public class FilePaths {

    //"storage/emulated/0"
    public String ROOT_DIR = Environment.getExternalStorageDirectory().getPath();

    public String PICTURES = ROOT_DIR + "/Pictures";
    public String CAMERA = ROOT_DIR + "/DCIM/camera";

    public String FIREBASE_IMAGE_STORAGE = "photos/users/";

}

/**
* Search a directory and return a list of all **directories** contained inside
* @param directory
* @return
*/
public static ArrayList<String> getDirectoryPaths(String directory){
    ArrayList<String> pathArray = new ArrayList<>();
    File file = new File(directory);
    File[] listfiles = file.listFiles();
    for(int i = 0; i < listfiles.length; i++){
        if(listfiles[i].isDirectory()){
            pathArray.add(listfiles[i].getAbsolutePath());
        }
    }
    return pathArray;
}
/**
     * Search a directory and return a list of all **files** contained inside
     * @param directory
     * @return
     */
    public static ArrayList<String> getFilePaths(String directory){
        ArrayList<String> pathArray = new ArrayList<>();
        File file = new File(directory);
 
        File[] listfiles = file.listFiles();
        Log.d(TAG, "getFilePaths: in file search");
 
        if(listfiles != null){
            for(int i = 0; i < listfiles.length; i++) {
                if (listfiles[i].isFile()) {//the error traces back here where it is telling me the listfiles are empty??
                    pathArray.add(listfiles[i].getAbsolutePath());
                } else {
                    Log.d(TAG, "getFilePaths: listfiles array is equal to null");
                }
            }
        }
        return pathArray;
    }

package com.example.networkusercommunication;

import android.content.Intent;
import android.provider.ContactsContract;
import android.service.autofill.Dataset;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.text.Editable;
import android.text.TextUtils;
import android.text.TextWatcher;
import android.text.format.DateFormat;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import com.example.networkusercommunication.adapter.AdapterChat;
import com.example.networkusercommunication.models.ModelChat;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
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.Query;
import com.google.firebase.database.ValueEventListener;
import com.squareup.picasso.Picasso;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Objects;

public class ChatActivity extends AppCompatActivity {

    // views from xml
    Toolbar toolbar;
    RecyclerView recyclerView;
    ImageView profileIv;
    TextView nameTv, userStatusTv;
    EditText messageEt;
    ImageButton sendBtn;

    //firebase auth
    FirebaseAuth firebaseAuth;

    FirebaseDatabase firebaseDatabase;
    DatabaseReference usersDbRef;

    // for checking if user has seen message or not
    ValueEventListener seenListener;
    DatabaseReference userRefForSeen;

    List<ModelChat> chatList;
    AdapterChat adapterChat;

    String hisUid;
    String myUid;
    String hisImage;


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

        // init views
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        toolbar.setTitle("");
        recyclerView = findViewById(R.id.chat_recyclerView);
        profileIv = findViewById(R.id.profileIv);
        nameTv = findViewById(R.id.nameTv);
        userStatusTv = findViewById(R.id.userStatusTv);
        messageEt = findViewById(R.id.messageEt);
        sendBtn = findViewById(R.id.sendBtn);

        // Layout ( LinearLayout ) for RecyclerView
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
        linearLayoutManager.setStackFromEnd(true);

        // recyclerview properties
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(linearLayoutManager);

        /* On clicking user from users list we have passed that user's UID using intent. So get that uid here to get the profile picture,
         * name and start chat with user */

        Intent intent = getIntent();
        hisUid = intent.getStringExtra("hisUid");

        // firebase auth instance
        firebaseAuth = FirebaseAuth.getInstance();

        firebaseDatabase = FirebaseDatabase.getInstance();
        usersDbRef = firebaseDatabase.getReference("Users");

        // search user to get that user's info
        Query userQuery = usersDbRef.orderByChild("uid").equalTo(hisUid);

