Android 实现双击退出应用程序

效果图：

实现代码：

package com.example.android12_zhangkai;

import android.content.DialogInterface;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private long firstTime=0;

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


    }


    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        switch (keyCode){
            case KeyEvent.KEYCODE_BACK:
                long secondTime=System.currentTimeMillis();
                if (secondTime-firstTime>2000){
                    Toast.makeText(MainActivity.this,"再按一次退出程序",Toast.LENGTH_SHORT).show();
                    firstTime=secondTime;
                    return true;
                }else{
                    System.exit(0);
                }
                break;
        }
        return super.onKeyUp(keyCode, event);
    }

}