第一种方式

// 定义一个变量，来标识是否退出
private static boolean isExit = false;
Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
    super.handleMessage(msg);
    isExit = false;
  }
 };

public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
    if (!isExit) {
        isExit = true;
        Toast.makeText(getApplicationContext(), "再按一次退出程序",
                Toast.LENGTH_SHORT).show();
        // 利用handler延迟发送更改状态信息
        mHandler.sendEmptyMessageDelayed(0, 2000);
    } else {
        exit(this);
    }
    return false;
}
return super.onKeyDown(keyCode, event);}

第二种方式

//保存点击的时间
private long exitTime = 0;
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
    if ((System.currentTimeMillis() - exitTime) > 2000) {
        Toast.makeText(getApplicationContext(), "再按一次退出程序",
                Toast.LENGTH_SHORT).show();
        exitTime = System.currentTimeMillis();
    } else {
                    exit();
                  }
    return false;
}
    return super.onKeyDown(keyCode, event);
}