HashMaps

A HashMap is a data structure that stores key-value pairs and uses a hash function to compute an index for fast data retrieval. It’s like a digital phone book where you can quickly find someone’s number (value) by looking up their name (key), regardless of how many contacts you have.

Properties

• Key-value pair storage: Each element consists of a unique key associated with a value.
• Uses hash function to compute index: Keys are converted to array indices using mathematical functions.
• Keys must be unique: No duplicate keys are allowed, but values can be duplicated.

Use Cases

• Caching systems: Storing frequently accessed data for quick retrieval.

Operations

• Search: Find a value using its key.
• Insertion: Add a new key-value pair.
• Deletion: Remove a key-value pair.
• Access: Retrieve a value using its key.
• Update: Modify the value associated with a key.

Advantages

• Very fast average-case operations: Most operations complete in constant time on average.
• Flexible key types: Can use various data types as keys (strings, numbers, objects).
• Dynamic sizing: Can grow and shrink as needed during runtime.

Disadvantages

• No ordering of elements: Elements are not stored in any predictable sequence.

Example

import java.util.HashMap;

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

// Insert key-value pairs
studentGrades.put("Swasti", 95);
studentGrades.put("Terrence", 87);
studentGrades.put("Shafeer", 82);

// Access values using keys
System.out.println(studentGrades.get("Swasti")); // Prints 95

// Update existing value
studentGrades.put("Swasti", 100);
System.out.println(studentGrades.get("Swasti")); // Prints 100

// Get size of the map
System.out.println(studentGrades.size()); // Prints 3

// Remove a key-value pair
studentGrades.remove("Shafeer");
System.out.println(studentGrades);

// Iterate through the map
for (String student : studentGrades.keySet()) {
  System.out.println(student + ": " + studentGrades.get(student));
}