How To Retrieve All The Keys In Firebase Android

I want to retrieve all the keys under single key, tried many things, but nothing work till now. This is my data structure I want to retrieve two keys which are under single key in

Solution 1:

All You have to do is do a nested query .! first find key of parent then get pass that key to next query and then find its children.!

List<user> userList=new ArrayList();

         mdatabaseRef.addValueEventListener(new ValueEventListener() {
      @Override     
      public void onDataChange(DataSnapshot dataSnapshot) {
         for (DataSnapshot child: dataSnapshot.getChildren()){
            String key = child.getKey();
             fun(key);
             }

      private void fun(String key){

        mdatabaseRef.child(key).addValueEventListener(new ValueEventListener() {
       @Override     
       public void onDataChange(DataSnapshot dataSnapshot) {for (DataSnapshot child: dataSnapshot.getChildren()){
    User user= postsnapshot.getValue(User.class);
userList.add(user);
     }

        }