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Indumathy
Indumathy

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Map Interface

#java #beginners #tutorial #programming

Introduction
In Java, Mapping means storing data in key–value pairs using the Map interface in the Java Collection Framework.
A key is used to identify a value. When we provide the key, we can easily retrieve the corresponding value.

Example:
Key → Student ID
Value → Student Name

So, when we give the student ID, we can get the student name easily.
Map Interface in Java

In Java, mapping is implemented using the Map interface.

Important Rules
Key
Duplicate keys are not allowed

Value
Duplicate values are allowed

Example:.

Key, Value

101, Kumar

102, Yogesh

103, Hari

Null Values

Only one null key is allowed (in HashMap)
Multiple null values are allowed

Common Classes in Map Interface

The most commonly used classes are:

  1. HashMap
  2. LinkedHashMap
  3. TreeMap****

HashMap in Java

HashMap is the most commonly used class in the Map interface.

Syntax

HashMap map = new HashMap<>();

Example:

import java.util.HashMap;

public class Sample {

public static void main(String[] args) {

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

    map.put(101, "Kumar");
    map.put(102, "Yogesh");
    map.put(103, "Hari");

    System.out.println(map);
}
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}
Output

{101=Kumar, 102=Yogesh, 103=Hari}

Important HashMap Methods

1. put() Method

Used to add key-value pairs.

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
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2. get() Method

Used to retrieve a value using the key.

Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
System.out.println(map.get(101));
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Output

Kumar

3. remove() Method

Used to remove an element from the map.

Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
map.remove(101);
System.out.println(map);
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Output
{102=Yogesh, 103=Hari}

You can also remove using key and value.

map.remove(101, "Kumar");
System.out.println(map);
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4. containsKey() Method

Checks whether the key exists in the map.

Example:


map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
System.out.println(map.containsKey(103));
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Output:
True

5. containsValue() Method

Checks whether the value exists in the map.
Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
System.out.println(map.containsValue("Hari"));
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Output:

True

6. size() Method

Returns the number of elements in the map.
Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
System.out.println(map.size());
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Output:
3

7. isEmpty() Method

Checks whether the map is empty or not.

Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
System.out.println(map.isEmpty());
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Output:
False

8. clear() Method

Removes all elements from the map.

Example:

map.put(101, "Kumar");
map.put(102, "Yogesh");
map.put(103, "Hari");
map.clear();
System.out.println(map);
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Output:
{}

Conclusion:

The Map Interface in Java is used to store data in key–value pairs. It allows quick retrieval of values using keys.
Among the different implementations:
HashMap → Fast and widely used
LinkedHashMap → Maintains insertion order
TreeMap → Stores keys in sorted order

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