nullpointerexception - Attempt to invoke virtual method 'java.io.File android.content.Context.getCacheDir()' on a null object reference -


this question has answer here:

i trying use volley , send request check data after login.
have special class 'api' where's volley staff.
trying after checking validation in loginactivity (default android studio template) want create api instance in loginactivity , send request server.
testing volley, later want create 1 api instance in main class.

here's api class:

import android.app.application; import android.util.log;  import com.android.volley.*; import com.android.volley.toolbox.volley; import com.android.volley.toolbox.jsonobjectrequest; import org.json.jsonobject; import java.lang.string;  public class api extends application {      protected string serverurl;     protected string get;     protected string post;     protected requestqueue queue;      public api(){         setserverurl("http://localhost:63424");         setget("http://localhost:63424/api");         setpost("http://localhost:63424/api");         queue = volley.newrequestqueue(this);     }      protected void sendget (string request)     {         jsonobjectrequest getrequest = new jsonobjectrequest(request.method.get, request, null,                 new response.listener<jsonobject>()                 {                     @override                     public void onresponse(jsonobject response) {                         // display response                         log.d("responseapi", response.tostring());                     }                 },                 new response.errorlistener()                 {                     @override                     public void onerrorresponse(volleyerror error) {                         log.d("error.responseapi", error.tostring());                     }                 }         );         queue.add(getrequest);     }       //logowanie     public boolean checklogin (string login, string password){         setget(getget()+"/search_users_/login=/"+login + "/");         sendget(getget());          return true;      }      public string getserverurl() {         return serverurl;     }      public void setserverurl(string serverurl) {         serverurl = serverurl;     }      public string getget() {         return get;     }      public void setget(string get) {         this.get = get;     }      public string getpost() {         return post;     }      public void setpost(string post) {         this.post = post;     }  }

here trying create api instance in loginactivity:

 public class userlogintask extends asynctask<void, void, boolean> {          private final string memail;         private final string mpassword;          userlogintask(string email, string password) {             memail = email;             mpassword = password;         }          @override         protected boolean doinbackground(void... params) {             // todo: attempt authentication against network service.              try {                 // simulate network access.                 thread.sleep(2000);             } catch (interruptedexception e) {                 return false;             }             log.d("api", "zaraz utworze api");             api api = new api();             return api.checklogin(memail, mpassword);              // todo: register new account here.          }          @override         protected void onpostexecute(final boolean success) {             mauthtask = null;             showprogress(false);              if (success) {                 finish();             } else {                 mpasswordview.seterror(getstring(r.string.error_incorrect_password));                 mpasswordview.requestfocus();             }         }          @override         protected void oncancelled() {             mauthtask = null;             showprogress(false);         }     }

this solution not work, error:

fatal exception: asynctask #1                                                             process: pawel.cooker, pid: 2646                                                             java.lang.runtimeexception: error occurred while executing doinbackground()                                                                 @ android.os.asynctask$3.done(asynctask.java:353)                                                                 @ java.util.concurrent.futuretask.finishcompletion(futuretask.java:383)                                                                 @ java.util.concurrent.futuretask.setexception(futuretask.java:252)                                                                 @ java.util.concurrent.futuretask.run(futuretask.java:271)                                                                 @ android.os.asynctask$serialexecutor$1.run(asynctask.java:245)                                                                 @ java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1162)                                                                 @ java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:636)                                                                 @ java.lang.thread.run(thread.java:764)                                                              caused by: java.lang.nullpointerexception: attempt invoke virtual method 'java.io.file android.content.context.getcachedir()' on null object reference                                                                 @ android.content.contextwrapper.getcachedir(contextwrapper.java:256)                                                                 @ com.android.volley.toolbox.volley.newrequestqueue(volley.java:43)                                                                 @ com.android.volley.toolbox.volley.newrequestqueue(volley.java:78)                                                                 @ pawel.cooker.api.<init>(api.java:17)                                                                 @ pawel.cooker.loginactivity$userlogintask.doinbackground(loginactivity.java:320)                                                                 @ pawel.cooker.loginactivity$userlogintask.doinbackground(loginactivity.java:299)                                                                 @ android.os.asynctask$2.call(asynctask.java:333)                                                                 @ java.util.concurrent.futuretask.run(futuretask.java:266)                                                                 @ android.os.asynctask$serialexecutor$1.run(asynctask.java:245)                                                                  @ java.util.concurrent.threadpoolexecutor.runworker(threadpoolexecutor.java:1162)                                                                  @ java.util.concurrent.threadpoolexecutor$worker.run(threadpoolexecutor.java:636)                                                                  @ java.lang.thread.run(thread.java:764) 

there's problem volley, have no idea what's wrong.

there's problem volley

no, there's problem api class.

make sure put in manifest

<application     android:name=".api"

after that

  1. don't use constructor. use oncreate . or, don't use application, , use volley-scoped singleton -- see example documentation
  2. replace localhost actual server address 

public class api extends application {      public static final string serverurl = "http://<use real server here>:63424";     public static final string = serverurl+"/api";     public static final string post = serverurl+"/api";     protected requestqueue queue;      private final api minstance;      private api() { }      public api getinstance() {         return minstance;     }      @override     public void oncreate(){         super.oncreate();         minstance = this;         queue = volley.newrequestqueue(this);     }
  1. you can't return blocking value async functions. use callbacks pass results

protected void sendget (string url, response.listener<jsonobject> onsuccess) {     jsonobjectrequest getrequest = new jsonobjectrequest(request.method.get, url, null,             onsuccess,               new response.errorlistener()             {                 @override                 public void onerrorresponse(volleyerror error) {                     log.d("error.responseapi", error.tostring());                 }             }     );     queue.add(getrequest); }  public void checklogin (string login, response.listener<jsonobject> onsuccess){     sendget(get+"/search_users_/login=/"+login, onsuccess); }
  1. don't use new api() when should instead use ((api) getapplicationcontext()) or above implementation, api.getinstance()

for example, 

api api = api.getinstance().checklogin(memail, new response.listener<jsonobject>() {     @override     public void onresponse(jsonobject response) {         // todo: register new account here.     } });
  1. most importantly here, don't need asynctask when using volley. it's asynchronous

on related note, if using restful apis, retrofit might more useful


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