        // get user picture and name
        userQuery.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                // check until required info is received
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    // get data
                    String name = "" + ds.child("name").getValue();
                    hisImage = "" + ds.child("image").getValue();
                    String typingStatus = "" + ds.child("typingTo").getValue();

                    // check typing status
                    if (typingStatus.equals(myUid)) {
                        userStatusTv.setText("schreibt...");
                    } else {

                        // get value of onlineStatus
                        String onlineStatus = "" + ds.child("onlineStatus").getValue();
                        if (onlineStatus.equals("online")) {
                            userStatusTv.setText(onlineStatus);
                        } else {
                            // convert timestamp to proper time date
                            // convert time stamp to dd/mm/yyyy hh:mm am/pm
                            // Es gibt verschiedene Zeitstellungen ( auch Deutschland bzw. restliche Welt )
                            Calendar cal = Calendar.getInstance(Locale.ENGLISH);
                            cal.setTimeInMillis(Long.parseLong(onlineStatus));
                            String dateTime = DateFormat.format("dd/MM/yyyy hh:mm aa", cal).toString();
                            userStatusTv.setText("Zuletzt online um: " + dateTime);
                        }

                    }

                    // set data
                    nameTv.setText(name);

                    try {
                        // image received, set it to imageview in toolbar
                        Picasso.get().load(hisImage).placeholder(R.drawable.ic_default_img_white).into(profileIv);

                    } catch (Exception e) {
                        // there is exception getting picture, set default picture
                        Picasso.get().load(R.drawable.ic_default_img_white).into(profileIv);

                    }

                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

        // click button to send message
        sendBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // get text from edit text
                String message = messageEt.getText().toString().trim();

                // check if text is empty or not
                if (TextUtils.isEmpty(message)) {
                    // text empty
                    Toast.makeText(ChatActivity.this, "Leere Nachricht kann nicht versenden werden", Toast.LENGTH_SHORT).show();

                } else {
                    // text not empty
                    sendMessage(message);

                }

            }
        });

        // check edit text change listener
        messageEt.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                if (s.toString().trim().length() == 0 ) {
                    checkTypingStatus("noOne");
                } else {
                    checkTypingStatus(hisUid); // uid of receiver
                }
            }

            @Override
            public void afterTextChanged(Editable s) {

            }
        });

        readMessages();

        seenMessage();

    }

    private void seenMessage() {
        userRefForSeen = FirebaseDatabase.getInstance().getReference("Chats");
        seenListener = userRefForSeen.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    ModelChat chat = ds.getValue(ModelChat.class);

                    if ( chat.getReceiver().equals(myUid) && chat.getSender().equals(hisUid)) {

                        HashMap<String, Object> hasSeenHashMap = new HashMap<>();
                        hasSeenHashMap.put("isSeen", true);
                        ds.getRef().updateChildren(hasSeenHashMap);
                    }
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void readMessages() {
        chatList = new ArrayList<>();
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Chats");
        dbRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                chatList.clear();
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    ModelChat chat = ds.getValue(ModelChat.class);
                    if (chat.getReceiver().equals(myUid) ||
                            (chat.getReceiver().equals(hisUid) && chat.getSender().equals(myUid))) {
                        chatList.add(chat);
                    }

                    // adapter
                    adapterChat = new AdapterChat(ChatActivity.this, chatList, hisImage);
                    adapterChat.notifyDataSetChanged();

                    // set adapter to recyclerview
                    recyclerView.setAdapter(adapterChat);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void sendMessage(String message) {

        /* "Chats" node will be created, that will contain all chats. Whenever user
         * sends message it will create a new child in "Chats" node and that child
         * will contain the following key values.
         * sender: UID of sender
         * receiver: UID of receiver
         * message: the actual message
         */

        DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();

        String timestamp = String.valueOf(System.currentTimeMillis());

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("sender", myUid);
        hashMap.put("receiver", hisUid);
        hashMap.put("message", message);
        hashMap.put("timestamp", timestamp);
        hashMap.put("isSeen", false);
        databaseReference.child("Chats").push().setValue(hashMap);

