java - 您如何实现回调以及回调的目的是什么？

Question

在这个项目中，我使用一个接口(interface)创建了两个类。我想在我的线程完成任务时打印 1 到 100。我认为使用回调我可以使用以下方法实现它:

MyScreen.java

package com.stackoverflow;

public final class MyScreen extends MainScreen
{
    /**
     * Creates a new MyScreen object
     */
    public MyScreen()
    {        
        // Set the displayed title of the screen       
       task tt=new task();
       tt.start();
       /// Here I think callback is used ...
       for (int i = 0; i < 100; i++) {
        System.out.println("i"+i);
       }
    }
}

GetDataCallBack.java

package mypackage;

public interface GetDataCallBack {
    public void getCount(int count);    
}

task.java

package mypackage;

public class task extends Thread {
    int count=0;
    public void run() {
        for (int i = 0; i < 150000; i++) {
            count+=i;
            System.out.println(count);
        }
            //How to send callback ?
    }
}

Answer 1

Java使用Observable提供回调机制和 Observer 。阅读 javdocs 了解更多信息:

以下示例取自:http://www.java2s.com/Code/Java/Design-Pattern/AsimpledemoofObservableandObserver.htm

/*
 * Copyright (c) Ian F. Darwin, http://www.darwinsys.com/, 1996-2002.
 * All rights reserved. Software written by Ian F. Darwin and others.
 * $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS''
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 * Java, the Duke mascot, and all variants of Sun's Java "steaming coffee
 * cup" logo are trademarks of Sun Microsystems. Sun's, and James Gosling's,
 * pioneering role in inventing and promulgating (and standardizing) the Java 
 * language and environment is gratefully acknowledged.
 * 
 * The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
 * inventing predecessor languages C and C++ is also gratefully acknowledged.
 */

import java.util.Observable;
import java.util.Observer;

/**
 * A simple demo of Observable->Observer
 * 
 * @author Ian Darwin
 * @version $Id: ObservDemo.java,v 1.3 2003/12/29 19:52:22 ian Exp $
 */
public class ObservDemo extends Object {
  MyView view;

  MyModel model;

  public ObservDemo() {

    view = new MyView();

    model = new MyModel();
    model.addObserver(view);

  }

  public static void main(String[] av) {
    ObservDemo me = new ObservDemo();
    me.demo();
  }

  public void demo() {
    model.changeSomething();
  }

  /** The Observer normally maintains a view on the data */
  class MyView implements Observer {
    /** For now, we just print the fact that we got notified. */
    public void update(Observable obs, Object x) {
      System.out.println("update(" + obs + "," + x + ");");
    }
  }

  /** The Observable normally maintains the data */
  class MyModel extends Observable {
    public void changeSomething() {
      // Notify observers of change
      setChanged();
      notifyObservers();
    }
  }
}