Collections Quiz in Java - MCQ - Multiple Choice Questions

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This post contains a few useful Java collections framework multiple-choice questions to self-test your knowledge of Java collections framework classes and interfaces.

The answer and explanation have been given for each MCQ.

1. Which interface provides key-value pair?

a. List
b. Set
c. Map
d. Collection

Answer

c. Map

Explanation 

In Java, elements of Map are stored in key/value pairs. Keys are unique values associated with individual Values.

A map cannot contain duplicate keys. And, each key is associated with a single value.

2. What is the implementation of the List interface?

a. HashMap
b. HashSet
c. LinkedList
d. LinkedHashSet

Answer

c. LinkedList

3. What are the implementation classes of the List interface?

a. ArrayList
b. HashSet
c. LinkedList
d. Vector

Answer

a, c, d

4. What are the implementation classes of the Set interface?

a. HashSet
b. LinkedHashSet
c. ArrayList
d. TreeSet

Answer

a,b,d

5. What are the implementation classes of the Map interface?

a. HashMap
b. LinkedHashMap
c. TreeMap
d. All of above

Answer

d. All of above

6. What are concurrent-aware interfaces?

a. List
b. BlockingQueue
c. ConcurrentMap
d. SortedMap

Answer

b, c 

7. What are concurrent-aware implementation classes?

a. TreeMap
b. CopyOnWriteArrayList
c. PriorityBlockingQueue
d. ConcurrentHashMap

    Answer

    b,c,d

    8. Choose the correct option based on this program:

    import java.util.*;
class UtilitiesTest {
    public static void main(String[] args) {
        List < int > intList = new ArrayList < > ();
        intList.add(10);
        intList.add(20);
        System.out.println("The list is: " + intList);

    A. It prints the following: The list is: [10, 20] 

    B. It prints the following: The list is: [20, 10] 

    C. It results in a compiler error 

    D. It results in a runtime exception

    Answer

    C. It results in a compiler error

    Explanation

    You cannot specify primitive types along with generics, so List needs to be changed to List<Integer>.

    9. What is the output of the following program?

    import java.util.*;
class UtilitiesTest {
    public static void main(String[] args) {
        List < Integer > intList = new LinkedList < > ();
        List < Double > dblList = new LinkedList < > ();
        System.out.println("First type: " + intList.getClass());
        System.out.println("Second type:" + dblList.getClass());
    }
}

    A. It prints the following: 

    First type: class java.util.LinkedList 

    Second type:class java.util.LinkedList 

    B. It prints the following: 

    First type: class java.util.LinkedList 

    Second type:class java.util.LinkedList 

    C. It results in a compiler error 

    D. It results in a runtime exception

    Answer

    A. It prints the following: 

    First type: class java.util.LinkedList 

    Second type:class java.util.LinkedList

    Explanation

    Due to type erasure, after compilation, both types are treated as the same LinkedList type

    10. What is the output of the following program?

    public class Question_7_1 {
    public static void main(String[] args) {
        ArrayDeque<Integer> deque =
              new ArrayDeque<Integer>();
        deque.push(1);
        deque.push(2);
        deque.push(3);
        deque.poll();
        System.out.println(deque);
    }
}
    A. [1, 2, 3]
    B. [1, 2]
    C. [2, 1]
    D. An exception occurs at runtime

    Answer

    The correct answer is C.

    Explanation

    The push() inserts the element at the front of the deque. After pushing 1, 2, 3 the queue looks like [3, 2, 1].

    The poll() retrieves and removes the first element of this deque, 3 in this case.

    11. Which of the following options can throw a NullPointerException?

    A.

    TreeSet<String> s = new TreeSet<>();
s.add(null);

    B.

    HashMap<String, String> m = new HashMap<>();
m.put(null, null);

    C.

    ArrayList<String> arr = new ArrayList<>();
arr.add(null);

    D.

    HashSet<String> s = new HashSet<String>();
s.add(null);

    Answer

    The correct answer is A.

    Explanation

    TreeSet doesn't allow null values because when you add an object if no Comparator is passed to the constructor of the TreeSet (like in this case), this class assumes that the object implements Comparable and tries to call the compareTo() method.

    12. Which of these maintains insertion order?

    a.List
    b.Set
    c.All
    d.None

    Answer

    a. List

    13. Which Interface does not allow duplicates?

    a. List
    b. Set
    c. Map
    d. SortedMap

    Answer

    b. Set

    14. Which data structure ArrayList internally uses?

    a. LinkedList
    b. Array
    c. Doubly LinkedList
    d. None

    Answer

    b. Array

    15. HashSet internally uses?

    a.Set
    b.HashMap
    c.List
    d.Collection

    Answer

    b.HashMap

    16. Which is the root interface of the Java Collection framework hierarchy?

    a.Collection
    b.Root
    c.Collections
    d.List/Set

    Answer

    a.Collection

    17. Which is best suited to a multi-threaded environment?

    a.WeakHashMap
    b.Hashtable
    c.HashMap
    d.ConcurrentHashMap

    Answer

    d.ConcurrentHashMap

    18. Which of these is synchronized?

    a.TreeMap
    b.HashMap
    c.Hashtable
    d.All

    Answer

    c.Hashtable

    19. Which of these classes should be preferred to be used as a key in a HashMap?

    a.String
    b.Integer
    c.Double
    d.Any of these

    Answer

    a.String

    20. Iterator returned by CopyOnWriteArraySet is

    a.Fail-fast
    b.Fail-safe
    c.none

    Answer

    b.Fail-safe

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