public class T {
    public static void main(String[] args) {
        CallBack callback = new CallBackImpl();

        // 两次耗时操作，时间不确定
        new T().doSomeThing(url, callback);
        new T().doSomeThing(url2, callback);
    }
}

class CallBackImpl implements CallBack {

    @Override
    public void onFinish(String result) {
        System.out.println("onFinish");
    }

}

interface CallBack {
    public void onFinish();
}