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Map Interface in Java

Last Updated : 23 Apr, 2025
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In Java, the Map Interface is part of the java.util package and represents a mapping between a key and a value. The Java Map interface is not a subtype of the Collections interface. So, it behaves differently from the rest of the collection types.

Key Features:

  • No Duplicates in Keys: Keys should be unique, but values can be duplicated.
  • Null Handling: It allows one null key in implementations like HashMap and LinkedHashMap, and allows multiple null values in most implementations.
  • Thread-Safe Alternatives: Use ConcurrentHashMap for thread-safe operations. Also, wrap an existing map using Collections.synchronizedMap() for synchronized access.

The Map data structure in Java is implemented by two interfaces:

  • Map Interface
  • SortedMap Interface

The three primary classes that implement these interfaces are,

  • HashMap
  • TreeMap
  • LinkedHashMap

Now, let us go through a simple example first to understand the concept.

Example:

Java 
// Java Program Implementing HashMap
import java.util.HashMap;
import java.util.Map;

public class Geeks {
    public static void main(String[] args) {
        
        // Create a Map using HashMap
        Map<String, Integer> m = new HashMap<>();

        // Adding key-value pairs to the map
        m.put("Geek1", 1);
        m.put("Geek2", 2);
        m.put("Geek3", 3);

        System.out.println("Map elements: " + m);
    }
}

Output
Map elements: {Geek3=3, Geek2=2, Geek1=1}


We must know that why and when to use Maps.

Maps are perfect to use for key-value mapping such as dictionaries. Some common scenarios are as follows: 

  • A map of error codes and their descriptions.
  • A map of zip codes and cities.
  • A map of managers and employees. Each manager (key) is associated with a list of employees (value) he manages.
  • A map of classes and students. Each class (key) is associated with a list of students (value).

Map Interface in Java


Creating Map Objects

Since Map is an interface, objects cannot be created of the type map. We always need a class that extends this map in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Map. 

Syntax: Defining Type-safe Map:

Map<String, Integer> hm = new HashMap<>(); // Type-safe map storing String keys and Integer values

Characteristics of a Map Interface

  • A map cannot contain duplicate keys and each key can map to at most one value. Some implementations allow null key and null values like the HashMap and LinkedHashMap, but some do not like the TreeMap.
  • The order of a map depends on the specific implementations. For example, TreeMap and LinkedHashMap have predictable orders, while HashMap does not.

Methods in Java Map Interface

MethodsAction Performed 
clear()This method is used in Java Map Interface to clear and remove all of the elements or mappings from a specified Map collection.
containsKey(Object)This method is used in Map Interface in Java to check whether a particular key is being mapped into the Map or not. It takes the key element as a parameter and returns True if that element is mapped in the map.
containsValue(Object)This method is used in Map Interface to check whether a particular value is being mapped by a single or more than one key in the Map. It takes the value as a parameter and returns True if that value is mapped by any of the keys in the map.
entrySet()This method is used in Map Interface in Java to create a set out of the same elements contained in the map. It basically returns a set view of the map or we can create a new set and store the map elements into them.
equals(Object)This method is used in Java Map Interface to check for equality between two maps. It verifies whether the elements of one map passed as a parameter is equal to the elements of this map or not.
get(Object)This method is used to retrieve or fetch the value mapped by a particular key mentioned in the parameter. It returns NULL when the map contains no such mapping for the key.
hashCode()This method is used in Map Interface to generate a hashCode for the given map containing keys and values.
isEmpty()This method is used to check if a map is having any entry for key and value pairs. If no mapping exists, then this returns true.
keySet()This method is used in Map Interface to return a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.
put(Object, Object)This method is used in Java Map Interface to associate the specified value with the specified key in this map.
putAll(Map)This method is used in Map Interface in Java to copy all of the mappings from the specified map to this map.
remove(Object)This method is used in Map Interface to remove the mapping for a key from this map if it is present in the map.
size()This method is used to return the number of key/value pairs available in the map.
values()This method is used in Java Map Interface to create a collection out of the values of the map. It basically returns a Collection view of the values in the HashMap.
getOrDefault(Object key, V defaultValue)Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)If the specified key is not already associated with a value or is associated with null, associate it with the given non-null value.
putIfAbsent(K key, V value)If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current associate value.

