Android双击返回退出应用的写法

private boolean mCanExit = false;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(keyCode==KeyEvent.KEYCODE_BACK && event.getAction()==KeyEvent.ACTION_DOWN){
        if (mCanExit) {
            finish();
            System.exit(0);
            return true;
        }
        mCanExit = true;
        Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
        mHandler.postDelayed(new Runnable() {
            @Override
            public void run() {
                mCanExit = false;
            }
        }, 2000);
        return true;
    }
    return super.onKeyDown(keyCode, event);
}