p4

 activity_main.xml

<?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:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="changing wall paper"
        android:textAlignment="center"
        android:textAllCaps="true"
        android:textColor="#8E24AA"
        android:textSize="34sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.516"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.096" />

    <Button
        android:id="@+id/btn_wall"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="click here for changing wallpaper"
        android:textAllCaps="true"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.494"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.349" />

</androidx.constraintlayout.widget.ConstraintLayout>

mainactivity.java

package com.example.wallpaper;


import androidx.appcompat.app.AppCompatActivity;

import android.app.WallpaperManager;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import java.io.IOException;
import java.util.Random;
import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends AppCompatActivity implements
        View.OnClickListener {

    Button btn;
    boolean running;
    int[] ia = new int[]{
            R.drawable.img1,R.drawable.img2,R.drawable.img3,R.drawable.img4,
            R.drawable.img5};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn = (Button) findViewById(R.id.btn_wall);
        btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        if(!running){
            new Timer().schedule(new MyTimer(),0,1000);
            running = true;

        }
    }

    private class MyTimer extends TimerTask {

        @Override
        public void run() {
            try{
                WallpaperManager wallpaperManager =WallpaperManager.getInstance(getBaseContext());
                Random random = new Random();

                wallpaperManager.setBitmap(BitmapFactory.decodeResource(getResources(),ia[random.nextInt(5)]));
            } catch (IOException e) {

            }
        }
    }
}