OCP Generics and Collections Questions and Answers

Question 1

Which of the following statements is true regarding the behavior of generics in Java at runtime?

Accepted Answer

Generic type information is discarded by the compiler through a process called type erasure.

Answer

The JVM maintains a separate class file for each specific instantiation of a generic type.

Answer

Runtime reflection can be used to determine the exact parameterized type of a generic collection.

Answer

A `List` and a `List` are treated as distinct types by the JVM, preventing runtime casting errors.

Question 2

A developer needs to create a list of strings that will not be modified after its creation. Which of the following is the most appropriate way to initialize this list in modern Java (9 and later)?

Accepted Answer

List<String> list = List.of("A", "B");

Answer

List list = new ArrayList<>(); list.add("A"); list.add("B");

Answer

List list = Collections.unmodifiableList(new ArrayList<>(Arrays.asList("A", "B")));

Answer

List list = new LinkedList<>(List.of("A", "B"));

Question 3

What is the primary difference between the `Comparable` and `Comparator` interfaces in Java?

Accepted Answer

`Comparable` defines a single, natural ordering for a class, whereas `Comparator` allows for multiple, external sorting strategies.

Answer

`Comparator` is used for natural ordering, while `Comparable` is used for custom, external sorting logic.

Answer

A class can implement multiple `Comparable` interfaces but only one `Comparator`.

Answer

The `compare()` method of `Comparator` takes one argument, while the `compareTo()` method of `Comparable` takes two.

Question 4

Examine the following code snippet: ```java NavigableSet set = new TreeSet<>(); set.add(10); set.add(20); set.add(30); set.add(40); System.out.println(set.lower(30)); System.out.println(set.floor(30)); System.out.println(set.higher(30)); ``` What will be the output?

Accepted Answer

20
30
40

Answer

20
20
40

Answer

null
30
40

Answer

30
30
40

Question 5

Which generic wildcard should be used in a method signature `void process(List<?> list)` to allow the method to add `Integer` objects to the list?

Accepted Answer

List<? super Integer>

Answer

List<? extends Number>

Answer

List<?>

Answer

List<Object>

Question 6

Which of the following is a key characteristic of the `java.util.Set` interface?

Accepted Answer

It does not allow duplicate elements.

Answer

It guarantees the order of elements based on insertion time.

Answer

It allows for duplicate elements.

Answer

It provides index-based access to its elements via a `get(int index)` method.