连续点击返回键，退出app实例源码（android）

package com.mingrisoft;import android.app.Activity;import android.content.DialogInterface;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.view.KeyEvent;import android.view.View;import android.widget.Toast;public class MainActivity extends Activity { private long exitTime = 0; //退出时间变量值 @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } @Override public boolean onKeyDown(int keyCode, KeyEvent event) { //判断是否单击了返回按键 if (keyCode == KeyEvent.KEYCODE_BACK) { exit(); //创建并调用退出方法 return true; //拦截返回键 } return super.onKeyDown(keyCode, event); } public void exit() { if ((System.currentTimeMillis() - exitTime) > 2000) { //计算按键时间差是否大于两秒 Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show(); exitTime = System.currentTimeMillis(); } else { finish(); System.exit(0); //销毁强制退出 } }}