How to get all leaf nodes of a tree?

Use recursion.

• if the node itself is a leaf, return it
• otherwise, return all the leaf-nodes of its children

Something like this (not tested):

public Set<TreeNode<T>> getAllLeafNodes() {
    Set<TreeNode<T>> leafNodes = new HashSet<TreeNode<T>>();
    if (this.children.isEmpty()) {
        leafNodes.add(this);
    } else {
        for (TreeNode<T> child : this.children) {
            leafNodes.addAll(child.getAllLeafNodes());
        }
    }
    return leafNodes;
}