How do I correctly assign my root node to a value?

Question

I am trying to create a Binary Tree from a Hashmap, and in order to do so, I'm trying to assign the root node to a specific Node in the Hashmap, basically the entry which has the entry.getValue() as null.

I have been able to convert my keys into nodes from ints, now I'm trying to set the key which has null as it's value to be the root.

I have also declared a root as a Binary Tree type, here is my code:

public static BinaryTreeNode parentMapToTree(Map<Integer, Integer> map) {
    HashMap<BinaryTreeNode, Integer> l = new HashMap<>();
    for(Map.Entry<Integer, Integer> entry : map.entrySet()){
      l.put(new BinaryTreeNode(entry.getKey()), entry.getValue());
    }
    BinaryTreeNode root = null;
    for(Map.Entry<Integer, Integer> entry : map.entrySet()){
      if (entry.getValue() == null);{
        root == l.get(entry.getKey());
      }

    }
  }

Can anyone please explain how to declare that particular entry as the root node for me to build by Binary Tree upon?