 

Example:

Java 
// Java Program to Demonstrate
// Working of Map interface

// Importing required classes
import java.util.*;

// Main class
class Geeks {

    // Main driver method
    public static void main(String args[])
    {
        // Creating an empty HashMap
        Map<String, Integer> hm
            = new HashMap<String, Integer>();

        // Inserting pairs in above Map
        // using put() method
        hm.put("a", new Integer(100));
        hm.put("b", new Integer(200));
        hm.put("c", new Integer(300));
        hm.put("d", new Integer(400));

        // Traversing through Map using for-each loop
        for (Map.Entry<String, Integer> me :
             hm.entrySet()) {

            System.out.print(me.getKey() + ":");
            System.out.println(me.getValue());
        }
    }
}

Output
a:100
b:200
c:300
d:400


Hierarchy of Map Interface in Java

Classes that implement the Map interface are shown in the below:

Map Interface in Java


1. HashMap 

HashMap is a part of Java’s collection since Java 1.2. It provides the basic implementation of the Map interface of Java. It stores the data in (Key, Value) pairs. To access a value one must know its key. This class uses a technique called Hashing. Hashing is a technique of converting a large String to a small String that represents the same String. A shorter value helps in indexing and faster searches. Let’s see how to create a map object using this class.

Example:

Java 
// Java Program to illustrate the Hashmap Class

// Importing required classes
import java.util.*;

// Main class
public class Geeks {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty HashMap
        Map<String, Integer> m = new HashMap<>();

        // Inserting entries in the Map
        // using put() method
        m.put("Sweta", 10);
        m.put("Amiya", 30);
        m.put("Gudly", 20);

        // Iterating over Map
        for (Map.Entry<String, Integer> e : m.entrySet())

            System.out.println(e.getKey() + " "
                               + e.getValue());
    }
}

Output
Amiya 30
Sweta 10
Gudly 20


2. LinkedHashMap

LinkedHashMap is just like HashMap with the additional feature of maintaining an order of elements inserted into it. HashMap provided the advantage of quick insertion, search, and deletion but it never maintained the track and order of insertion which the LinkedHashMap provides where the elements can be accessed in their insertion order. Let’s see how to create a map object using this class.

Example:  

Java 
// Java Program to Illustrate the LinkedHashmap Class
import java.util.*;

public class Geeks {

    public static void main(String[] args)
    {

        // Creating an empty LinkedHashMap
        Map<String, Integer> m = new LinkedHashMap<>();

        // Inserting pair entries in above Map
        // using put() method
        m.put("Sweta", 10);
        m.put("Amiya", 30);
        m.put("Gusly", 20);

        // Iterating over Map
        for (Map.Entry<String, Integer> e : m.entrySet())

            // Printing key-value pairs
            System.out.println(e.getKey() + " "
                               + e.getValue());
    }
}

Output
Sweta 10
Amiya 30
Gusly 20


3. TreeMap

The TreeMap in Java is used to implement the Map interface and NavigableMap along with the Abstract Class. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. This proves to be an efficient way of sorting and storing the key-value pairs. The ordering maintained by the TreeMap must be consistent with the equals() method if a custom comparator is not used.

Example:  

Java 
// Java Program to Illustrate TreeMap Class

// Importing required classes
import java.util.*;

// Main class
public class Geeks {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty TreeMap
        Map<String, Integer> m = new TreeMap<>();

        // Inserting custom elements in the Map
        // using put() method
        m.put("Sweta", 10);
        m.put("Amiya", 30);
        m.put("Gudly", 20);

        // Iterating over Map using for each loop
        for (Map.Entry<String, Integer> e : m.entrySet())

            System.out.println(e.getKey() + " "
                               + e.getValue());
    }
}

Output
Amiya 30
Gudly 20
Sweta 10


Performing Different Operations using Map Interface and HashMap Class

Now, let’s see how to perform a few frequently used operations on a Map using the widely used HashMap class.

1. Adding Elements

To add an element to the map, we can use the put() method. The insertion order is not retained in the hashmap. Internally, for every element, a separate hash is generated and the elements are indexed based on this hash to make it more efficient.

