Map Interface in Java

In Java, Map Interface is present in java.util package represents a mapping between a key and a value. Java Map interface is not a subtype of the Collection interface. Therefore it behaves a bit differently from the rest of the collection types. A map contains unique keys.

Geeks, the brainstormer should have been why and when to use Maps.

Maps are perfect to use for key-value association mapping such as dictionaries. The maps are used to perform lookups by keys or when someone wants to retrieve and update elements by keys. 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).

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 hm = new HashMap(); 
// Obj is the type of the object to be stored in Map

Characteristics of a Map Interface

1. 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.
2. The order of a map depends on the specific implementations. For example, TreeMap and LinkedHashMap have predictable orders, while HashMap does not.
3. There are two interfaces for implementing Map in Java. They are Map and SortedMap, and three classes: HashMap, TreeMap, and LinkedHashMap.

Methods in Java Map Interface

 Example:

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

Classes that implement the Map interface are depicted in the below media and described later as follows:

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 

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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  

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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 storing order maintained by the treemap must be consistent with equals just like any other sorted map, irrespective of the explicit comparators. Let’s see how to create a map object using this class.

Example  

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Performing Operations using Map Interface and HashMap Class

Since Map is an interface, it can be used only with a class that implements this interface. Now, let’s see how to perform a few frequently used operations on a Map using the widely used HashMap class. 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. 

1. Adding Elements

In order to add an element to the map, we can use the put() method. However, 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 

{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. Since 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 wish to change. 

Example 

Java

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

In order 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 GFG {        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");            // Initial Map         System.out.println(hm1);            hm1.remove(new Integer(4));            // Final Map         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 GFG {     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

5. Count the Occurrence of numbers using Hashmap

In this code, 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 

Java

// Java program to Count the Occurrence  // of numbers using Hashmap import java.util.*;    class HelloWorld {     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> aa = new ArrayList();         for (int i = 0; i < a.length; i++) {             aa.add(a[i]);         }            HashMap<Integer, Integer> h = new HashMap();            // counting occurrence of numbers         for (int i = 0; i < aa.size(); i++) {             h.putIfAbsent(aa.get(i), Collections.frequency(                                          aa, aa.get(i)));         }         System.out.println(h);     } }

Output

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

FAQs in Java Map Interface

Q1. What is a map interface in Java?

Answer:

The map contains key-value pairs, where we access elements in the map using key values.

Q2. What are the types of map interfaces in Java?

Answer:

There are 3 map interface implementations HashMap, LinkedHashMap, and TreeMap.

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