        // reset edittext after sending message
        messageEt.setText("");


    }

    private void checkUserStatus() {

        // get current user
        FirebaseUser user = firebaseAuth.getCurrentUser();
        if (user != null) {
            // user is signed in stay here
            //set email of logged in user
            //mProfileTv.setText(user.getEmail());
            myUid = user.getUid(); // currently signed in user's uid

        } else {
            // user not signed in, go to main activity
            startActivity(new Intent(this, MainActivity.class));
            finish();
        }
    }

    private void checkOnlineStatus(String status) {
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Users").child(myUid);
        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("onlineStatus", status);

        // update value of onlineStatus of current user
        dbRef.updateChildren(hashMap);
    }

    private void checkTypingStatus(String typing) {
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Users").child(myUid);
        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("typingTo", typing);

        // update value of onlineStatus of current user
        dbRef.updateChildren(hashMap);
    }

    @Override
    protected void onStart() {
        checkUserStatus();

        // set online
        checkOnlineStatus("online");
        super.onStart();
    }

    @Override
    protected void onPause() {
        super.onPause();
        // get timestamp
        String timestamp = String.valueOf(System.currentTimeMillis());
        // set offline with last seen time stamp
        checkOnlineStatus(timestamp);
        checkTypingStatus("noOne");
        userRefForSeen.removeEventListener(seenListener);
    }

    @Override
    protected void onResume() {

        // set online
        checkOnlineStatus("online");
        super.onResume();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        // hide searchview, as we don't need it here
        menu.findItem(R.id.action_search).setVisible(false);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_logout) {
            firebaseAuth.signOut();
            checkUserStatus();
        }

        return super.onOptionsItemSelected(item);
    }
}

package com.example.networkusercommunication.models;

public class ModelChat {

    String message, receiver, sender, timestamp;
    boolean isSeen;

    public ModelChat() {
    }

    public ModelChat(String message, String receiver, String sender, String timestamp, boolean isSeen) {
        this.message = message;
        this.receiver = receiver;
        this.sender = sender;
        this.timestamp = timestamp;
        this.isSeen = isSeen;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public String getReceiver() {
        return receiver;
    }

    public void setReceiver(String receiver) {
        this.receiver = receiver;
    }

    public String getSender() {
        return sender;
    }

    public void setSender(String sender) {
        this.sender = sender;
    }

    public String getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(String timestamp) {
        this.timestamp = timestamp;
    }

    public boolean isSeen() {
        return isSeen;
    }

    public void setSeen(boolean seen) {
        isSeen = seen;
    }
}

package com.example.networkusercommunication.adapter;

import android.app.AlertDialog;
import android.content.ContentValues;
import android.content.Context;
import android.content.DialogInterface;
import android.provider.ContactsContract;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.text.format.DateFormat;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import com.example.networkusercommunication.R;
import com.example.networkusercommunication.models.ModelChat;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
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.Query;
import com.google.firebase.database.ValueEventListener;
import com.squareup.picasso.Picasso;

import java.util.Calendar;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;

public class AdapterChat extends RecyclerView.Adapter<AdapterChat.MyHolder> {

    private static final int MSG_TYPE_LEFT = 0;
    private static final int MSG_TYPE_RIGHT = 1;
    Context context;
    List<ModelChat> chatList;
    String imageUrl;

    FirebaseUser fUser;

    public AdapterChat(Context context, List<ModelChat> chatList, String imageUrl) {
        this.context = context;
        this.chatList = chatList;
        this.imageUrl = imageUrl;
    }

    @NonNull
    @Override
    public MyHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
        // inflate layouts: row_chat_left.xml for receiver, row_chat_right.xml for sender
        if (i == MSG_TYPE_RIGHT) {
            View view = LayoutInflater.from(context).inflate(R.layout.row_chat_right, viewGroup, false);
            return new MyHolder(view);
        } else {
            View view = LayoutInflater.from(context).inflate(R.layout.row_chat_left, viewGroup, false);
            return new MyHolder(view);
        }

