Java Map Interview Questions

Here, we have listed the most important top 17 Java Map interview questions with the best possible answers. These map interview questions always asks in any java technical tests and interviews from freshers, and experienced.

We have also covered some important coding interview questions based on the map that are always asked in java technical tests in various companies. So, get answered these questions.

Map Interview Questions with Answers

1. What is a Map in Java?

Answer: A map in Java is a container object that stores elements as key and value pairs. A key is a unique element (object) that serves as an “index” in the map.

The element that is associated with a key is called value. A map stores the values associated with keys.

2. Can we store primitive type values in Map keys and values?

Answer: In the map, both keys and values must be objects, not primitive type.

3. What is one-to-one mapping in Java?

Answer: Each key maps to only one value. This type of mapping is called one-to-one mapping in java.

4. Can we have duplicate keys in Map?

Answer: No, all keys must be unique, but values may be duplicate (i.e. we can store the same value to several keys).

5. What are the classes that implement Map interface?

Answer: Java classes that implement Map interface are:

• AbstractMap
• EnumMap
• HashMap
• TreeMap
• LinkedHashMap
• WeakHashMap
• IdentityHashMap

6. Does Map extend the Collections framework in Java?

Answer: Map is a part of the Java collections framework, but it does not extend the collection interface.

Answer: The fundamental differences between a Set and a Map in Java are:

1. A Set does not allow to insert duplicate elements. While a Map does not allow using duplicate keys, but it allows to insert duplicate values for unique keys.

2. A Set allows to insert only one null value. In a Map we can have a single null key at most and many null values.

3. A Set does not maintain any order of elements. LinkedHashSet that is subclasses of Set interface can sort elements in an order.

A Map does not maintain any order of its elements. TreeMap that implements Map interface store elements of the map in the ascending order of keys.

8. How will you decide when to use a List, Set or a Map collection in Java?

Answer: (a) If you want a collection that does not store duplicate values, then use a Set.

(b) If you want to access elements operations based on an index value frequently, then use a List. E.g. ArrayList.

(c) If you want to maintain the insertion order of elements in a collection, then use a List based collection.

(d) For fast search operation based on a key-value pair, use a HashMap based collection.

(e) If you want to maintain a collection of elements in a natural sorted order, the use a TreeSet based collection.

9. How to create Map object in Java?

Answer: You can create an object of the map using any of its three concrete classes: HashMap, LinkedHashMap, or TreeMap.

The general syntax to create a map object is as follows:

a) Map<K, V> map = new HashMap<>(); // It creates an empty map.
b) Map<K, V> map = new HashMap<>(Map m); // It creates a map with initializing elements of m.

Answer: Yes, we can use any user-defined class as Map key.

11. Which method will you use to add an entry with a specified key and value to the map?

Answer: Map provides a method named put(K key, V value) that adds an entry with a specified key and value to a map.

12. Which method will you use to get the value for the specified key in the map?

Answer: get(Object key) method

13. What is the output of the following code?

import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) 
{
  Map<Integer, String> map = new HashMap<>();
  map.put(101, "Red");
  map.put(103, "Green");
  map.put(102, "Yellow");
 
  Map<Integer,String> map2 = new HashMap<>();
  map2.put(115, "Brown");
  map2.put(120, "Purple");
  map.putAll(map2);
  System.out.println(map);
 }
}

Output:
       {115=Brown, 101=Red, 102=Yellow, 103=Green, 120=Purple}

14. What is the output of the following program?

import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) 
{
  Map<Integer, String> map = new HashMap<>();
  map.put(101, "Red");
  map.put(103, "Green");
  map.put(102, "Yellow");
  map.remove(104);
  map.remove(106,"Pink");
  System.out.println(map);
 }
}

Output:
      {101=Red, 102=Yellow, 103=Green}

15. Is there any error in the program? If not, what is the output of program code?

import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) 
{
  Map<String, String> map = new HashMap<>();
  map.put("A", "Apple");
  map.put("B", "Boy");
  map.put("C", "Cat");
  map.replace("C", "Element");
  map.remove("A","Apple");
  System.out.println(map);
 }
}

Output:
      {B=Boy, C=Element}

16. Is code will compile successfully? If yes, what will be the output of the following code?

import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) 
{
  Map<String, String> map = new HashMap<>();
  map.put("V", "Violet");
  map.put("I", "Indigo");
  map.put("B", "Blue");
  map.put("G", "Green");
  map.put("Y", "Yellow");
 
  String value = map.get("B");
  System.out.println(value);
  boolean entryKey = map.containsKey("B");
  System.out.println(entryKey);
  boolean entryValue = map.containsValue("Brown");
  System.out.println(entryValue);
 }
}

Output:
      Blue
      true
      false

17. What are the ways to iterate a Map in Java?

Answer: Go to this tutorial for better answer: How to iterate a Map in Java

In this tutorial, you have covered the top 17 Map interview questions with the best possible answers. Hope that you will have understood all answers of these map interview questions and practiced all basic programs.
All the best!!!