How do I show a toast when my check box is checked or unchecked?

Question

I want to show a toast every time the checkbox is checked or unchecked. The code below that isn't commented out runs but the toast doesn't appear. The code that is commented out runs but the app force closes with the following logcat:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CheckBox.setOnCheckedChangeListener(android.widget.CompoundButton$OnCheckedChangeListener)' on a null object reference

Here is my code:

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    CheckBox checkBoxOne;

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

        checkBoxOne = findViewById(R.id.checkBoxOne);
        if (checkBoxOne.isChecked()) {
            Toast.makeText(this, "Yes", Toast.LENGTH_SHORT).show();
        }
/*
        checkBoxOne.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                Toast.makeText(MainActivity.this, "Hello pal", Toast.LENGTH_SHORT).show();
            }
        });
*/
    }
}

.xml file:

<?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">

    <CheckBox
        android:id="@+id/checkBoxOne"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="24dp"
        android:text="CheckBox"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

try this code:

package com.example.testdemo;

import android.os.Bundle;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {


    CheckBox chk;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chk = findViewById(R.id.chk1);
        chk.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                if (isChecked) {
                    Toast.makeText(MainActivity.this, "Checkbox is Checked", Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(MainActivity.this, "Checkbox is not Checked", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

}

Answer 2

Try your checkbox with checkBox.setChecked(true ro false);

for example :

CheckBox checkBoxOne;

    checkBoxOne.setChecked(false) {
            Toast.makeText(this, "Yes", Toast.LENGTH_SHORT).show();
               }