activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    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">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:adjustViewBounds="true"
        android:src="@drawable/your_image" />

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/bitmapToBase64Btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:text="Bitmap to Base64" />

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/base64ToBitmapBtn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:text="Base64 to Bitmap" />
</LinearLayout>

MainActivity.java

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.os.Bundle;
import android.util.Base64;
import android.view.View;
import android.widget.ImageView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.AppCompatButton;

import java.io.ByteArrayOutputStream;

public class MainActivity extends AppCompatActivity {

    private String base64String = "";

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

        final ImageView imageView = findViewById(R.id.imageView);
        final AppCompatButton base64ToBitmap = findViewById(R.id.base64ToBitmapBtn);
        final AppCompatButton bitmapToBase64 = findViewById(R.id.bitmapToBase64Btn);

        bitmapToBase64.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                // getting Bitmap from ImageView
                final Bitmap imageBitmap = ((BitmapDrawable) imageView.getDrawable()).getBitmap();

                // converting to Base64
                base64String = bitmapToBase64(imageBitmap);

                // remove image from ImageView
                imageView.setImageBitmap(null);
            }
        });

        base64ToBitmap.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                // getting bitmap from base64 string
                Bitmap bitmap = base64ToBitmap(base64String);

                // setting bitmap to ImageView
                imageView.setImageBitmap(bitmap);
            }
        });
    }

    private String bitmapToBase64(Bitmap bitmap) {

        // initialize byte stream to store bytes from bitmap
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

        // compress Bitmap and getting bytes
        bitmap.compress(Bitmap.CompressFormat.JPEG, 100, byteArrayOutputStream);

        // getting byte array from output stream
        byte[] bytes = byteArrayOutputStream.toByteArray();

        // get base64 encoded string
        final String base64String = Base64.encodeToString(bytes, Base64.DEFAULT);

        return base64String;
    }

    private Bitmap base64ToBitmap(String base64String) {

        // decode base64 string
        byte[] bytes = Base64.decode(base64String, Base64.DEFAULT);

        // Initialize bitmap
        Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);

        return bitmap;
    }
}