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Java Queue and Map Interface

Here are 10 essential multiple-choice questions on Java Queue and Map Interface, covering key concepts.

Last Updated : Apr 2, 2025

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Question 1

Which statement about the Queue interface in Java is correct?

A
Queue allows duplicate elements but does not allow null
B
Queue follows LIFO (Last In, First Out) order
C
Queue is an implementation of Deque
D
Queue follows FIFO (First In, First Out) order

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Question 2

What is the time complexity of offer(), poll(), and peek() operations in PriorityQueue?

A
O(1), O(1), O(1)
B
O(log n), O(log n), O(1)
C
O(n), O(n), O(n)
D
O(log n), O(n), O(log n)

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Question 3

What will be the output of the following code?

Java

import java.util.*;
public class Test {
    public static void main(String[] args) {
        Queue<Integer> queue = new PriorityQueue<>();
        queue.add(30);
        queue.add(10);
        queue.add(50);
        queue.add(20);
        System.out.println(queue.poll());
    }
}

A
10
B
30
C
50
D
20

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Question 4

Which of the following statements is true about Deque in Java?

A
Deque allows insertion and removal only from one end
B
Deque can function as both a Queue and a Stack
C
Deque is an abstract class
D
Deque does not allow duplicate elements

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Question 5

Which of the following is not a valid implementation of Map in Java?

A
HashMap
B
Hashtable
C
LinkedListMap
D
TreeMap

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Question 6

What will be the output of the following code?

Java

import java.util.*;
public class Test {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("A", 10);
        map.put("B", 20);
        map.put("A", 30);
        System.out.println(map.get("A"));
    }
}

A
10
B
20
C
30
D
Compilation Error

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Question 7

What is the key difference between HashMap and LinkedHashMap?

A
LinkedHashMap maintains insertion order, while HashMap does not
B
HashMap is slower than LinkedHashMap
C
HashMap allows duplicate keys, while LinkedHashMap does not
D
LinkedHashMap does not allow null keys, while HashMap does

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Question 8

What will be the output of the following code?

Java

import java.util.*;
public class Test {
    public static void main(String[] args) {
        Map<Integer, String> map = new LinkedHashMap<>();
        map.put(3, "Three");
        map.put(1, "One");
        map.put(2, "Two");
        System.out.println(map.keySet());
    }
}

A
[1, 2, 3]
B
[3, 1, 2]
C
[2, 1, 3]
D
Unpredictable order

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Question 9

Which of the following statements about TreeMap is correct?

A
TreeMap maintains insertion order
B
TreeMap allows null keys
C
TreeMap sorts keys in ascending order by default
D
TreeMap uses a hash table for storage

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Question 10

What will be the output of the following code?

Java

import java.util.*;
public class Test {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("X", 100);
        map.put("Y", 200);
        map.put(null, 300);
        System.out.println(map.get(null));
    }
}

A
300
B
null
C
Compilation Error
D
Runtime Exception

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