Android: Call Method from another Class which star

问题:

as the title already said, I’m trying to call a method LoginSuccess() from the Class JSON_Factroy_Return_Handler.java which then should start an Intent. I always get a java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference.

To make it more clear what I want to do, I’m going to post the important code snippet.

Login_Activity.java

public void Login(View view) { //Button Login pressed calls Login-Method
    ...
    new JSON_Factory_Test().execute(result); //Calls AsyncTask-Class
    ...
}


public void LoginSuccess() {
    Intent mainIntent = new Intent(getApplicationContext(), Main_Activity.class);       
    startActivity(mainIntent);
    finish();
}

JSON_Factory_Test.java

public class JSON_Factory_Test extends AsyncTask<String, String, JSONObject> {

    @Override
    protected JSONObject doInBackground(String... args) {
        ...
        return jsonobject;
    }

    @Override
    protected void onPostExecute(JSONObject jsonobject) {
        new JSON_Factory_Return_Handler().JSON_Factory_Retrun(jsonobject);
    }

}

JSON_Factroy_Return_Handler.java

public class JSON_Factory_Return_Handler {
    ...
    public void JSON_Factory_Retrun(JSONObject jsonresult) {
        ...
        if(jsonParams.get("result").toString().equals("TRUE")) {
            new Login_Activity().LoginSuccess();
        } else {
            ...     
        }
        ...
    }

}

When I then run this Project I alway get a NullPointerException!

FATAL EXCEPTION: main
Process: com.example.project, PID: 2855
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:105)
    at com.example.project.main.Login_Activity.LoginSuccess(Login_Activity.java:138)
    at com.example.project.json.JSON_Factory_Return_Handler.JSON_Factory_Retrun(JSON_Factory_Return_Handler.java:27)
    at com.example.project.json.JSON_Factory_Test.onPostExecute(JSON_Factory_Test.java:49)
    at com.example.project.json.JSON_Factory_Test.onPostExecute(JSON_Factory_Test.java:1)
    at android.os.AsyncTask.finish(AsyncTask.java:632)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5070)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:836)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:631)

---

I also tried this for the Login_Activity.java but this also won't work either

public void LoginSuccess() {
    Intent mainIntent = new Intent(Login_Activity.this, Main_Activity.class);       
    Login_Activity.this.startActivity(mainIntent);
    finish();
}

When I then run this Project I also get a NullPointerException!

    FATAL EXCEPTION: main
Process: com.example.project, PID: 3187
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
    at android.content.ContextWrapper.getPackageName(ContextWrapper.java:131)
    at android.content.ComponentName.<init>(ComponentName.java:77)
    at android.content.Intent.<init>(Intent.java:4011)
    at com.example.project.main.Login_Activity.LoginSuccess(Login_Activity.java:138)
    at com.example.project.json.JSON_Factory_Return_Handler.JSON_Factory_Retrun(JSON_Factory_Return_Handler.java:27)
    at com.example.project.json.JSON_Factory_Test.onPostExecute(JSON_Factory_Test.java:49)
    at com.example.project.json.JSON_Factory_Test.onPostExecute(JSON_Factory_Test.java:1)
    at android.os.AsyncTask.finish(AsyncTask.java:632)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5070)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:836)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:631)

I know that I could do everything in one Class but I need the AsyncTask more than once. And I also need the JSON_Factory_Return_Handler.java because this Class checks the returned values and then it should call new methods.

If you guys need more information I would be happy if I could give them to you :)

Thanks for your help in advance, Cheers 3dDi

回答1:

((Login_Activity())mContext).LoginSuccess();

where mContext is your Login_Activity reference

you need pass activity reference