Contact.java

package codepath.com.recyclerviewfun;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Contact {

    private int mId;

    private String mName;
    private boolean mOnline;

    public Contact(int id, String name, boolean online) {
        mId = id;
        mName = name;
        mOnline = online;
    }

    public int getId() {
        return mId;
    }

    public String getName() {
        return mName;
    }

    public boolean isOnline() {
        return mOnline;
    }

    private static int lastContactId = 0;

    public static void resetContactId() {
        lastContactId = 0;
    }

    public static List<Contact> createContactsList(int numContacts, int offset) {
        List<Contact> contacts = new ArrayList<Contact>();

        for (int i = 1; i <= numContacts; i++) {
            contacts.add(new Contact(i + offset, randomIdentifier(), i <= numContacts / 2));
        }

        return contacts;
    }


    public static String randomIdentifier() {
        final java.util.Random rand = new java.util.Random();
        // class variable
        final String lexicon = "ABCDEFGHIJKLMNOPQRSTUVWXYZ12345674890";

        // consider using a Map<String,Boolean> to say whether the identifier is being used or not
        final Set<String> identifiers = new HashSet<String>();

        StringBuilder builder = new StringBuilder();
        while(builder.toString().length() == 0) {
            int length = rand.nextInt(5)+5;
            for(int i = 0; i < length; i++) {
                builder.append(lexicon.charAt(rand.nextInt(lexicon.length())));
            }
            if(identifiers.contains(builder.toString())) {
                builder = new StringBuilder();
            }
        }
        return builder.toString();
    }
}

ContactAdapter.java

package codepath.com.recyclerviewfun;

import android.content.Context;
import android.support.v7.util.DiffUtil;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

import java.util.List;

// Create the basic adapter extending from RecyclerView.Adapter
// Note that we specify the custom ViewHolder which gives us access to our views
public class ContactsAdapter extends
        RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

    // Store a member variable for the contacts
    private List<Contact> mContacts;

    // Pass in the contact array into the constructor
    public ContactsAdapter(List<Contact> contacts) {
        mContacts = contacts;
    }

    // Provide a direct reference to each of the views within a data item
    // Used to cache the views within the item layout for fast access
    public static class ViewHolder extends RecyclerView.ViewHolder {
        // Your holder should contain a member variable
        // for any view that will be set as you render a row
        public TextView nameTextView;
        public Button messageButton;

        // We also create a constructor that accepts the entire item row
        // and does the view lookups to find each subview
        public ViewHolder(View itemView) {
            // Stores the itemView in a public final member variable that can be used
            // to access the context from any ViewHolder instance.
            super(itemView);

            nameTextView = (TextView) itemView.findViewById(R.id.contact_name);
           //messageButton = (Button) itemView.findViewById(R.id.message_button);
        }
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        Context context = parent.getContext();
        LayoutInflater inflater = LayoutInflater.from(context);

        View contactView = inflater.inflate(R.layout.item_contact, parent, false);

        ViewHolder viewHolder = new ViewHolder(contactView);
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(ViewHolder viewHolder, int position) {
        Contact contact = mContacts.get(position);

        TextView textView = viewHolder.nameTextView;
        textView.setText(contact.getName());

        Button button = viewHolder.messageButton;

        if (position % 2 == 0) {
            textView.setBackground(viewHolder.nameTextView.getContext().getResources().getDrawable(R.color.colorPrimaryDark));
        } else {
            textView.setBackground(viewHolder.nameTextView.getContext().getResources().getDrawable(R.color.colorAccent));
        }
    }

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

item_contact.xml

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent" android:layout_height="wrap_content"
    android:orientation="vertical">

    <TextView
        android:id="@+id/contact_name"
        android:background="@color/colorAccent"
        android:layout_width="0dp" 
        android:layout_height="wrap_content"
        android:layout_marginLeft="0dp"
        android:paddingBottom="5dp"
        android:layout_marginRight="0dp" android:layout_marginTop="0dp"
        app:layout_constraintHorizontal_bias="0.0" app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/guideline2"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintWidth_default="wrap"
        tools:text="CONSTRAINT LAYOUT Y U NO WORK !!!!"/>

    <android.support.constraint.Guideline
        android:id="@+id/guideline2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.6"/>

</android.support.constraint.ConstraintLayout>

MainActivity.java

package codepath.com.recyclerviewfun;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.view.View;

import java.util.List;

public class MainActivity extends AppCompatActivity {

    List<Contact> allContacts;
    ContactsAdapter adapter;
    RecyclerView rvItems;

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

        rvItems = (RecyclerView) findViewById(R.id.rvContacts);
        allContacts = Contact.createContactsList(10, 0);
        adapter = new ContactsAdapter(allContacts);
        rvItems.setAdapter(adapter);
        final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
        rvItems.setLayoutManager(linearLayoutManager);
    }

    public void clearItems(View view) {
        allContacts.clear();
        Contact.resetContactId();
        allContacts.addAll(Contact.createContactsList(10, 0));
        adapter.notifyDataSetChanged();
    }
}