Generics and Collections Flashcards

What is the result of compiling and running the following code? ```java List list = new ArrayList<>(); list.add("A"); List objList = list; objList.add(42); System.out.println(list.get(1)); ```

Answer: Compile error: incompatible types

A List cannot be assigned to List because generic types are invariant in Java, causing a compile-time error.

Which method signature correctly uses a lower-bounded wildcard to accept a List of Number or any supertype?

Answer: void process(List list)

The lower-bounded wildcard `? super Number` accepts List, List, or any list whose type is a supertype of Number.

What does the Collections.unmodifiableList() method return?

Answer: A view that throws UnsupportedOperationException on mutation

Collections.unmodifiableList() returns a view backed by the original list; structural modifications through the view throw UnsupportedOperationException.

Which statement about TreeMap is correct?

Answer: It maintains keys in natural sorted order

TreeMap stores keys in their natural ordering (or by a provided Comparator), unlike HashMap which uses hashing.

Given `Map> map`, which generic method call compiles without an unchecked warning?

Answer: map.put("k", Arrays.asList(1, 2, 3))

Arrays.asList(1, 2, 3) infers List from the declared map type, so no unchecked warning is generated.

What happens when you call Iterator.remove() before calling Iterator.next()?

Answer: It throws IllegalStateException

Iterator.remove() requires a prior call to next(); calling remove() without it throws IllegalStateException.

Which collection class is most appropriate for a LIFO (last-in, first-out) stack in modern Java?

Answer: java.util.ArrayDeque

ArrayDeque is recommended over the legacy Stack class for LIFO operations because it is faster and not synchronized.