{"id":26916,"date":"2021-03-30T02:20:00","date_gmt":"2021-03-29T20:50:00","guid":{"rendered":"https:\/\/www.mygreatlearning.com\/blog\/hashmap-in-java\/"},"modified":"2025-02-13T21:24:21","modified_gmt":"2025-02-13T15:54:21","slug":"hashmap-in-java","status":"publish","type":"post","link":"https:\/\/www.mygreatlearning.com\/blog\/hashmap-in-java\/","title":{"rendered":"HashMap in Java: A Complete Guide"},"content":{"rendered":"\n<p>HashMap is a prominent key-value data structure in <a href=\"https:\/\/www.mygreatlearning.com\/blog\/java-tutorial-for-beginners\/\">Java<\/a> that supports efficient updates, along with fast retrieval, insertion, and deletion capabilities. The <code>java.util<\/code> package provides HashMap as an efficient lookup tool, making it suitable for applications such as caching, database indexing, and frequency counting.<\/p>\n\n\n\n<p>In this tutorial, we\u2019ll <strong>deep dive into HashMap<\/strong>, covering its <strong>definition, internal workings, key operations, iteration techniques, and performance optimizations<\/strong>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"what-is-hashmap-in-java\"><strong>What is HashMap in Java?<\/strong><\/h2>\n\n\n\n<p>A HashMap is a part of Java\u2019s <strong>Map<\/strong> interface that stores data in key-value pairs. It uses hashing to map keys to values, making retrieval operations very fast (average time complexity of O(1)).<\/p>\n\n\n\n<p><strong>Key Features of HashMap<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Allows null keys and multiple null values.<\/li>\n\n\n\n<li>Unordered \u2013 the insertion order is not maintained.<\/li>\n\n\n\n<li>Allows unique keys; duplicate keys overwrite previous values.<\/li>\n\n\n\n<li>Non-thread-safe (use ConcurrentHashMap for thread safety).<\/li>\n<\/ul>\n\n\n\n<p>While <code>HashMap<\/code> is a key-value store, you may also want to learn about <strong><a href=\"https:\/\/www.mygreatlearning.com\/blog\/hashset-in-java\/\">HashSet in Java<\/a><\/strong>, which is a set-based collection in Java that utilizes hashing.<\/p>\n\n\n\n    <div class=\"courses-cta-container\">\n        <div class=\"courses-cta-card\">\n            <div class=\"courses-cta-header\">\n                <div class=\"courses-learn-icon\"><\/div>\n                <span class=\"courses-learn-text\">Academy Pro<\/span>\n            <\/div>\n            <p class=\"courses-cta-title\">\n                <a href=\"https:\/\/www.mygreatlearning.com\/academy\/premium\/master-java-programming\" class=\"courses-cta-title-link\">Java Programming Course<\/a>\n            <\/p>\n            <p class=\"courses-cta-description\">Learn Java the right way! Our course teaches you essential programming skills, from coding basics to complex projects, setting you up for success in the tech industry.<\/p>\n            <div class=\"courses-cta-stats\">\n                <div class=\"courses-stat-item\">\n                    <div class=\"courses-stat-icon courses-user-icon\"><\/div>\n                    <span>16.05 Hrs<\/span>\n                <\/div>\n                <div class=\"courses-stat-item\">\n                    <div class=\"courses-stat-icon courses-star-icon\"><\/div>\n                    <span>3 Projects<\/span>\n                <\/div>\n            <\/div>\n            <a href=\"https:\/\/www.mygreatlearning.com\/academy\/premium\/master-java-programming\" class=\"courses-cta-button\">\n                Learn Java Programming\n                <div class=\"courses-arrow-icon\"><\/div>\n            <\/a>\n        <\/div>\n    <\/div>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"how-hashmap-works-internally\"><strong>How HashMap Works Internally<\/strong><\/h2>\n\n\n\n<p>Internally, HashMap in Java uses an <strong>array of linked lists<\/strong> (known as a bucket array). The key\u2019s <strong>hash code<\/strong> determines which bucket it will be placed into. If multiple keys produce the same hash code (collision), Java resolves it using <strong>separate chaining<\/strong> (linked list) or <strong>red-black tree<\/strong> (Java 8+ for large buckets).<\/p>\n\n\n\n<p><strong>Steps Involved in HashMap Operations:<\/strong><\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li>The hashCode() method generates a unique integer (hash) for a given key.<\/li>\n\n\n\n<li>The hash is used to compute an <strong>index<\/strong> in the internal bucket array.<\/li>\n\n\n\n<li>If the index is empty, a new key-value pair is inserted. If not, Java handles collisions using chaining (linked list or tree structure).<\/li>\n\n\n\n<li>When a key is retrieved, equals() is used to match the exact key.