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"

Answer 1

use `findViewById` within onCreate method
i.e.

public class MainActivity extends AppCompatActivity {

TextView a;

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

a = findViewById(R.id.textView);
Button b = findViewById(R.id.button);
b.setOnClickListener(e -> {
changeNum();
});
}

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

}