按二次back键退出程序应用的实现

package com.loaderman.twoexitdemo;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.view.KeyEvent;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            isExit = false;
        }
    };

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

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        exit();
        return event.equals(KeyEvent.KEYCODE_BACK );
    }

    private static boolean isExit = false;

    private void exit() {
        if (!isExit) {
            isExit = true;
            Toast.makeText(this, "再次点击退出程序", Toast.LENGTH_SHORT).show();
            handler.sendEmptyMessageDelayed(0, 2000);
        } else {
            finish();
            System.exit(0);
        }
    }
}