Oracle Java OCP Generics and Collections Quiz

1. Which statement correctly declares a raw type?

List list = new ArrayList();

2. Which wildcard allows both reading and writing to a collection?

No wildcard (specific type parameter)

3. What is the time complexity of get() operation in a HashMap?

O(1) average case

O(n) always

O(log n)

O(n log n)

4. Which interface does HashMap implement?

Map and Cloneable

List and Serializable

Set and Comparable

Queue and Iterator

5. What is the purpose of the Collections.unmodifiableList() method?

Create a read-only view of a list

Remove all elements from a list

Sort a list in descending order

Convert a list to an array

6. In a generic method declaration, where do type parameters appear?

Between < > before the return type

After the method name

Inside the parameter list

In the throws clause

7. Which of the following is a fail-fast iterator behavior?

Throws ConcurrentModificationException if collection is modified during iteration

Silently skips newly added elements

Creates a snapshot of the collection

Automatically locks the collection during iteration

8. What does the Comparator interface provide?

A way to define custom sorting order for objects

Automatic sorting of all collections

Thread-safe comparison operations

Memory-efficient storage of objects

9. Which Collection method throws ClassCastException at runtime?

Add(E e) on a raw type list

Iterator() on any Collection

Contains(Object o) on a typed list

Size() on an empty set

10. What is a covariant wildcard, and what is its syntax?

Represents optional types

11. What is the primary benefit of using generics in Java?

Compile-time type safety and elimination of casting

Faster runtime performance

Automatic garbage collection

Reduced memory usage

12. In generics, what does type erasure mean?

Generic type information is removed at compile time

All objects are deleted from memory

The JVM creates separate bytecode for each type

Type parameters are converted to primitives

13. What does TreeSet use to order elements?

The Comparable interface or a Comparator

Insertion order

Hash codes

Object memory addresses

14. Which Map implementation maintains insertion order?

LinkedHashMap

TreeMap

WeakHashMap

IdentityHashMap

15. What is the difference between ArrayList and LinkedList?

ArrayList uses arrays; LinkedList uses doubly-linked nodes

ArrayList is thread-safe; LinkedList is not

LinkedList is faster for random access

ArrayList cannot store null values

16. Which Collection interface does NOT guarantee order?

Set

List

Queue

Vector

17. What is the purpose of bounded type parameters like ?

Restrict type arguments to those implementing a specific interface

Allow only primitive types

Prevent instantiation of generic objects

Improve garbage collection efficiency

18. In the declaration List

Any subclass of Number

Only Number itself

Nothing (it is read-only)

Any type that extends Object

19. What does the wildcard '?' represent in generics?

An unknown type

Any primitive type

A required type parameter

The Object class

20. Which declaration correctly defines a generic class with two type parameters?

Public class Box { }

Public class Box[T, U] { }