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//Callbacks.java
 
interface Incrementable
{
    void increment();
}
 
class Callee1 implements Incrementable
{
    private int i=0;
    public void increment()
    {
        i++;
        System.out.println(i); 
    } 
}
 
class MyIncrement
{
    public void increment()
    {
        System.out.println("Other operation");
    }
    public static void f(MyIncrement mi)
    {
        mi.increment();                             // 這個是不是 Callback??
    }
}
 
class Callee2 extends MyIncrement
{
    private int i=0;
    private void incr()
    {
        i++;
        System.out.println(i);
    }
    private class Closure implements Incrementable  //這個名稱是 Closure，
    {                                                                 //不過為什麼呢？？
        public void increment() { incr(); }     // <---- 這個是 callback ??    
    }
    Incrementable getCallbackReference()
    {
        return new Closure();
    }
    
}
 
class Caller
{
    private Incrementable callbackReference;
    Caller(Incrementable cbh)
    {
        callbackReference=cbh;
    }
    void go()
    {
        callbackReference.increment();   
    }
}
 
public class Callbacks
{
    public static void main(String[] args)
    {
        Callee1 c1=new Callee1();
        Callee2 c2=new Callee2();
        MyIncrement.f(c2);
        Caller caller1=new Caller(c1);
        Caller caller2=new Caller(c2.getCallbackReference());
        caller1.go();
        caller1.go(); 
        caller2.go();
        caller2.go(); 
    }
}