App crashes on startup after adding ImageView

Question

I am running a basic app with one ImageView. It runs until I add the following

ImageView gohan = findViewById(R.id.gohanView);

These are the only things that I've done:

Created project
Created empty activity
Added ImageView to activity
Imported image to drawable folder
Attached image to src of ImageView
RUN (Everything works at this point)
I then add the code above to my MainActivity class and the app will not run anymore.

Java

package com.example.david.dragonballz;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

    ImageView gohan = findViewById(R.id.gohanView);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

XML

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/gohanView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="69dp"
        android:layout_marginEnd="114dp"
        android:layout_marginLeft="114dp"
        android:layout_marginRight="114dp"
        android:layout_marginStart="114dp"
        android:layout_marginTop="69dp"
        android:src="@drawable/gohan"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

Logcat

04-18 13:02:51.416 15392-15392/com.example.david.dragonballz E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.david.dragonballz, PID: 15392
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.david.dragonballz/com.example.david.dragonballz.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6938)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:117)
        at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:149)
        at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:56)
        at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:31)
        at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:31)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:198)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:183)
        at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
        at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
        at com.example.david.dragonballz.MainActivity.<init>(MainActivity.java:9)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1180)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2829)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6938) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

Here is example of WORKING code from the Android classes I'm taking. Why does this work for him and used to work for me? EXAMPLE

put ImageView gohan = findViewById(R.id.gohanView); inside oncreate method after.setContentView. — Dhaval Patel, Apr 18 '18 at 17:09

score 1 · Answer 1 · edited Feb 20 '19 at 14:27

1

Never find id of any views before onCreate() method because the xml layout is loaded on onCreate().

setContentView(R.layout.activity_main);

find id in onCreate()

gohan = (ImageView) findViewById(R.id.gohanView);

edited Feb 20 '19 at 14:27

George Mulligan

11,813
6
37
50

answered Apr 18 '18 at 18:24

CodeForMe

11
7

Quick learner · Answer 2 · 2018-04-18T18:09:14.047

0

Declare this

public ImageView gohan;

Add this line in oncreate()

gohan = (ImageView)findViewById(R.id.gohanView);

after this line

 setContentView(R.layout.activity_main);

Finally your onCreate()

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
    gohan = (ImageView)findViewById(R.id.gohanView);

 gohan.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // do something 
           gohan.animate().alpha(0).setDuration(2000);
            }
        });
    }

ALSO NOW ACCESS THIS IMAGEVIEW ANYWHERE

edited Apr 18 '18 at 18:09

answered Apr 18 '18 at 17:10

Quick learner

10,632
4
45
55

Thanks! That allowed it to run but now I can't access "gohan" from another class. I'm not sure what changed but I used to be able to have the ImageView in the MainActivity class and then access it when creating classes within the MainActivity. Does that make sense? – INKALAXY Apr 18 '18 at 17:27
@INKALAXY, You can just declare the object as `ImageView gohan` in your activity and assign it in `onCreate` method. What do you mean by accessing it with classes within your activity? Why you wanted to do that? – Uma Sankar Apr 18 '18 at 17:39
Ok. So you're saying this should work: `public class MainActivity extends AppCompatActivity { ImageView gohan; public void gohanClick (View view) { gohan.animate().alpha(0).setDuration(2000); } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); ImageView gohan = findViewById(R.id.gohanView); } }` – INKALAXY Apr 18 '18 at 17:50
okay wait let me update again – Quick learner Apr 18 '18 at 17:51
updated my answer – Quick learner Apr 18 '18 at 17:54
Thanks again! I'm starting to get the hang of it now. However, I am still confused why the way I did it before is now not working. I have added a screenshot (low quality, sorry) to my original question. The code that I screenshot was working for the instructor and for me up until yesterday. Any ideas? – INKALAXY Apr 18 '18 at 22:19
you have not done correctly – Quick learner Apr 19 '18 at 05:00

App crashes on startup after adding ImageView

2 Answers2