package com.example.clickexittest; 

  import android.app.Activity; 
  import android.os.Bundle; 
  import android.os.Handler; 
  import android.os.Message; 
  import android.util.Log; 
  import android.view.KeyEvent; 
  import android.widget.Toast; 

  public class MainActivity extends Activity { 

    private static final String TAG = MainActivity_Exit.class.getSimpleName(); 

    // 定义一个变量，来标识是否退出 
    private static boolean isExit = false; 

    private static Handler mHandler = new Handler() { 

      @Override 
      public void handleMessage(Message msg) { 
        super.handleMessage(msg); 
        isExit = false; 
      } 
    }; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activity_main); 

    } 

    @Override 
    public boolean onKeyDown(int keyCode, KeyEvent event) { 
      if (keyCode == KeyEvent.KEYCODE_BACK) { 
        exit(); 
        return true; 
      } 
      return super.onKeyDown(keyCode, event); 
    } 

    private void exit() { 
      if (!isExit) { 
        isExit = true; 
        Toast.makeText(getApplicationContext(), "再按一次后退键退出程序", 
            Toast.LENGTH_SHORT).show(); 
        // 利用handler延迟发送更改状态信息 
        mHandler.sendEmptyMessageDelayed(0, 2000); 
      } else { 

        Log.e(TAG, "exit application"); 

        this.finish(); 
      } 
    } 
  }

 package com.example.clickexittest; 

  import android.app.Activity; 
  import android.os.Bundle; 
  import android.util.Log; 
  import android.view.KeyEvent; 
  import android.widget.Toast; 

  public class MainActivity extends Activity { 

    private static final String TAG = MainActivity.class.getSimpleName(); 

    private long clickTime = 0; //记录第一次点击的时间 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activity_main); 

    } 

    @Override 
    public boolean onKeyDown(int keyCode, KeyEvent event) { 
      if (keyCode == KeyEvent.KEYCODE_BACK) { 
        exit(); 
        return true; 
      } 
      return super.onKeyDown(keyCode, event); 
    } 

    private void exit() { 
      if ((System.currentTimeMillis() - clickTime) > 2000) { 
        Toast.makeText(getApplicationContext(), "再按一次后退键退出程序", 
            Toast.LENGTH_SHORT).show(); 
        clickTime = System.currentTimeMillis(); 
      } else { 
        Log.e(TAG, "exit application"); 
        this.finish(); 
  //     System.exit(0); 
      } 
    } 
  }