Telusko Docs

1. Introduction

The Map interface in Java is used to store data in key-value pairs. Unlike List and Set, a Map is not considered a direct subtype of Collection.

In a map:

Each key must be unique
Values may be duplicated
A key maps to exactly one value

Example:

import java.util.HashMap;
import java.util.Map;

public class Demo {
    public static void main(String[] args) {
        Map<Integer, String> students = new HashMap<>();

        students.put(101, "Navin");
        students.put(102, "Kiran");
        students.put(103, "Riya");

        System.out.println(students);
    }
}

2. Why Map Is Useful

Use Map when you want fast association between a key and its value.

Common examples:

Student ID to student name
Username to password hash
Product ID to price
Country code to country name

3. Important Features

Stores key-value pairs
Keys are unique
Values can repeat
Some implementations allow null
Different implementations have different ordering behavior

4. Common Map Implementations

4.1 `HashMap`

No guaranteed order
Fast average performance
Allows one null key and multiple null values

4.2 `LinkedHashMap`

Maintains insertion order
Slightly more overhead than HashMap

4.3 `TreeMap`

Stores keys in sorted order
Slower than HashMap
Useful when sorted keys are needed

4.4 `Hashtable`

Legacy class
Synchronized
Usually avoided in modern Java

5. Creating a Map

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

This means:

Key type is String
Value type is Integer

6. Common Methods

6.1 `put()`

Adds or updates a key-value pair.

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

marks.put("Math", 95);
marks.put("Science", 88);
marks.put("Math", 99);

If the key already exists, the old value is replaced.

6.2 `get()`

Retrieves value by key.

System.out.println(marks.get("Math")); // 99

6.3 `remove()`

marks.remove("Science");

6.4 `containsKey()` and `containsValue()`

System.out.println(marks.containsKey("Math"));
System.out.println(marks.containsValue(99));

6.5 `size()`, `isEmpty()`, `clear()`

System.out.println(marks.size());
System.out.println(marks.isEmpty());
marks.clear();

7. Iterating Over a Map

There are different ways to iterate.

7.1 Using `keySet()`

for (String subject : marks.keySet()) {
    System.out.println(subject + " -> " + marks.get(subject));
}

7.2 Using `entrySet()`

This is usually the most practical way.

for (Map.Entry<String, Integer> entry : marks.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}

7.3 Using `values()`

for (Integer mark : marks.values()) {
    System.out.println(mark);
}

8. Duplicate Keys and Values

Duplicate keys are not allowed:

Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(1, "B");

System.out.println(map);

Output:

{1=B}

The second value replaces the first one.

Duplicate values are allowed:

map.put(2, "B");

This is valid.

9. `null` in Map

Whether null is allowed depends on the implementation.

Example with HashMap:

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

map.put(null, "Admin");
map.put("theme", null);

System.out.println(map);

10. Map Is Not a Collection

This is an important interview and theory point.

Map is part of the Java Collections Framework, but it does not extend the Collection interface.

That is because a map stores pairs, not individual elements.

11. Practical Example

import java.util.HashMap;
import java.util.Map;

public class WordCounter {
    public static void main(String[] args) {
        String[] words = {"java", "python", "java", "c", "python", "java"};

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

        for (String word : words) {
            frequency.put(word, frequency.getOrDefault(word, 0) + 1);
        }

        System.out.println(frequency);
    }
}