What is Map in Java?

A Map in Java is a part of the Collections Framework that stores data in the form of key-value pairs.

• Each key is unique.


• Values can be duplicated.


• A key is used to retrieve its corresponding value.


• Map is available in the java.util package.


• It does not extend the Collection interface.

Real-Time Example

Think of a student's record:

Here:

• Roll Number = Key


• Student Name = Value

A key uniquely identifies a value.

Why Use Map?

Map is useful when you need:

• Fast searching
  


• Unique identifiers
  


• Key-based data storage
  


• Efficient lookup operations

Examples

• Employee ID → Employee Details


• Username → Password


• Product ID → Product Information


• Roll Number → Student Details


• Country Code → Country Name

Syntax

Map<KeyType, ValueType> map = new HashMap<>();

Example

Map<Integer, String> students = new HashMap<>();

• Integer → Key


• String → Value

Creating a Map

import java.util.*;



public class Main {

    public static void main(String[] args) {



        Map<Integer, String> map = new HashMap<>();



        map.put(101, "Harini");

        map.put(102, "Rahul");

        map.put(103, "Kaviya");



        System.out.println(map);

    }

}

Output

{101=Harini, 102=Rahul, 103=Kaviya}

Important Methods in Map

1. put()

Used to insert key-value pairs.

map.put(101, "Harini");

Example

Map<Integer,String> map = new HashMap<>();



map.put(1, "Java");

map.put(2, "Python");



System.out.println(map);

Output

{1=Java, 2=Python}

2. get()

Retrieves value using key.

System.out.println(map.get(1));

Output

Java

3. remove()

Removes entry based on key.

map.remove(1);

Output

{2=Python}

4. containsKey()

Checks whether a key exists.

System.out.println(map.containsKey(2));

Output

true

5. containsValue()

Checks whether a value exists.

System.out.println(map.containsValue("Python"));

Output

true

6. size()

Returns number of entries.

System.out.println(map.size());

Output

2

7. isEmpty()

Checks if map is empty.

System.out.println(map.isEmpty());

Output

false

8. clear()

Removes all entries.

map.clear();

Traversing a Map

Using keySet()

Map<Integer,String> map = new HashMap<>();



map.put(1, "Java");

map.put(2, "Python");

map.put(3, "C++");



for(Integer key : map.keySet()) {

    System.out.println(key);

}

Output

1

2

3

Using values()

for(String value : map.values()) {

    System.out.println(value);

}

Output

Java

Python

C++

Using entrySet()

Most commonly used approach.

for(Map.Entry<Integer,String> entry : map.entrySet()) {

    System.out.println(entry.getKey() + " : " + entry.getValue());

}

Output

1 : Java

2 : Python

3 : C++

Implementations of Map Interface

1. HashMap

Features

• Most commonly used


• No insertion order maintained


• Allows one null key


• Allows multiple null values


• Fast performance

Example

Map<Integer,String> map = new HashMap<>();



map.put(101, "Harini");

map.put(102, "Kaviya");



System.out.println(map);

2. LinkedHashMap

Features

• Maintains insertion order


• Slightly slower than HashMap


• Allows null key and values

Example

Map<Integer,String> map = new LinkedHashMap<>();



map.put(3, "C");

map.put(1, "A");

map.put(2, "B");



System.out.println(map);

Output

{3=C, 1=A, 2=B}

3. TreeMap

Features

• Stores keys in sorted order


• Does not allow null keys


• Based on Red-Black Tree

Example

Map<Integer,String> map = new TreeMap<>();



map.put(3, "C");

map.put(1, "A");

map.put(2, "B");



System.out.println(map);

Output

{1=A, 2=B, 3=C}