Interfaces for Callbacks（Java中回调）

Interfaces for Callbacks

A common technique in the C language is to pass a pointer to a function in the argument list of another function. The receiving function can invoke the passed function using the pointer. This approach is referred to as a callback.

通常在C中通过指针作为函数参数传递给其他函数，被调用的函数可以调用传递过来的指针来实现回调。
for example:

void  callback( void ) {
　　　//todo:write your code in here.
　　　//回调时要做的事
   
}
typedef   void  (*)( void ) _callback;  //定义类型
void  AnotherProc(_callback Callbacked)
{
    Callbacked();
}

void  main( void )
{
    AnotherProc(callback);
}

Callbacks are very useful in situations where you want to invoke different functions without needing to know what particular function is being invoked. For example, a plotting function could receive a pointer to a function that takes a value on the horizontal axis as an argument and returns a value for the vertical axis. The plotting function could then plot any such function that is passed to it.

Java, however, does not provide for pointers (direct memory addresses) to methods. Instead, interfaces are used for callbacks. In this case, a method holds an interface reference in its argument list and then invokes a method in the interface.

(The Method class in the Reflection package can represent a method and can invoke it in a general manner. However, it is not recommended to use Method for callbacks. It is rather clumsy and it also loses the type checking on the arguments and return type during the compilation.)

In the following code, we see that the aFunc(Switchable obj) method invokes the getState() method of the Switchable interface. An instance of any class that implements the Switchable interface can be passed, thus providing the same generality as the pointer callbacks in C.

 1 public   class  TestCallBack
 2 { 
 3    public   static   void  main(String [] args)
 4    {
 5     Switchable []switches  =   new  Switchable[ 2 ];
 6     switches[ 0 ]  =   new  Relay();
 7     switches[ 1 ]  =   new  Relay();
 8     switches[ 2 ]  =   new  Valve();;
 9
10      for (  int  i = 0 ; i < 3 ; i ++ )
11      { 
12       aFunc(switches[i]);
13     }  
14   }
15    //  Pass Switchable objects and call their getState() 
16    void  aFunc( Switchable obj)
17    { 
18        if (obj.getState()) doSomething();
19   }
20    other code
21 }  
22

See previous example for the Switchable,
Relay, & Valve definitions.