How to search if a username exist in the given firebase database?

Question

{
 users:
  {
    apple:
     {
       username :  apple
       email    :  apple@xy.com
       uid      :  tyutyutyu
     }
    mango:
     {
       username :  man         


        

Answer 1

EDIT: New answer, old one still below.

I would get rid of your method "checkFirebaseForUsername" because it will always return 0, no matter what.

What you need to do is this:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
ref.child("users").child("username").addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
     if(dataSnapshot.exists()){
      // use "username" already exists
      // Let the user know he needs to pick another username.
    } else {
      // User does not exist. NOW call createUserWithEmailAndPassword
      mAuth.createUserWithPassword(...);
      // Your previous code here.

    }                               
  }

  @Override
  public void onCancelled(DatabaseError databaseError) {

  }
});

Old Answer:

{
 users:
  {
    apple[X]:
     {
       username :  apple[Y]
       email    :  apple@xy.com
       uid      :  tyutyutyu
     }
    mango:
     {
       username :  mango
       email    :  mango@xy.com
       uid      :  erererer
     }
  }
}

If for example, the node apple[X] will always have the same name as the child property "username":apple[Y], then it is as simple as this.

DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
ref.child("users").child("username").addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
     if(dataSnapshot.exists()){
      // use "username" already exists
    } else {
      // "username" does not exist yet.
    }                               
  }

  @Override
  public void onCancelled(DatabaseError databaseError) {

  }
});

however, if say, the node apple[X] can have a different value than the property apple[Y], and you want to see if any node exists where the "username" property is the same, then you will need to do a query.

 Query query = FirebaseDatabase.getInstance().getReference().child("users").orderByChild("username").equalTo("usernameToCheckIfExists");
 query.addListenerForSingleValueEvent(new ValueEventListener() {
      @Override
      public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.getChildrenCount() > 0) {
            // 1 or more users exist which have the username property "usernameToCheckIfExists"
         }
       }

      @Override
      public void onCancelled(DatabaseError databaseError) {

      }
  });