所謂回調，就是客戶程序C調用服務程序S中的某個函數A，然后S又在某個時候反過來調用C中的某個函數B，對于C來說，這個B便叫做回調函數。

例：  
  1.class   A,class   B  
  2.class   A實現接口operate  
  3.class   B擁有一個參數為operate接口類型的函數test(operate   o)  
  4.class   A運行時調用class   B中test函數,以自身傳入參數  
  5.class   B已取得A，就可以隨時回調A所實現的operate接口中的方法

舉個例子來說:

public interface InterestingEvent
{

    // This is just a regular method so it can return something or

    // take arguments if you like.

    public void interestingEvent ();

}

public class EventNotifier
{

    private InterestingEvent ie;

    private boolean somethingHappened;

    public EventNotifier (InterestingEvent event)
    {

        // Save the event object for later use.

        ie = event;

        // Nothing to report yet.

        somethingHappened = false;

    }

    // 

    public void doWork ()
    {

        // Check the predicate, which is set elsewhere.

        if (somethingHappened)
        {

            // Signal the even by invoking the interface's method.

            ie.interestingEvent ();

        }

        //
    }

    //

}

public class CallMe implements InterestingEvent
{

    private EventNotifier en;

    public CallMe ()
    {

        // Create the event notifier and pass ourself to it.

        en = new EventNotifier (this);

    }

    // Define the actual handler for the event.

    public void interestingEvent ()
    {

        // Wow!  Something really interesting must have occurred!

        // Do something

    }

    //

}