<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"creating-a-hashmap-in-java-syntax-example\"><strong>Creating a HashMap in Java (Syntax &amp; Example)<\/strong><\/h2>\n\n\n\n<p>To create a HashMap, we use the following syntax:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nimport java.util.HashMap;\n\npublic class HashMapExample {\n    public static void main(String&#x5B;] args) {\n        \/\/ Creating a HashMap\n        HashMap&amp;lt;String, Integer&gt; studentMarks = new HashMap&amp;lt;&gt;();\n\n        \/\/ Adding key-value pairs\n        studentMarks.put(&quot;Alice&quot;, 85);\n        studentMarks.put(&quot;Bob&quot;, 92);\n        studentMarks.put(&quot;Charlie&quot;, 78);\n\n        \/\/ Retrieving a value\n        System.out.println(&quot;Alice&#039;s Marks: &quot; + studentMarks.get(&quot;Alice&quot;));\n\n        \/\/ Checking if a key exists\n        System.out.println(&quot;Is Bob in the HashMap? &quot; + studentMarks.containsKey(&quot;Bob&quot;));\n\n        \/\/ Removing an entry\n        studentMarks.remove(&quot;Charlie&quot;);\n\n        \/\/ Printing the final HashMap\n        System.out.println(&quot;Final HashMap: &quot; + studentMarks);\n    }\n}\n<\/pre><\/div>\n\n\n<h2 class=\"wp-block-heading\" id=\"important-hashmap-methods-and-operations\"><strong>Important HashMap Methods and Operations<\/strong><\/h2>\n\n\n\n<p>Here are some essential operations on HashMap:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong>Method<\/strong><\/td><td><strong>Description<\/strong><\/td><td><strong>Example<\/strong><\/td><\/tr><tr><td><code>put(K key, V value)<\/code><\/td><td>Inserts a key-value pair.<\/td><td><code>map.put(\"John\", 25);<\/code><\/td><\/tr><tr><td><code>get(Object key)<\/code><\/td><td>Retrieves a value using a key.<\/td><td><code>map.get(\"John\");<\/code><\/td><\/tr><tr><td><code>remove(Object key)<\/code><\/td><td>Removes a key-value pair.<\/td><td><code>map.remove(\"John\");<\/code><\/td><\/tr><tr><td><code>containsKey(Object key)<\/code><\/td><td>Checks if a key exists.<\/td><td><code>map.containsKey(\"John\");<\/code><\/td><\/tr><tr><td><code>containsValue(Object value)<\/code><\/td><td>Checks if a value exists.<\/td><td><code>map.containsValue(25);<\/code><\/td><\/tr><tr><td><code>size()<\/code><\/td><td>Returns the number of key-value pairs.<\/td><td><code>map.size();<\/code><\/td><\/tr><tr><td><code>isEmpty()<\/code><\/td><td>Checks if the map is empty.<\/td><td><code>map.isEmpty();<\/code><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p><a href=\"https:\/\/www.mygreatlearning.com\/blog\/collection-in-java\/\">Java collections<\/a> like HashMap often work hand-in-hand with other interfaces, such as the <a href=\"https:\/\/www.mygreatlearning.com\/blog\/java-queue-interface\/\">Queue Interface<\/a>, for managing elements in specific orders<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"iterating-over-a-hashmap\"><strong>Iterating Over a HashMap<\/strong><\/h2>\n\n\n\n<p>There are multiple ways to iterate over a HashMap:<\/p>\n\n\n\n<p><strong>1. Using <\/strong><strong>for-each<\/strong><strong> loop (Entry Set)<\/strong><\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nfor (Map.Entry&amp;lt;String, Integer&gt; entry : studentMarks.entrySet()) {\n    System.out.println(entry.getKey() + &quot; : &quot; + entry.getValue());\n}\n<\/pre><\/div>\n\n\n<p><strong>2. Using <\/strong><strong>keySet()<\/strong><strong> (Iterate Over Keys)<\/strong><\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nfor (String key : studentMarks.keySet()) {\n    System.out.println(&quot;Key: &quot; + key);\n}\n<\/pre><\/div>\n\n\n<p><strong>3. Using <\/strong><strong>values()<\/strong><strong> (Iterate Over Values)<\/strong><\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nfor (Integer value : studentMarks.values()) {\n    System.out.println(&quot;Value: &quot; + value);\n}\n<\/pre><\/div>\n\n\n<p><strong>4. Using <\/strong><strong>forEach()<\/strong><strong> (Lambda Expression)<\/strong><\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nstudentMarks.forEach((key, value) -&gt; System.out.println(key + &quot; : &quot; + value));\n<\/pre><\/div>\n\n\n<p>The use of lambda expressions in HashMap operations can be better understood through <a href=\"https:\/\/www.mygreatlearning.com\/blog\/functional-interfaces-and-lambda-expressions-in-java\/\">Functional Interfaces and Lambda Expressions in Java<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"hashmap-vs-other-map-implementations\"><strong>HashMap vs Other Map Implementations<\/strong><\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong>Feature<\/strong><\/td><td><strong>HashMap<\/strong><\/td><td><strong>TreeMap<\/strong><\/td><td><strong>LinkedHashMap<\/strong><\/td><\/tr><tr><td>Ordering<\/td><td>No ordering<\/td><td>Sorted by key<\/td><td>Insertion order maintained<\/td><\/tr><tr><td>Performance<\/td><td>O(1) for get()<\/td><td>O(log n) for get()<\/td><td>O(1) for get()<\/td><\/tr><tr><td>Null Keys<\/td><td>1 allowed<\/td><td>Not allowed<\/td><td>1 allowed<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"performance-and-use-cases\"><strong>Performance and Use Cases<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time Complexity<\/strong>:\n<ul class=\"wp-block-list\">\n<li><strong>Average: <\/strong>O(1) for put(), get(), and remove().