Why List.of(...) Refuses add()

A list built with the shortest syntax available, passed to a method that appends one thing to it. The code compiles and throws the first time it runs.

java

List<String> refs = List.of("A", "B");
refs.add("C");

text

Exception in thread "main" java.lang.UnsupportedOperationException

List.of(...) returns an unmodifiable list. It implements List, so add exists and must compile — and its implementation throws, because there is no other way for an interface method to say "not here".

Three ways to build a list, and what each one permits
Written	Can be added to	Accepts null
`new ArrayList<>()`	Yes	Yes
`List.of(a, b)`	No — throws	No — throws on construction
`Arrays.asList(a, b)`	`set` yes, `add` no	Yes
`new ArrayList<>(List.of(a, b))`	Yes	Yes

The exception is a design decision rather than an oversight: a single List type that every caller can accept is worth more than a separate interface for lists that cannot grow. Wrap in new ArrayList<>(...) when you need to modify, and prefer List.of everywhere you do not — a collection that cannot change is one fewer thing to reason about.