Add new key-value pair child in Firebase DB

addNewNode.java

DatabaseReference testRef = mRef.child("test");

ref.runTransaction(new Transaction.Handler() {
  @Override
  public Transaction.Result doTransaction(MutableData currentData) {
    currentData.child("my new child").setValue(1);
    return Transaction.success(currentData);
  }
  @Override
  public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
    //This method will be called once with the results of the transaction.
  }
});