Android实现首次点击返回键提示信息，第二次点击退出应用

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode == KeyEvent.KEYCODE_BACK){
            long secondClickBack = System.currentTimeMillis();
            if(secondClickBack - firstClickBack >1500){
                Toast.makeText(this, "再按一次退出每一天", 0).show();
                firstClickBack = secondClickBack;
                return true;
            }else{
                MyApplication.getInstance().exitApp();
                return true;
            }
        }
        return super.onKeyDown(keyCode, event);
    }

package com.zsx.base;

import java.util.ArrayList;

import android.app.Activity;
import android.app.Application;

public class MyApplication extends Application {
    public static ArrayList<Activity> list;
    private static MyApplication myApp = null;

    //要保证每个Activity中使用的MyApplication都是同一个，就用到了单例模式
    //单例模式：只有一个对象，也就是上面的那个myApp只实例化一次
    //由于MyApplication继承自Application，复写的构造函数访问权限不能低于父类的，所以不能为private
    //所以我们在这里实现一个伪单例模式
    public static MyApplication getInstance(){
        if(myApp == null ){
            myApp = new MyApplication();
            list = new ArrayList<Activity>();
        }
        return myApp;
    }
    /**
     * 添加activity到数组中
     * @param activity
     */
    public void addActivity(Activity activity){
        list.add(activity);
    }

    /**
     * 退出应用，结束所有activity
     */
    public void exitApp(){
        for (Activity activity : list) {
            activity.finish();
        }
    }
}