How to Implement HashMap in Java from Scratch

When working with Java, having a good understanding of data structures is essential. One such data structure is the HashMap, which allows efficient storage and retrieval of key-value pairs.

In this article, we will learn how to implement a HashMap from scratch in Java, step by step.

Understanding HashMap

What is a HashMap?

A HashMap is a data structure that stores key-value pairs and provides constant-time complexity for basic operations like insertion, deletion, and retrieval. It is part of the Java Collections Framework and is widely used in various applications.

How does HashMap work?

HashMap internally uses an array of linked lists to store the key-value pairs. Each element in the array is called a “bucket” and can contain multiple entries. The key is hashed to determine the index of the bucket. In case of hash collisions, where two or more keys hash to the same index, a linked list is used to handle collisions.

Advantages of using HashMap

• Fast access and retrieval of values based on keys.
• Flexible size, as it adjusts dynamically based on the number of elements.
• Support for null keys and values.
• Efficient operations for inserting, deleting, and updating key-value pairs.

Implementing HashMap

Now let’s walk through the steps to implement a HashMap from scratch in Java.

Step 1: Creating the HashMap class

We start by creating a new Java class named “HashMap” with the necessary attributes and methods. Our HashMap class will have a fixed size, defined by the number of buckets.

Step 2: Defining the Entry class

Next, we define a nested class called “Entry” that represents each key-value pair in the HashMap. The Entry class will have fields for the key, value, and a reference to the next Entry in case of collisions.

Step 3: Implementing the put() method

The put() method is used to insert a key-value pair into the HashMap. We first calculate the hash code of the key, then find the index of the corresponding bucket. If the bucket is empty, we create a new Entry and assign it to the bucket. If there is a collision, we append the new Entry to the linked list at that bucket.

Step 4: Implementing the get() method

The get() method retrieves the value associated with a given key from the HashMap. We calculate the hash code of the key, find the bucket, and iterate through the linked list to find the Entry with the matching key. If found, we return the corresponding value.

Step 5: Implementing the remove() method

The remove() method removes the key-value pair associated with a given key from the HashMap. We calculate the hash code of the key, find the bucket, and iterate through the linked list to find the Entry with the matching key. If found, we remove the Entry from the linked list.

Testing the HashMap

To ensure our implementation of HashMap is functioning correctly, we should write test cases to cover different scenarios. We can test the put(), get(), and remove() methods with various keys and values, including null values, and verify that the expected results are obtained.

Conclusion

In this article, we have learned how to implement a HashMap from scratch in Java. By understanding the internal workings and following the step-by-step approach, we can create our own custom HashMap. HashMaps provide efficient storage and retrieval of key-value pairs, making them valuable data structures in Java programming.

FAQs

What is the time complexity of HashMap operations?

The time complexity of basic HashMap operations like put(), get(), and remove() is typically O(1) on average. However, in the case of hash collisions, the complexity can be O(n), where n is the number of elements in the linked list at a particular bucket.

Can a HashMap contain duplicate keys?

No, a HashMap cannot contain duplicate keys. If a duplicate key is inserted, the existing entry associated with that key will be replaced with the new entry.

How does a HashMap handle null keys and values?

A HashMap can have at most one null key and multiple null values. The null key is hashed to index 0, and null values can be stored in any bucket.

Is HashMap thread-safe?

No, the standard implementation of HashMap in Java is not thread-safe. If multiple threads concurrently access and modify a HashMap, it can lead to inconsistent results. To achieve thread-safety, you can use the ConcurrentHashMap class.

How do HashMaps handle resizing?

HashMaps have an initial capacity and load factor. When the number of elements exceeds the product of the capacity and load factor, the internal array is resized to accommodate more elements. This process involves rehashing all the existing entries and redistributing them to new buckets, which can be computationally expensive.