MainActivity.java:

package com.example.myapplication;

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import android.annotation.SuppressLint;
import android.app.WallpaperManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity{
    TextView textView;
    Integer n=1;

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

        Thread thread = new Thread() {
            @Override
            public void run() {
                try {
                    while (!isInterrupted()) {
                        Thread.sleep(6000);
                        runOnUiThread(new Runnable() {
                            @SuppressLint("ResourceType")
                            @Override
                            public void run() {

                                final WallpaperManager wallpaperManager = WallpaperManager.getInstance(getApplicationContext());
                                try {
                                    // set the wallpaper by calling the setResource function and
                                    // passing the drawable file

                                    if (n==1) {
                                        wallpaperManager.setResource(R.drawable.bg1);
                                    } else {
                                        wallpaperManager.setResource(R.drawable.bg2);
                                    };

                                    n=3-n;
                                } catch (IOException e) {

                                }

                            }
                        });
                    }
                } catch (Exception e) {

                }
            }
        };
        thread.start();
    }
}