What does 'unsupportedoperationexception' mean in java?

This runtime exception is thrown when a method is invoked which isn't supported by the object instance or in the current context. It's often seen with unmodifiable collections. Example:

List list = Collections.unmodifiableList(new ArrayList<>());
list.add("test");

Solution:

List modifiableList = new ArrayList<>();
modifiableList.add("test");

Always make sure you're using the appropriate type of collection or understand the limitations of the one you're working with.