<p>In this article of HashMap performance Improvement Changes in Java 8, we will discuss an interesting change made in Java 8. The Oracle made this change in <strong><a href="https://dineshonjava.com/hashmap-class-in-collection-framework/">HashMap</a> </strong>due to performance factor. The HashMap has frequently used map implementation for general purpose as we have discussed it in <strong><a href="https://dineshonjava.com/choosing-the-right-map-to-use-in-java/">choosing the right map implementation in Java article</a></strong>.</p>
<p><img class="aligncenter size-full wp-image-4326" src="https://dineshonjava.com/wp-content/uploads/2018/10/hashmap-change-in-java8.jpg" alt="HashMap Performance Improvement Changes in Java 8" width="1920" height="1080" /></p>
<p>First of all, you must be aware of the <strong><a href="https://dineshonjava.com/internal-working-of-hashmap-in-java/">internal implementation of the HashMap</a></strong> to understand this change in HashMap in Java 8. Also, we have looked at the <strong><a href="https://dineshonjava.com/internal-working-of-linkedhashmap-in-java/">LinkedHasHMap internal implementation</a></strong> and <strong><a href="https://dineshonjava.com/internal-working-of-treemap-in-java/">TreeMap internal implementation</a></strong>.</p>
<h2>HashMap in Java</h2>
<p>The HashMap class extends AbstractMap and implements Map interface. HashMap is a key and value collection in java. The HashMap stores the data in key and value format. It provides the basic implementation of the Map interface of Java. We can put a value with using a key and also we can access this value using that key. It uses a hash table to store the map. This allows the execution time of the get() and the put() methods to remain the same.</p>
<h2>HashMap performance Improvement Changes in Java 8</h2>
<p>Here I am going to discuss JDK 8’s new strategy for dealing with <em>Hash</em> collisions. Earlier before Java 8, the performance of the <em>HashMap</em> was poor due to the hash collision, it degrades the performance of HashMap significantly. This change can be notifiable only when if you are using the HashMap for a large number of elements, <em>why (see below points)?</em></p>
<p>The traversal of <em>HashMap,</em> get(), and other methods lookup time of <em>HashMap</em> have a negative impact due to hash collisions. This situation you can face when multiple keys end up in the same bucket, then values along with their keys are placed in a linked list. So, the retrieval time of elements from HashMap increases from <em><strong>O(1)</strong> </em>to <em><strong>O(n)</strong></em>. Because the linked list has to be traversed to get the entry in the worst case scenario.</p>
<p>But <strong>Java 8</strong> has come with the following new strategy for <em>HashMap</em> objects in case of high collisions.</p>
<ul>
<li>To address this issue, Java 8 hash elements use <strong>balanced trees instead of linked lists</strong> after a certain <strong>threshold</strong> is reached. Which means HashMap starts with storing Entry objects in a linked list but after the number of items in a hash becomes larger than a certain threshold. The hash will change from using a linked list to a balanced tree.</li>
<li>Above changes ensure the performance of <strong><em>O(log(n))</em></strong> in worst case scenarios and O(1) with proper hashCode().</li>
<li>The alternative String hash function added in Java 7 has been removed.</li>
</ul>
<h2>Summary</h2>
<p>Java 8 came with this new improvement in the <em>HashMap</em>, <em>LinkedHashMap</em>, and <em>ConcurrentHashMap</em>. All other map implementation is as it is.</p>
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