点击两次back退出app

 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back键退出app",

           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();

     }

   }

 }