Android 返回键退出的两种方式

package com.example.onemoreexit;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.KeyEvent;

public class MainActivity extends Activity {

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

// 第一种方法：打击弹出对话框，提示是否退出，可以自己设置提示语言，以及图片
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
switch (keyCode) {
case KeyEvent.KEYCODE_BACK:
AlertDialog.Builder build = new AlertDialog.Builder(this);
build.setTitle("系统提示").setMessage("确定要退出吗？");
build.setPositiveButton("确定",
new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
finish();
}
});
build.setNegativeButton("取消",
new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
}
}).show();
break;
}
return super.onKeyDown(keyCode, event);
}

// 第二种方法（再按一次退出程序）两秒内双击返回键退出app
private long exitTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK
&& event.getAction() == KeyEvent.ACTION_DOWN) {
if ((System.currentTimeMillis() - exitTime) > 2000) {
Toast.makeText(getApplicationContext(), "再按一次退出程序",
Toast.LENGTH_SHORT).show();
exitTime = System.currentTimeMillis();
} else {
finish();
System.exit(0);
}
return true;
}
return super.onKeyDown(keyCode, event);
}
}