Javamedium

`List.of(...)`: what kind of list does it create and a common gotcha?

Answer

`List.of(...)` creates an unmodifiable (immutable) list. If you try to add/remove elements, you get `UnsupportedOperationException`. A common gotcha: it also does not allow null elements (it throws `NullPointerException` on creation).

List<String> xs = List.of("a", "b");
// xs.add("c"); // throws UnsupportedOperationException

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): java, collections, immutability, list
JVM: memory (heap/stack), GC, and what drives latency.
Contracts: equals/hashCode/toString, mutability and consequences.
Performance: boxing, allocations, collections, inlining.
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

Here’s an additional example (building on the short answer):

List<String> xs = List.of("a", "b");
// xs.add("c"); // throws UnsupportedOperationException

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?