Question

public void findClassNodesMatching(String lowerCaseSearchText, List<? extends DefaultMutableTreeNode> foundNodes) {
    findClassNodesMatching(lowerCaseSearchText, (DefaultMutableTreeNode) getRoot(), foundNodes);
}

private void findClassNodesMatching(String lowerCaseSearchText, DefaultMutableTreeNode node, List<? extends DefaultMutableTreeNode> foundNodes) {
    String nodeLabel = node.toString().toLowerCase();
    if (nodeLabel.indexOf(lowerCaseSearchText) >= 0) {
        foundNodes.add(node);
    }
}

Why does this code give an error

The method add(capture#2-of ? extends DefaultMutableTreeNode) in the
type List is not
applicable for the arguments (DefaultMutableTreeNode)

The error is at the line of foundNodes.add(node);

Answer 1

You can add to a list where you only know the type as a List<? extends Something> constraint. Imagine you could:

List<Banana> bananas = new ArrayList<Banana>();
// This is fine, right?
List<? extends Fruit> fruit = bananas;

// We don't want to be able to add an apple to a list of bananas!
fruit.add(new Apple());

// ... as otherwise this is no longer safe
Banana bananas = bananas.get(0);

It’s exactly the same thing in your case – imagine you passed in some List<SomeVerySpecificKindOfNode>; you shouldn’t be able to add any DefaultMutableTreeNode to that.

It’s not clear what your context is, but just changing your code to use List<DefaultMutableTreeNode> in both places may well be enough to fix it. It depends on the callers.