    }

    @Override
    public void onBindViewHolder(@NonNull MyHolder myHolder, final int i) {

        // get data
        String message = chatList.get(i).getMessage();
        String timeStamp = chatList.get(i).getTimestamp();

        // convert time stamp to dd/mm/yyyy hh:mm am/pm
        // Es gibt verschiedene Zeitstellungen ( auch Deutschland bzw. restliche Welt )
        Calendar cal = Calendar.getInstance(Locale.ENGLISH);
        cal.setTimeInMillis(Long.parseLong(timeStamp));
        String dateTime = DateFormat.format("dd/MM/yyyy hh:mm aa", cal).toString();

        // set data
        myHolder.messageTv.setText(message);
        myHolder.timeTv.setText(dateTime);
        try {
            Picasso.get().load(imageUrl).into(myHolder.profileIv);
        } catch (Exception e) {

        }

        // click to show delete dialog
        myHolder.messageLAyout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // show delete message confirm dialog
                AlertDialog.Builder builder = new AlertDialog.Builder(context);
                builder.setTitle("Delete");
                builder.setMessage("Are you sure to delete this message?");
                // delete button
                builder.setPositiveButton("Delete", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        deleteMessage(i);
                    }
                });

                // cancel delete button
                builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        // dismiss dialog
                        dialog.dismiss();
                    }
                });

                // create and show dialog
                builder.create().show();
            }
        });

        // set seen/delivered status of message
        if (i == chatList.size() - 1) {
            if (chatList.get(i).isSeen()) {
                myHolder.isSeenTv.setText("Gesehen");
            } else {
                myHolder.isSeenTv.setText("Versendet");
            }
        } else {
            myHolder.isSeenTv.setVisibility(View.GONE);
        }
    }

    private void deleteMessage(int position) {

        final FirebaseUser myUID = FirebaseAuth.getInstance().getCurrentUser();

        String msgTimeStamp = chatList.get(position).getTimestamp();
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Chats");
        Query query = dbRef.orderByChild("timestamp").equalTo(msgTimeStamp);
        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot ds:dataSnapshot.getChildren()) {

                    if (ds.child("sender").getValue().equals(myUID)) {

                        // Set value of message in "This message was deleted"
                        HashMap<String, Object > hashMap = new HashMap<>();
                        hashMap.put("message", "This message was deleted");
                        ds.getRef().updateChildren(hashMap);

                        Toast.makeText(context, "message deleted", Toast.LENGTH_SHORT).show();

                    } else  {
                        Toast.makeText(context, "Only your messages can be deleted", Toast.LENGTH_SHORT).show();
                    }

                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });




    }


    @Override
    public int getItemCount() {
        return chatList.size();
    }

    @Override
    public int getItemViewType(int position) {
        // get currently signed in user
        fUser = FirebaseAuth.getInstance().getCurrentUser();
        if (chatList.get(position).getSender().equals(fUser.getUid())) {
            return MSG_TYPE_RIGHT;
        } else {
            return MSG_TYPE_LEFT;
        }
    }

    // view holder class
    class MyHolder extends RecyclerView.ViewHolder {

        // views
        ImageView profileIv;
        TextView messageTv, timeTv, isSeenTv;
        LinearLayout messageLAyout; // for click listener to show delete


        public MyHolder(@NonNull View itemView) {
            super(itemView);

            // init views
            profileIv = itemView.findViewById(R.id.profileIv);
            messageTv = itemView.findViewById(R.id.messageTv);
            timeTv = itemView.findViewById(R.id.timeTv);
            isSeenTv = itemView.findViewById(R.id.isSeenTv);
            messageLAyout = itemView.findViewById(R.id.messageLayout);


        }
    }
}