Collections Framework Flashcards

What is the behavior of `TreeMap.subMap(fromKey, toKey)` with respect to the boundary keys?

Answer: Inclusive on fromKey, exclusive on toKey

`TreeMap.subMap(fromKey, toKey)` returns a view with `fromKey` inclusive and `toKey` exclusive, matching the convention of standard range methods.

Which class provides a thread-safe variant of `HashMap` without synchronizing the entire map?

Answer: ConcurrentHashMap

`ConcurrentHashMap` uses segment-level or bucket-level locking to allow concurrent reads and partial concurrent writes without locking the full map.

Which `Map` method was added in Java 8 and computes a new value only if the key is absent?

Answer: computeIfAbsent()

`computeIfAbsent(key, mappingFunction)` invokes the function to compute a value only when the key is not already present, and stores the result.

What exception is thrown when you attempt to add a null element to a `TreeSet`?

Answer: NullPointerException

`TreeSet` uses natural ordering or a `Comparator`, and comparing `null` causes a `NullPointerException` when the first comparison is attempted.

Which of the following collection factory methods allows duplicate elements?

Answer: List.of(1, 1)

`List.of()` permits duplicate elements, whereas `Set.of()` and `Map.of()` with duplicate keys/elements throw `IllegalArgumentException` at construction time.

When using `Collections.sort(List list)`, what must type `T` satisfy?

Answer: T must implement Comparable

`Collections.sort()` requires elements to implement `Comparable` so their natural ordering can be used; otherwise, a `ClassCastException` is thrown at runtime.

Which `Queue` method adds an element and throws an exception if capacity is exceeded, rather than returning false?

Answer: add()

`add()` throws `IllegalStateException` when the queue is full, while `offer()` returns `false` on failure and is preferred for capacity-constrained queues.