Question

I have a class like:

  class BSTNode<K extends Comparable, V> {
    K key;
    BSTNode(K key, V value) { ... }
  }

Then I am using

node.key.compareTo(root.key) >= 0

Where node and root are BSTNode. In that line, I am getting an unchecked error. Why?

warning: [unchecked] unchecked call to compareTo(T) as a member of the raw type Comparable
      } else if (node.key.compareTo(root.key) >= 0) { // new node >= root
                                   ^
  where T is a type-variable:
    T extends Object declared in interface Comparable
1 warning

In my understanding, as defined in BSTNode, K should extend/implement Comparable. So node.key.compareTo(root.key) should be OK?

Answer 1

Comparable is also generified. Try the following:

class BSTNode<K extends Comparable<? super K>, V> { ... }

Also, make sure to use the proper types in your declarations:

// will cause the warning
BSTNode root = new BSTNode<Integer, Integer>(1, 1);
// will NOT cause the warning
BSTNode<Integer, Integer> root = new BSTNode<Integer, Integer>(1, 1);