title: Check if a key already exist in firebase
body: <p>I have some problems related to my database, and the docs seems a lot of confusing i can't figure out how to solve that, basicly i have my user data in the firebase, and i want to see if in my user with some id i already have the key username if yes i want to update with some data of my form, if no i want to add a value based on my form, but when i want to know how to check if the id already exist, i did this:</p>
  
  <pre><code>  auth = FirebaseAuth.getInstance();
          database = FirebaseDatabase.getInstance();
          myRef = database.getReference();
          userId = auth.getCurrentUser().getUid();
          user =  myRef.child("Users").child(userId);
          Log.d("test1",String.valueOf(user.child("age").getKey()));
          if(user.child("username").co){
              usernameTxt.setText(user.child("username").toString());
          }
          if(user.child("age") != -1){
              usernameTxt.setText(user.child("age").toString());
          }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/jtUGF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jtUGF.png" alt="data"></a></p>