme();
                    ^
PrivateInterface.java:31: f() in PrivateInterface.InterfaceTest is not
defined in a public class or interface; cannot be accessed from outside package
pi.f();
          ^
2 errors