Example: 

Java 
// Java program to demonstrate
// the working of Map interface
import java.util.*;

class Geeks {
    public static void main(String args[])
    {
        // Default Initialization of a
        // Map
        Map<Integer, String> hm1 = new HashMap<>();

        // Initialization of a Map
        // using Generics
        Map<Integer, String> hm2
            = new HashMap<Integer, String>();

        // Inserting the Elements
        hm1.put(1, "Geeks");
        hm1.put(2, "For");
        hm1.put(3, "Geeks");

        hm2.put(new Integer(1), "Geeks");
        hm2.put(new Integer(2), "For");
        hm2.put(new Integer(3), "Geeks");

        System.out.println(hm1);
        System.out.println(hm2);
    }
}

Output
{1=Geeks, 2=For, 3=Geeks}
{1=Geeks, 2=For, 3=Geeks}


2. Changing Element

After adding the elements if we wish to change the element, it can be done by again adding the element with the put() method. The elements in the map are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we want to change. 

Example: 

Java 
// Java program to demonstrate
// the working of Map interface
import java.util.*;

class Geeks {
    public static void main(String args[])
    {

        // Initialization of a Map
        // using Generics
        Map<Integer, String> hm1
            = new HashMap<Integer, String>();

        // Inserting the Elements
        hm1.put(new Integer(1), "Geeks");
        hm1.put(new Integer(2), "Geeks");
        hm1.put(new Integer(3), "Geeks");

        System.out.println("Initial Map: " + hm1);

        hm1.put(new Integer(2), "For");

        System.out.println("Updated Map: " + hm1);
    }
}

Output
Initial Map: {1=Geeks, 2=Geeks, 3=Geeks}
Updated Map: {1=Geeks, 2=For, 3=Geeks}


3. Removing Elements

To remove an element from the Map, we can use the remove() method. This method takes the key value and removes the mapping for a key from this map if it is present in the map.

Example:  

Java 
// Java program to demonstrate
// the working of Map interface
import java.util.*;

class Geeks {

    public static void main(String args[])
    {

        // Initialization of a Map
        // using Generics
        Map<Integer, String> hm1
            = new HashMap<Integer, String>();

        // Inserting the Elements
        hm1.put(new Integer(1), "Geeks");
        hm1.put(new Integer(2), "For");
        hm1.put(new Integer(3), "Geeks");
        hm1.put(new Integer(4), "For");

        System.out.println(hm1);

        hm1.remove(new Integer(4));

        System.out.println(hm1);
    }
}

Output
{1=Geeks, 2=For, 3=Geeks, 4=For}
{1=Geeks, 2=For, 3=Geeks}


4. Iterating through the Map

There are multiple ways to iterate through the Map. The most famous way is to use a for-each loop and get the keys. The value of the key is found by using the getValue() method.

Example:  

Java 
// Java program to demonstrate
// the working of Map interface
import java.util.*;

class Geeks {
    public static void main(String args[])
    {

        // Initialization of a Map
        // using Generics
        Map<Integer, String> hm1
            = new HashMap<Integer, String>();

        // Inserting the Elements
        hm1.put(new Integer(1), "Geeks");
        hm1.put(new Integer(2), "For");
        hm1.put(new Integer(3), "Geeks");

        for (Map.Entry mapElement : hm1.entrySet()) {
            int key = (int)mapElement.getKey();

            // Finding the value
            String value = (String)mapElement.getValue();

            System.out.println(key + " : " + value);
        }
    }
}

Output
1 : Geeks
2 : For
3 : Geeks


Example: Count the Occurrence of Numbers using Hashmap

In this example, we are using putIfAbsent() along with Collections.frequency() to count the exact occurrence of numbers. In many programs, you need to count the occurrence of a particular number or letter. You use the following approach to solve those types of problems.

Example:

Java 
// Java program to Count the Occurrence 
// of numbers using Hashmap
import java.util.*;

class Geeks {
    public static void main(String[] args)
    {
        int a[] = { 1, 13, 4, 1, 41, 31, 31, 4, 13, 2 };

        // put all elements in arraylist
        ArrayList<Integer> al = new ArrayList();
        for (int i = 0; i < a.length; i++) {
            al.add(a[i]);
        }

        HashMap<Integer, Integer> hm = new HashMap();

        // counting occurrence of numbers
        for (int i = 0; i < al.size(); i++) {
            hm.putIfAbsent(al.get(i), Collections.frequency(
                                         al, al.get(i)));
        }
        System.out.println(hm);
    }
}

Output
{1=2, 2=1, 4=2, 41=1, 13=2, 31=2}


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