Android实现点击两次返回键退出

package com.gaolei.exitdemo;import android.app.Activity;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.KeyEvent;import android.widget.Toast;public class MainActivity extends Activity {  // 定义一个变量，来标识是否退出  private static boolean isExit = false;  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 false;    }    return super.onKeyDown(keyCode, event);  }  private void exit() {    if (!isExit) {      isExit = true;      Toast.makeText(getApplicationContext(), "再按一次退出程序",          Toast.LENGTH_SHORT).show();      // 利用handler延迟发送更改状态信息      mHandler.sendEmptyMessageDelayed(0, 2000);    } else {      finish();      System.exit(0);    }