Java HashMap explained

In Java, HashMap is a class in the java.util package that implements the Map interface and represents a mapping of keys to values using a hash table. A HashMap allows you to store key-value pairs and provides methods for inserting, deleting, and manipulating the elements of the map.

HashMap is an implementation of the Map interface and uses a hash table to store the elements of the map. A hash table is a data structure that uses a hash function to map keys to indices in an array, allowing for fast insertion, deletion, and lookup of elements.

Here is an example of how to use the HashMap class:

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

public class Main {
  public static void main(String[] args) {
    // Create a map
    Map<String, Integer> map = new HashMap<>();

    // Add key-value pairs to the map
    map.put("A", 1);
    map.put("B", 2);
    map.put("C", 3);

    // Get the value for a specific key
    int value = map.get("B");  // returns 2

    // Check if the map contains a specific key
    boolean containsA = map.containsKey("A");  // returns true
    boolean containsD = map.containsKey("D");  // returns false

    // Get the size of the map
    int size = map.size();  // returns 3
  }
}

In this example, we create a map of strings to integers using a HashMap and add key-value pairs to the map using the put() method. We then use the get() method to get the value for a specific key and the containsKey() method to check if the map contains a specific key. We also use the size() method to get the size of the map.

HashMap is a fast and efficient implementation of the Map interface and is commonly used in Java applications. It is an essential part of the Java Collections Framework and is used in a wide range of applications.