Android截击未知构造函数

Android volley unknown constructor

我正在尝试使用 android 截击。到目前为止,这是我的代码:

JsonObjectRequest getRequest = new JsonObjectRequest(Request.Method.GET, url, null,
        new Response.Listener<JSONObject>()
        {
            @Override
            public void onResponse(JSONObject response) {
                System.out.println(response);
            }
        },
        new Response.ErrorListener()
        {
            @Override
            public void onErrorResponse(VolleyError error) {
                System.out.println(error);
            }
        }
);

这里是我关于凌空抽射的介绍:

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonObjectRequest;
import com.android.volley.toolbox.Volley;

然而,这给我带来了 "Cannot resolve constructor" 错误。有什么想法吗?

null 转换为 String

JsonObjectRequest getRequest = new JsonObjectRequest(Request.Method.GET, url, (String)null,
        new Response.Listener<JSONObject>()
        {
            @Override
            public void onResponse(JSONObject response) {
                System.out.println(response);
            }
        },
        new Response.ErrorListener()
        {
            @Override
            public void onErrorResponse(VolleyError error) {
                System.out.println(error);
            }
        }
);