Android按返回键退出程序的两种方式

package com.example.onemoreexit;import android.app.Activity;import android.app.AlertDialog;import android.content.DialogInterface;import android.os.Bundle;import android.view.KeyEvent;import android.widget.Toast;public class MainActivity extends Activity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);}  // 1.以AlertDialog的方式  @Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {switch (keyCode) {case KeyEvent.KEYCODE_BACK:AlertDialog.Builder build = new AlertDialog.Builder(this);build.setTitle("提示").setMessage("确定退出？");build.setPositiveButton("确定", new DialogInterface.OnClickListener() {@Overridepublic void onClick(DialogInterface dialog, int which) {finish();}});build.setNegativeButton("取消", null).show();break;}return super.onKeyDown(keyCode, event);} // 2.以Toast的方式/*private long exitTime = 0;@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {if ((System.currentTimeMillis() - exitTime) > 2000) {Toast.makeText(getApplicationContext(), "再按一次退出", Toast.LENGTH_SHORT).show();exitTime = System.currentTimeMillis();} else {finish();System.exit(0);}return true;}return super.onKeyDown(keyCode, event);}*/}