Category: React • Beginner
Published on 15 Feb 2026
Explanation
What is a Map? A Map<K, V> stores data in key–value pairs. • K → Key • V → Value Each key maps to exactly one value. Duplicate keys are not allowed, but duplicate values are allowed. #white- Example: 101 → "John" 102 → "Emma" 103 → "David"
Code Example
Explanation
Common Map Implementation classes: #white-HashMap: Fastest, most used #white-LinkedHashMap: Insertion order #white-TreeMap: Sorted order
Code Example
public static void main(String[] args) {
#white- // Creating a HashMap
Map<Integer, String> students = new HashMap<>();
#white- // Adding elements
students.put(101, "John");
students.put(102, "Emma");
students.put(103, "David");
System.out.println(students.get(101));
cont..
Explanation
Code Example
cont..
#white-// Removing element
students.remove(103);
#white-// Printing all entries
for (Map.Entry<Integer, String> entry : students.entrySet()) {
System.out.println(entry.getKey() + " : " + entry.getValue());
}
}
#white-Output:
Student 101: John
Contains key 102? true
101 : John
102 : Emma
Explanation
#white-Sorted Map: Keys are automatically sorted.
Code Example
import java.util.TreeMap;
import java.util.Map;
public class TreeMapExample {
public static void main(String[] args) {
Map<Integer, String> numbers = new TreeMap<>();
numbers.put(3, "Three");
numbers.put(1, "One");
numbers.put(2, "Two");
System.out.println(numbers);
}}
#white-Output:
{1=One, 2=Two, 3=Three}
Explanation
#white-Iterating in Different Ways:
Code Example
Using keySet():
for (Integer key : students.keySet()) {
System.out.println(key + " -> " + students.get(key));
}
Using values():
for (String value : students.values()) {
System.out.println(value);
}
}
Using forEach (Java 8+):
students.forEach((key, value) ->
System.out.println(key + " : " + value)
);
Explanation
Important Map Methods put(K, V) Insert key-value get(K) Get value by key remove(K) Delete by key containsKey(K) Check key containsValue(V) Check value size() Number of elements clear() Remove all
Code Example
Explanation
Word Frequency Program
Code Example
import java.util.HashMap;
import java.util.Map;
public class WordCount {
public static void main(String[] args) {
String text = "apple banana apple orange banana apple";
String[] words = text.split(" ");
Map<String, Integer> wordCount = new HashMap<>();
cont..
Explanation
Code Example
cont..
for (String word : words) {
wordCount.put(word, wordCount.getOrDefault(word, 0) + 1);
}
System.out.println(wordCount);
}
}
Output:
{apple=3, banana=2, orange=1}