Toast

 MainActivity -

package com.example.toast;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import android.view.View;

import android.widget.Button;

import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    Button show;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        show=findViewById(R.id.showToast);

        show.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                Toast.makeText(getApplicationContext(),"Welcome TO Android",Toast.LENGTH_SHORT).show();

            }

        });

    }

}

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activity_main -

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout 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"

    android:gravity="center"

    android:orientation="vertical"

    tools:context=".MainActivity">

    <Button

        android:id="@+id/showToast"

        android:layout_width="121dp"

        android:layout_height="68dp"

        android:layout_gravity="center"

        android:background="@color/colorPrimary"

        android:text="Show Toast"

        android:textColor="#fff" />

</LinearLayout>

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AndroidManifest -

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

    package="com.example.toast">

    <application

        android:allowBackup="true"

        android:icon="@mipmap/ic_launcher"

        android:label="@string/app_name"

        android:roundIcon="@mipmap/ic_launcher_round"

        android:supportsRtl="true"

        android:theme="@style/AppTheme">

        <activity android:name=".MainActivity">

            <intent-filter>

                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />

            </intent-filter>

        </activity>

    </application>

</manifest>