This is the Activity to which I show all my images.

HomeScreen.java

package com.kelynnjeri.memories;

import android.content.ContentResolver;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;

import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.google.firebase.storage.FirebaseStorage;
import com.google.firebase.storage.StorageReference;
import com.google.firebase.storage.UploadTask;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.util.Log;
import android.view.View;
import android.webkit.MimeTypeMap;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

public class HomeScreen extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private RecyclerView.LayoutManager layoutManager;
    private StorageReference storageRef;
    private DatabaseReference dbReference;
    private ImageAdapter mImageAdapter;
    private List<Uploads> mUploadsList;
    private static final int REQUEST_CODE = 21;
    private static final String TAG = "HomeScreen";
    private Uri imageUri;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home_screen);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        Log.d(TAG, "HomeScreen Created");

        mRecyclerView = findViewById(R.id.recyclerview);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        mUploadsList = new ArrayList<>();

        dbReference = FirebaseDatabase.getInstance().getReference("images");
        dbReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for(DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                    Uploads upload = postSnapshot.getValue(Uploads.class);
                    mUploadsList.add(upload);
                }
                mImageAdapter = new ImageAdapter(HomeScreen.this, mUploadsList);
                mRecyclerView.setAdapter(mImageAdapter);

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Toast.makeText(HomeScreen.this, databaseError.getMessage(), Toast.LENGTH_SHORT).show();

            }
        });




        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent addImage = new Intent(Intent.ACTION_GET_CONTENT);
                addImage.setType("image/*");
                addImage.putExtra(Intent.EXTRA_LOCAL_ONLY, true);
                startActivityForResult(addImage.createChooser(addImage, "Add Image"), REQUEST_CODE);
                uploadImage();
            }
        });

        storageRef = FirebaseStorage.getInstance().getReference();
    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode == REQUEST_CODE && resultCode == RESULT_OK) {
            imageUri = data.getData();
        }
    }

    private String getImageExtension(Uri uri) {
        ContentResolver contentResolver = getContentResolver();
        MimeTypeMap mime = MimeTypeMap.getSingleton();
        return  mime.getExtensionFromMimeType(contentResolver.getType(uri));
    }

    private void uploadImage() {
        if(imageUri != null) {
            StorageReference imageReference = storageRef.child("images/" + System.currentTimeMillis() + "." + getImageExtension(imageUri));
            imageReference.putFile(imageUri)
            .addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                @Override
                public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                    Toast.makeText(HomeScreen.this, "Upload Successful.", Toast.LENGTH_LONG).show();

                }
            })
            .addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception e) {
                    Toast.makeText(HomeScreen.this, "Upload Failed!!!" + e.getMessage(), Toast.LENGTH_SHORT).show();

                }
            });


        } else {
            Toast.makeText(this, "No File Selected.", Toast.LENGTH_SHORT).show();
        }
    }




}