实现点击两次返回键退出

//第一步，定义一个变量，用于标识是否退出
boolean isExit;

//第二步，重写Activity中onKeyDown方法
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        exit();
        return false;
    } else {
        return super.onKeyDown(keyCode, event);
    }
}

//第三步，写一个退出方法
public void exit(){
    if (!isExit) {
        isExit = true;
        Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
        mHandler.sendEmptyMessageDelayed(0, 2000);
    } else {
        Intent intent = new Intent(Intent.ACTION_MAIN);
        intent.addCategory(Intent.CATEGORY_HOME);
        startActivity(intent);
        System.exit(0);
    }
}

//第四步，根据exit()方法中的的消息，写一个Handler
Handler mHandler = new Handler() {

    @Override
    public void handleMessage(Message msg) {
        // TODO Auto-generated method stub
        super.handleMessage(msg);
        isExit = false;
    }
 
};