HashMap

HashMap in Java

What Is It?
A HashMap in Java is a part of the Java Collections Framework that stores data in key-value pairs. Each element in a HashMap consists of a key and a value, where the key is unique, and the value can be accessed using that key. This makes it very efficient for lookups, updates, and insertions.

How It Works
HashMap uses a technique called hashing, which allows it to compute an index where each key-value pair should be stored. Because of this, the time it takes to retrieve, add, or delete an entry is typically constant, making HashMap highly efficient.

Syntax for Creating a HashMap
Here’s how you create a HashMap:

import java.util.HashMap; // Import statement

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

Example of Using a HashMap
Let’s use a HashMap to store the number of pets people own:

HashMap<String, Integer> pets = new HashMap<>();
pets.put("Alice", 3);  // Alice owns 3 pets
pets.put("Bob", 1);    // Bob owns 1 pet
pets.put("Carol", 2);  // Carol owns 2 pets

System.out.println(pets.get("Alice"));  // Prints: 3
System.out.println(pets.containsKey("Bob"));  // Prints: true
pets.remove("Carol");  // Removes Carol's entry

In this example, each person’s name is a key, and the number of pets they own is the value. We can easily add to, access, check the existence of, or remove entries.

Common Methods

• put(key, value): Adds a key-value pair to the map or updates the value if the key already exists.
• get(key): Returns the value associated with the key, or null if the key does not exist.
• remove(key): Removes the key and its associated value from the map.
• containsKey(key): Checks if the map contains a specific key.
• keySet(): Returns a set of all keys in the map.
• values(): Provides a collection of all values in the map.

When to Use It
Use a HashMap when you need to quickly look up data using unique keys. It’s ideal for cases where you want to manage and access data without needing an ordered list.

Simple Tips

• HashMap is not synchronized, which means it’s not safe for use by multiple threads without external synchronization.
• The keys in a HashMap must have consistent implementations of hashCode() and equals() methods to function properly.
• HashMap allows one null key and multiple null values.

HashMap is a powerful tool for managing data where you want fast and efficient access to elements using keys. It simplifies tasks that involve searching, updating, or maintaining large sets of data based on unique identifiers.