<\/li>\n\n\n\n<li><strong>Worst case:<\/strong> O(n) (when all elements fall in the same bucket).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<p><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Use Cases<\/strong>:\n<ul class=\"wp-block-list\">\n<li>Caching (quick lookups).<\/li>\n\n\n\n<li>Counting word frequencies in text analysis.<\/li>\n\n\n\n<li>Implementing LRU (Least Recently Used) Cache.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"common-mistakes-and-best-practices\"><strong>Common Mistakes and Best Practices<\/strong><\/h2>\n\n\n\n<p><strong>Mistakes<\/strong><\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Using mutable keys<\/strong> \u2013 If keys are mutable, their hashCode() may change, making them untraceable.<\/li>\n\n\n\n<li><strong>Not handling null keys\/values properly<\/strong> \u2013 Be cautious when dealing with null keys and values.<\/li>\n\n\n\n<li><strong>Ignoring initial capacity and load factor<\/strong> \u2013 Setting a proper capacity avoids frequent rehashing.<\/li>\n<\/ol>\n\n\n\n<p><strong>Best Practices<\/strong><\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li>Use ConcurrentHashMap in multi-threaded environments.<\/li>\n\n\n\n<li>Choose the right initial capacity to minimize rehashing.<\/li>\n\n\n\n<li>Override equals() and hashCode() correctly for custom objects.<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"frequently-asked-questions\"><strong>Frequently Asked Questions<\/strong><\/h2>\n\n\n\n<p><strong>1. Why is HashMap not thread-safe?<\/strong><\/p>\n\n\n\n<p>HashMap is not synchronized, meaning multiple threads modifying it simultaneously may lead to <strong>race conditions<\/strong> and <strong>data inconsistency<\/strong>. Use <code> ConcurrentHashMap <\/code>for thread-safe operations.<\/p>\n\n\n\n<p>You can learn more about handling concurrency in Java through <strong><a href=\"https:\/\/www.mygreatlearning.com\/blog\/threads-in-java\/\">Threads in Java<\/a><\/strong>.<\/p>\n\n\n\n<p><strong>2. What happens if two keys have the same hash code?<\/strong><\/p>\n\n\n\n<p>If two keys generate the same hash, they are stored in the same bucket using <strong>separate chaining<\/strong> (linked list or tree structure in Java 8+). The <code>equals()<\/code> method then determines the correct key-value pair.<\/p>\n\n\n\n<p><strong>3. What is the default capacity and load factor of HashMap?<\/strong><\/p>\n\n\n\n<p>By default, HashMap has a <strong>capacity of 16<\/strong> and a <strong>load factor of 0.75<\/strong>. This means resizing occurs when 75% of the capacity is filled.<\/p>\n\n\n\n<p><strong>4. Can we store a <\/strong><strong>null<\/strong><strong> key in HashMap?<\/strong><\/p>\n\n\n\n<p>Yes, HashMap allows <strong>one null key<\/strong> and multiple null values. However, Hashtable does not allow null keys or values.<\/p>\n\n\n\n<p><strong>5. How does HashMap resize itself?<\/strong><\/p>\n\n\n\n<p>When the number of elements exceeds <code>capacity * loadFactor <\/code>, the size doubles, and all entries are <strong>rehashed<\/strong> into a new bucket array. This process is costly, so setting an optimal initial capacity is recommended.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>HashMap in Java is a powerful key-value data structure used for fast retrieval and efficient updates. This tutorial covers its internal mechanics, essential operations, iteration methods, and performance considerations, making it a valuable tool for applications like caching, indexing, and frequency counting.<\/p>\n","protected":false},"author":41,"featured_media":27478,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_uag_custom_page_level_css":"","site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"set","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25860],"tags":[36826],"content_type":[],"class_list":["post-26916","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-software","tag-java"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.3 (Yoast SEO v27.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>HashMap in Java<\/title>\n<meta name=\"description\" content=\"Learn about HashMap in Java, from its structure and key operations to performance optimizations and use cases. 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