MainActivity.java

package com.myapplication.rohim.myapplication;

import android.graphics.drawable.Drawable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private ImageView appImageView;
    private Button appButton;
    private Drawable drawable;
    private Random random;
    private Drawable [] drawables = null;

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

        getSupportActionBar().setTitle("Random Image");

        appImageView = (ImageView) findViewById(R.id.imageView);
        appButton =(Button) findViewById(R.id.button);

        drawables = new Drawable[] {
                getResources().getDrawable(R.drawable.classroom),
                getResources().getDrawable(R.drawable.user)
        };

        appButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                random = new Random();
                int randomNumber = random.nextInt(drawables.length);
                drawable = drawables[randomNumber];
                appImageView.setImageDrawable(drawable);
            }
        });
    }
}