INT org.json.JSONArray.length（）上的一个空对象引用 [英] int org.json.JSONArray.length() on a null object reference
问题描述
我用凌空库连接到服务器，我有一个交易类和服务器类与此codeS：
I'm using volley library to connect to server and I have a Transaction class and a Server class with this codes:
横贯：
public class Trans extends Server {
public Object suggest(Context context) {
return connect("xxxxxxx", Request.Method.GET);
}
}
服务器：
public Object connect(String url, int method) {
final Object[] object = {null};
StringRequest postRequest;
postRequest = new StringRequest(method, url,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
object[0] = response;
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
}
) {
@Override
protected Map<String, String> getParams() throws AuthFailureError {
Map<String, String> params = new HashMap<String, String>();
return params;
}
@Override
public String getBodyContentType() {
return "application/x-www-form-urlencoded; charset=UTF-8";
}
};
int socketTimeout = 20000;
RetryPolicy policy = new DefaultRetryPolicy(socketTimeout,
DefaultRetryPolicy.DEFAULT_MAX_RETRIES,
DefaultRetryPolicy.DEFAULT_BACKOFF_MULT);
postRequest.setRetryPolicy(policy);
AppController.getInstance().addToRequestQueue(postRequest);
return object[0];
}
但是，当我试图让 JSONArray 与此：
@Override
protected void onResume() {
ArrayList<Suggests> arrayList = new ArrayList<>();
JSONArray jsonArray = (JSONArray) trans.suggest(MainActivity.this);
for (int i = 0; i < jsonArray.length(); i++) {
try {
JSONObject item = (JSONObject) jsonArray.get(i);
Suggests suggests = new Suggests();
suggests.title = item.getString("title");
suggests.type = item.getString("type");
arrayList.add(suggests);
} catch (JSONException e) {
e.printStackTrace();
}
}
super.onResume();
}
应用程序强制关闭，我有这个错误的logcat ：
java.lang.RuntimeException: Unable to resume activity {ir.aftabeshafa.shafadoc/ir.aftabeshafa.shafadoc.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int org.json.JSONArray.length()' on a null object reference
有什么问题，我应该如何解决？
What's the problem and how can I fix it?
推荐答案
去关建议，意见，
第1步定义将传递沿着你想要的结果的接口。在＆LT; T＆GT; 是一个泛型类型，这样你就可以回到你想要的任何数据。你会看到，在步骤2
Step 1 Define an interface that will pass-along the result you want. The <T> is a generic type, so you can return whatever data you want. You'll see that in Step 2.
public interface AsyncResponse<T> {
void onResponse(T response);
}
第2步更新服务器和跨类借此作为一个参数
Step 2 Update the Server and Trans classes to take this as a parameter
public class Server {
public void connect(String url, int method, final AsyncResponse<String> asyncResponse) {
StringRequest request = new StringRequest(method, url, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
if (asyncResponse != null) {
asyncResponse.onResponse(response);
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("VolleyError", error.getMessage());
Log.e("VolleyError", new String(error.networkResponse.data));
}
}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
return params;
}
@Override
public String getBodyContentType() {
return "application/x-www-form-urlencoded; charset=UTF-8";
}
};
int socketTimeout = 20000;
RetryPolicy policy = new DefaultRetryPolicy(socketTimeout,
DefaultRetryPolicy.DEFAULT_MAX_RETRIES,
DefaultRetryPolicy.DEFAULT_BACKOFF_MULT);
request.setRetryPolicy(policy);
AppController.getInstance().addToRequestQueue(request);
}
}
侧的问题：你是否真的需要在上下文？它不使用...
public class Trans extends Server {
public void suggest(Context context, AsyncResponse<String> asyncResponse) {
connect("xxxxxxxx", Request.Method.GET, asyncResponse);
}
}
第3步：使用像这样的方法来获得一个回调的结果。 //jsonplaceholder.typi$c$c.com/users HTTP测试一>，它返回10 User对象的JSONArray。
Step 3 Use your method like so to get a result in a callback. This was tested against the url http://jsonplaceholder.typicode.com/users, which returns a JSONArray of 10 User objects.
的 ArrayAdapter 加入了一个完整的例子，因为它没有出现在的ArrayList 的问题是有益的为了任何东西。
The ArrayAdapter was added for a full example since it didn't appear the ArrayList in the question was useful for anything.
public class MainActivity extends AppCompatActivity {
private ArrayList<User> arrayList;
private Trans trans;
private ArrayAdapter<User> adapter;
private ListView listView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
trans = new Trans();
arrayList = new ArrayList<User>();
adapter = new ArrayAdapter<User>(MainActivity.this, android.R.layout.simple_list_item_1, arrayList);
listView = (ListView) findViewById(android.R.id.list);
listView.setAdapter(adapter);
}
@Override
protected void onResume() {
trans.suggest(MainActivity.this, new AsyncResponse<String>() {
@Override
public void onResponse(String response) {
arrayList.clear(); // prevent duplicate data
try {
// The URL that was tested returns a JSONArray
// Change to JSONObject, if necessary
JSONArray jsonArray = new JSONArray(response);
for (int i = 0; i < jsonArray.length(); i++) {
JSONObject item = (JSONObject) jsonArray.get(i);
User user = new User();
user.username = item.getString("name");
user.email = item.getString("email");
arrayList.add(user);
}
} catch (JSONException e) {
e.printStackTrace();
}
// Notify since the arrayList has changed the adapter
adapter.notifyDataSetChanged();
}
});
super.onResume();
}
}
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