Map in Java

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:

Roll Number (Key)	Student Name (Value)
101	Harini
102	Rahul
103	Kaviya

Here:

• Roll Number = Key
• Student Name = Value

A key uniquely identifies a value.

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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

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Syntax

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

Example

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

• Integer → Key
• String → Value

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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}

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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}

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2. get()

Retrieves value using key.

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

Output

Java

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3. remove()

Removes entry based on key.

map.remove(1);

Output

{2=Python}

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4. containsKey()

Checks whether a key exists.

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

Output

true

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5. containsValue()

Checks whether a value exists.

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

Output

true

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6. size()

Returns number of entries.

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

Output

2

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7. isEmpty()

Checks if map is empty.

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

Output

false

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8. clear()

Removes all entries.

map.clear();

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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

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Using values()

for(String value : map.values()) {
    System.out.println(value);
}

Output

Java
Python
C++

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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++

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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);

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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}

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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}

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