Crash caused by use of findViewByID method

Question

I am trying to access views from the XML layout in the java activity. I made use of the findViewByID method to do so and be able to access a TextView. Whenever I run the program it c4rashes. The logcat references the line were the findViewByID method is used but I don't seem to get the problem.

package com.example.numberguessinggame;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

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

    TextView a = findViewById(R.id.textView);
    Button b = findViewById(R.id.button);

    public void changeNum(View view){
        a.setText("6 0 0");
    }

}


<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="changeNum"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.606" />

</androidx.constraintlayout.widget.ConstraintLayout>

--------- beginning of crash 2019-08-26 18:55:07.304 3605-3605/com.example.numberguessinggame E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.numberguessinggame, PID: 3605 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.numberguessinggame/com.example.numberguessinggame.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2718) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at androidx.appcompat.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249) at androidx.appcompat.app.AppCompatDelegate.create(AppCompatDelegate.java:182) at androidx.appcompat.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520) at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:191) at com.example.numberguessinggame.MainActivity.<init>(MainActivity.java:18) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1173) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2708) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Answer 1 · 2019-09-02T22:06:44Z

September 2, 2019 10:06pm

Hi Ibrahim,

your code is fine, however, pay attention to the scope and where to declare and initialize the button and textView. We declare the variables of type TextView and Button outside onCreate and we initialize them inside onCreate.

public class MainActivity extends AppCompatActivity {

    TextView a;
    Button b;

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


        a = findViewById(R.id.textView);
        b = findViewById(R.id.button);

    }

    public void changeNum (View view){
        a.setText("6 0 0");
    }
}

Answer 2 · 2019-08-27T15:03:54Z

August 27, 2019 3:03pm

Try adding a cast before hand maybe?

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Ibrahim Adeosun

Ibrahim Adeosun

Crash caused by use of findViewByID method

Ken Alger

Ken Alger

2 Answers

Fatemah Tavakoli

Fatemah Tavakoli

Ibrahim Adeosun

Ibrahim Adeosun

Daniel Prowse

Daniel Prowse

Ibrahim Adeosun

Ibrahim Adeosun