A special value that can be assigned to variables or returned by a function if no data was provided. You can use null to represent values that are missing or that do not have a defined data type.

Example

In a numeric context, null evaluates to 0. Equality tests can be performed with null. In this statement, a binary tree node has no left child, so the field for its left child could be set to null.

if (tree.left == null) {
 tree.left = new TreeNode();
}