HashMap in Java

HashMap is a part of the Java Collections Framework and is an implementation of the Map interface. It provides a key-value pair data structure, where elements are stored as a combination of keys and their associated values. HashMap does not allow duplicate keys, but it allows multiple values to be mapped to the same key. It offers constant-time performance for basic operations like adding, removing, and retrieving elements.

Declaration and Initialization

To use a HashMap in Java, you need to import the java.util.HashMap package. Here's an example of declaring and initializing a HashMap:

    
import java.util.HashMap;

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

        // Adding key-value pairs to the HashMap
        studentScores.put("Alice1", 90);
        studentScores.put("Bob1", 85);
        studentScores.put("Charlie1", 78);

        // No duplicate keys allowed, the value for "Bob1" will be updated
        studentScores.put("Bob1", 92);

        // Accessing values using keys
        System.out.println("Charlie1's score: " + studentScores.get("Charlie1")); // Output: Charlie1's score: 78

        // Checking if a key exists
        boolean exists = studentScores.containsKey("Alice1");
        System.out.println("Is Alice1 a student? " + exists); // Output: Is Alice1 a student? true

        // Removing key-value pairs
        studentScores.remove("Bob1");

        // Iterating over key-value pairs
        for (String student : studentScores.keySet()) {
            int score = studentScores.get(student);
            System.out.println(student + ": " + score);
        }
    }
}
    

Common HashMap Operations

Here are some commonly used operations with HashMaps:

• put(key, value): Adds a key-value pair to the HashMap.
• get(key): Retrieves the value associated with the specified key.
• containsKey(key): Checks if the HashMap contains a specific key.
• remove(key): Removes the key-value pair with the specified key.
• size(): Returns the number of key-value pairs in the HashMap.