我读过 What is a NullPointerException, and how do I fix it? Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference 还有一些，但我没有运气修理它。
这是我从排球错误监听器中得到的错误 -
com.android.volley.VolleyError：java.lang.NullPointerException：尝试调用虚方法＆＃39; int java.lang.String.length（）＆＃39;在空对象引用上
我对android很新，有人可以向我解释一下我的问题是什么原因并帮我修复它吗？
以下是发出截击请求的活动代码 -
public class CircleActivity extends AppCompatActivity implements SwipeRefreshLayout.OnRefreshListener {
private Toolbar toolbar;
private FloatingActionButton fab;
private ArrayList<Users> listUsers = new ArrayList<>();
SessionManager sessionManager;
BroadcastReceiver broadcastReceiver;
private RecyclerView listUsersCircle;
//swipe
private SwipeRefreshLayout swipeRefreshLayout;
//volley login
//mobile hotspot
//public static final String REGISTER_URL = "http://192.168.43.181/save/user/getPeopleList";
//ghar
public final String REGISTER_URL = "http://192.168.0.10/save/user/getPeopleList";
private CircleListAdapter circleListAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
sessionManager = new SessionManager(getApplicationContext());
if (sessionManager.isLoggedIn()) {
setContentView(R.layout.activity_circle);
toolbar = (Toolbar) findViewById(R.id.app_bar);
//tell android to use my toolbar
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);
listUsersCircle = (RecyclerView) findViewById(R.id.circle);
listUsersCircle.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
circleListAdapter = new CircleListAdapter(getApplicationContext());
listUsersCircle.setAdapter(circleListAdapter);
swipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipe_refresh_circle);
swipeRefreshLayout.setOnRefreshListener(this);
sendJsonRequest();
//get user_id from session
String user_id = sessionManager.getUserDetails().get("user_id");
//adding data to the intent
Intent intent = new Intent(getBaseContext), LocationService.class);
intent.putExtra("user_id", user_id);
startService(intent);
fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
String phoneNumber="123123123412";
String message ="Hello sms";
sendSMS(phoneNumber,message);
}
});
//location service
// startService(new Intent(getBaseContext(), LocationService.class));
//swipe to refresh
/* JSONObject obj = new JSONObject();
try {
obj.put("txtCircleId", "acca16d632");
} catch (JSONException e) {
e.printStackTrace();
}
Log.d("Nishon",obj.toString());
RequestQueue requestQueue = VolleySingleton.getsInstance().getRequestQueue();
JsonObjectRequest request = new JsonObjectRequest(Request.Method.POST,
URL_GET_CIRCLE_USERS,
obj,
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
Toast.makeText(getApplicationContext(), response.toString(), Toast.LENGTH_LONG).show();
Log.d("Nishon",response.toString());
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.d("Nishon", error.toString());
}
});
requestQueue.add(request);*/
} else {
Intent intent = new Intent(this, LoginActivity.class);
startActivity(intent);
finish();
}
}
private void sendSMS(String phoneNumber, String message)
{
PendingIntent pi = PendingIntent.getActivity(this, 0,
new Intent(this, SMS.class), 0);
SmsManager sms = SmsManager.getDefault();
sms.sendTextMessage(phoneNumber, null, message, pi, null);
}
public void sendJsonRequest() {
RequestQueue requestQueue = VolleySingleton.getsInstance().getRequestQueue();
StringRequest request = new StringRequest(Request.Method.POST, REGISTER_URL,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
listUsers = parseJsonResponse(response);
circleListAdapter.setCircleList(listUsers);
Log.d("d",response);
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(getApplicationContext(), error.toString(), Toast.LENGTH_LONG).show();
Log.d("d", error.toString());
}
}) {
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
String group_key= sessionManager.getUserDetails().get("group_key");
params.put("group_key", group_key);
return params;
}
};
requestQueue.add(request);
}
public ArrayList<Users> parseJsonResponse(String response) {
ArrayList<Users> listUsers = new ArrayList<>();
if (response != null || response.length() > 0) {
Toast.makeText(getApplicationContext(), response.toString(), Toast.LENGTH_LONG).show();
JSONObject jsonObject = null;
try {
jsonObject = new JSONObject(response.toString());
} catch (JSONException e) {
e.printStackTrace();
}
//extracting json array from response string
JSONArray jsonArray = null;
try {
if (jsonObject.has("users")) {
jsonArray = jsonObject.getJSONArray("users");
for (int z = 0; z < jsonArray.length(); z++) {
JSONObject currentUser = jsonArray.getJSONObject(z);
//String email = currentUser.getString("email");
String fname = currentUser.getString("first_name");
String lname = currentUser.getString("last_name");
Users user = new Users();
user.setFname(fname);
user.setLname(lname);
listUsers.add(user);
}
}
} catch (JSONException e) {
e.printStackTrace();
}
/* JSONObject jsonRow = null;
try {
jsonRow = jsonArray.getJSONObject(1);
} catch (JSONException e) {
e.printStackTrace();
}
//get value from jsonRow
try {
String resultStr = jsonRow.getString("email");
Log.d("Nishon", resultStr);
} catch (JSONException e) {
e.printStackTrace();
}*/
}
return listUsers;
}
@Override
protected void onPause() {
super.onPause();
finish();
}
public void showNotification(View view) {
}
@Override
public void onRefresh() {
swipeRefreshLayout.setRefreshing(true);
sendJsonRequest();
swipeRefreshLayout.setRefreshing(false);
}
/*@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_POWER) {
// Do something here...
event.startTracking(); // Needed to track long presses
return true;
}
return super.onKeyDown(keyCode, event);
}
*/
}
网络服务正在运作。
答案 0 :(得分：3)
这意味着您尝试获取length()的{{1}}为空。
罪魁祸首就是这条线：
String这意味着如果响应不为空 OR ，则其长度大于零。
你想要的是：
if (response != null || response.length() > 0) {
如果响应为空，这将使语句失败，因为如果是，则无法检查长度。