firebase , single and structure read and write

download the google-services.json file and add it to the app folder


build.gradle   <project>
============================
add to dependencies
        classpath 'com.google.gms:google-services:3.0.0'


build.gradle <app>
====================
add to dependencies
        compile 'com.google.firebase:firebase-core:9.0.2'
        compile 'com.google.firebase:firebase-database:9.0.2'

add to the bottom
apply plugin: 'com.google.gms.google-services'


activity_main.xml
======================
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter message....."
        android:id="@+id/helloMsg"/>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Send message..."
        android:onClick="sendMsg"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Read Message...."
        android:onClick="readMsg"/>
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Enter User Name....."
        android:id="@+id/uName"/>
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/uPass"
        android:hint="Enter User Pass"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Create User"
        android:onClick="createUser"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Get all"
        android:onClick="displayUsers"/>
</LinearLayout>



Users.java (for holding the structure of Json and db)
===========================================================
import android.content.Context;
import android.net.wifi.WifiInfo;
import android.net.wifi.WifiManager;
import android.widget.Toast;

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

/**
 * Created by app0811 on 05/06/2016.
 */
public class Users {
    public String userName;  //hold userName
    public String userPass;  //hold userPass
    public String userId;    //hold userId

    public Users(){}  //empty constructor, must have

    public Users(String userName, String userPass, String uid)
    {
        this.userName=userName;
        this.userPass=userPass;
        this.userId=uid;  //represent the UUID
    }

    public void saveUser()
    {
        //create an instance of User class
        Users user=new Users(userName,userPass,userId,context);

        //creating a connection to fire base
        FirebaseDatabase database=FirebaseDatabase.getInstance();

        //creating a reference to Users object
        DatabaseReference myRef=database.getReference("Users");

        //saving the user under the UUID
        myRef.child(userId).setValue(user);
    }
}

MainActivity.java
==================================

public class MainActivity extends AppCompatActivity {

    EditText myMsg;
    EditText uName;
    EditText uPass;
    Context context;
    final List<Users> myData=new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myMsg=(EditText)findViewById(R.id.helloMsg);
        uName=(EditText)findViewById(R.id.uName);
        uPass=(EditText)findViewById(R.id.uPass);
        context=this;
    }

    private String getMac()
    {
        //seeting WifiManager
        WifiManager manager = (WifiManager) getSystemService(Context.WIFI_SERVICE);
        //getting the info for the manager
        WifiInfo info = manager.getConnectionInfo();
        //getting mac address , physical address of the wifi card
        String address = info.getMacAddress();
        //returning the address
        return address;
    }

    private String getUUID()
    {
        //create a unique UUID
        UUID idOne = UUID.randomUUID();
        //returning the UUID
        return idOne.toString();
    }


    public void sendMsg(View v)  // Write a message to the database
    {
        //creating an instance to the database
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        //creating a reference to message object
        DatabaseReference myRef = database.getReference("message");
        //getting the message from the activity
        String myString = myMsg.getText().toString();
        //setting the value
        myRef.setValue(myString);
    }

    public void readMsg(View v)
    {
        //creating a connection to the database
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        //creating a reference to the database
        DatabaseReference myRef = database.getReference("message");
        //creating an event listener to the reference
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                String value = dataSnapshot.getValue(String.class);
                Toast.makeText(MainActivity.this, "Read:"+value, Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
                Toast.makeText(MainActivity.this, "Failed to read value.", Toast.LENGTH_SHORT).show();
            }
        });
    }

    public void createUser(View v)
    {
        //getting the info from the activity
        final String userName=uName.getText().toString();
        final String userPass=uPass.getText().toString();
        //creating new instance of the project
        Users user=new Users(userName,userPass,getUUID(),this);
        //calling inside method from the class to save the data
        user.saveUser();
    }

    public void displayUsers(View v)
    {
        //creating an instance to the database
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        //creating a reference to the database
        final DatabaseReference myRef = database.getReference("Users");

        //adding an event listener to the Event
        myRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            //method for data change
            public void onDataChange(DataSnapshot dataSnapshot) {
                //moving on entire data that we received from the database
                for (DataSnapshot item:dataSnapshot.getChildren()) {
                    //get a single value, and push it to User instance by the Users class build
                    Users recivedUser = item.getValue(Users.class);
                    //add the value to our list
                    myData.add(recivedUser);
                }
                //display the message to the screen
                Toast.makeText(context, "we got:"+myData.size(), Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                //handle in error case
            }
        });
    }

}