Many Java beginners struggle to choose the right data structure when they need fast data lookup without caring about the order of elements. HashMap in Java is built exactly for this purpose, letting you store and retrieve data using a unique key in constant time. Understanding how HashMap works internally and when to use it will make you a more confident and efficient Java developer.<\/p>\n\n\n\n
HashMap in Java<\/a> is a class in the java.util package that stores data as key-value pairs. Every key in a HashMap is unique, and each key maps to exactly one value. You use the key to store a value and the same key to retrieve it later.<\/p>\n\n\n\n Key characteristics of HashMap:<\/p>\n\n\n\n Read More: <\/strong>Java Data Structures Unlocked: A Layman\u2019s Guide with Easy Examples<\/strong><\/a><\/p>\n\n\n\n Understanding the internal working of HashMap is one of the most common Java interview topics. Here is how it works under the hood.<\/p>\n\n\n\n When you insert a key-value pair, Java<\/a> calls the hashCode() method on the key. This produces an integer called a hash code, which is used to calculate the index of a bucket in an internal array.<\/p>\n\n\n\n HashMap uses an array of linked lists called buckets. Each bucket holds one or more entries. When two keys produce the same bucket index, it is called a collision.<\/p>\n\n\n\n Java handles collisions using chaining. Multiple entries with the same bucket index are stored as a linked list at that index. From Java 8 onwards, when the number of entries in a single bucket exceeds 8, the linked list is converted to a balanced binary tree (Red-Black Tree) <\/a>for faster lookup.<\/p>\n\n\n\n When you call get(key), Java computes the hash code of the key, finds the correct bucket, and then uses equals() to find the exact entry within that bucket.<\/p>\n\n\n\n Creating a HashMap is straightforward. Here is the basic syntax:<\/p>\n\n\n\n\n
How Does HashMap Work Internally?<\/strong><\/h2>\n\n\n\n
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How to Create and Use a HashMap in Java<\/strong><\/h2>\n\n\n\n
import java.util.HashMap;
HashMap<String, Integer> studentMarks = new HashMap<>();
\/\/ Adding key-value pairs
studentMarks.put(“Arun”, 85);
studentMarks.put(“Priya”, 92);
studentMarks.put(“Rahul”, 78);
\/\/ Retrieving a value
System.out.println(studentMarks.get(“Priya”)); \/\/ Output: 92
\/\/ Checking if a key exists
System.out.println(studentMarks.containsKey(“Rahul”)); \/\/ Output: true
\/\/ Removing a key
studentMarks.remove(“Arun”);
\/\/ Iterating over the HashMap
for (String name : studentMarks.keySet()) {
System.out.println(name + “: ” + studentMarks.get(name));
}<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n