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);
}

}