Immutable collections: List.of, Set.of, Map.of

What you will learn

• Create immutable lists, sets and maps with List.of, Set.of and Map.of
• Predict the exceptions: UnsupportedOperationException, NullPointerException and IllegalArgumentException
• Explain why an immutable collection can still show changes inside its objects
• Protect a class with List.copyOf, Set.copyOf and Map.copyOf (a defensive copy)
• Choose between an immutable collection, an unmodifiable view and a copy

Topics in this lesson

1. List.of, Set.of and Map.of
2. Rules that surprise people
3. Immutable is not deep
4. Copying: List.copyOf and the defensive copy
5. Choosing: immutable, view or copy

Key points

• List.of, Set.of and Map.of make immutable collections: add, set, remove and put throw UnsupportedOperationException.
• They reject null; Set.of and Map.of reject repeats with IllegalArgumentException, and their order is not fixed.
• Immutable means the collection cannot change, not the objects inside it; make the items immutable too.
• List.copyOf is a snapshot (a defensive copy); unmodifiableList is a read-only view that follows the original.

Interview questions this lesson answers

• What is an immutable collection?
• What is the difference between List.of and Collections.unmodifiableList?
• Are the objects inside List.of(...) immutable too?
• Why does Set.of throw on a repeated item, and why should you not rely on its order?
• Can List.of hold null?
• What is a defensive copy, and how do you make one with collections?
• When would you choose List.copyOf over Collections.unmodifiableList?