{"id":1272,"date":"2022-02-26T07:26:27","date_gmt":"2022-02-26T07:26:27","guid":{"rendered":"https:\/\/mdr.foobrdigital.com\/?p=1272"},"modified":"2022-02-26T07:26:27","modified_gmt":"2022-02-26T07:26:27","slug":"hash-map","status":"publish","type":"post","link":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/2022\/02\/26\/hash-map\/","title":{"rendered":"Hash Map"},"content":{"rendered":"\n

In this tutorial, we will learn about the Java HashMap class and its various operations with the help of examples.<\/p>\n\n\n\n

The\u00a0HashMap<\/code>\u00a0class of the Java collections framework provides the functionality of the\u00a0hash table data structure.<\/p>\n\n\n\n

It stores elements in key\/value<\/strong> pairs. Here, keys<\/strong> are unique identifiers used to associate each value<\/strong> on a map.<\/p>\n\n\n\n

The HashMap<\/code> class implements the Map<\/a> interface.<\/p>\n\n\n\n

\"Java
Java HashMap Implementation<\/figcaption><\/figure>\n\n\n\n
\n\n\n\n

Create a HashMap<\/h2>\n\n\n\n

In order to create a hash map, we must import the java.util.HashMap<\/code> package first. Once we import the package, here is how we can create hashmaps in Java.<\/p>\n\n\n\n

\/\/ hashMap creation with 8 capacity and 0.6 load factor\nHashMap<K, V> numbers = new HashMap<>();<\/code><\/pre>\n\n\n\n
In the above code, we have created a hashmap named numbers<\/var>. Here, K<\/strong> represents the key type and V<\/strong> represents the type of values. For example,<\/p>\n\n\n\n
HashMap<String, Integer> numbers = new HashMap<>();<\/code><\/pre>\n\n\n\n
Here, the type of keys<\/strong> is String<\/code> and the type of values<\/strong> is Integer<\/code>.<\/p>\n\n\n\n
\n\n\n\n
Example 1: Create HashMap in Java<\/h3>\n\n\n\n
import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    \/\/ create a hashmap\n    HashMap<String, Integer> languages = new HashMap<>();\n\n    \/\/ add elements to hashmap\n    languages.put(\"Java\", 8);\n    languages.put(\"JavaScript\", 1);\n    languages.put(\"Python\", 3);\n    System.out.println(\"HashMap: \" + languages);\n  }\n}<\/code><\/pre>\n\n\n\n
Output<\/strong><\/p>\n\n\n\n
HashMap: {Java=8, JavaScript=1, Python=3}<\/samp><\/code><\/pre>\n\n\n\n
In the above example, we have created a HashMap<\/code> named languages<\/var>.<\/p>\n\n\n\n
Here, we have used the put()<\/code> method to add elements to the hashmap. We will learn more about the put()<\/code> method later in this tutorial.<\/p>\n\n\n\n
\n\n\n\n
Basic Operations on Java HashMap<\/h2>\n\n\n\n
The HashMap<\/code> class provides various methods to perform different operations on hashmaps. We will look at some commonly used arraylist operations in this tutorial:<\/p>\n\n\n\n
  • Add elements<\/li>
  • Access elements<\/li>
  • Change elements<\/li>
  • Remove elements<\/li><\/ul>\n\n\n\n
    \n\n\n\n
    1. Add elements to a HashMap<\/h3>\n\n\n\n
    To add a single element to the hashmap, we use the put()<\/code> method of the HashMap<\/code> class. For example,<\/p>\n\n\n\n
    import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    \/\/ create a hashmap\n    HashMap<String, Integer> numbers = new HashMap<>();\n\n    System.out.println(\"Initial HashMap: \" + numbers);\n    \/\/ put() method to add elements\n    numbers.put(\"One\", 1);\n    numbers.put(\"Two\", 2);\n    numbers.put(\"Three\", 3);\n    System.out.println(\"HashMap after put(): \" + numbers);\n  }\n}<\/code><\/pre>\n\n\n\n
    Output<\/strong><\/p>\n\n\n\n
    Initial HashMap: {}\nHashMap after put(): {One=1, Two=2, Three=3}<\/samp><\/code><\/pre>\n\n\n\n
    In the above example, we have created a HashMap<\/code> named numbers<\/var>. Here, we have used the put()<\/code> method to add elements to numbers.<\/p>\n\n\n\n
    Notice the statement,<\/p>\n\n\n\n
    numbers.put(\"One\", 1);<\/code><\/pre>\n\n\n\n
    Here, we are passing the String<\/code> value One<\/var> as the key and Integer<\/code> value 1<\/var> as the value to the put()<\/code> method.<\/p>\n\n\n\n
    Recommended Readings<\/strong><\/p>\n\n\n\n
    • Java HashMap put()<\/li>
    • Java HashMap putAll()<\/li>
    • Java HashMap putIfAbsent()<\/li><\/ul>\n\n\n\n
      \n\n\n\n
      2. Access HashMap Elements<\/h3>\n\n\n\n
      We can use the get()<\/code> method to access the value from the hashmap. For example,<\/p>\n\n\n\n
      import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    HashMap<Integer, String> languages = new HashMap<>();\n    languages.put(1, \"Java\");\n    languages.put(2, \"Python\");\n    languages.put(3, \"JavaScript\");\n    System.out.println(\"HashMap: \" + languages);\n\n    \/\/ get() method to get value\n    String value = languages.get(1);\n    System.out.println(\"Value at index 1: \" + value);\n  }\n}<\/code><\/pre>\n\n\n\n
      Output<\/strong><\/p>\n\n\n\n
      HashMap: {1=Java, 2=Python, 3=JavaScript}\nValue at index 1: Java<\/samp><\/code><\/pre>\n\n\n\n
      In the above example, notice the expression,<\/p>\n\n\n\n
      languages.get(1);<\/code><\/pre>\n\n\n\n
      Here, the get()<\/code> method takes the key<\/strong> as its argument and returns the corresponding value<\/strong> associated with the key.<\/p>\n\n\n\n
      We can also access the keys<\/strong>, values<\/strong>, and key\/value<\/strong> pairs of the hashmap as set views using keySet()<\/code>, values()<\/code>, and entrySet()<\/code> methods respectively. For example,<\/p>\n\n\n\n
      import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n    HashMap<Integer, String> languages = new HashMap<>();\n\n    languages.put(1, \"Java\");\n    languages.put(2, \"Python\");\n    languages.put(3, \"JavaScript\");\n    System.out.println(\"HashMap: \" + languages);\n\n    \/\/ return set view of keys\n    \/\/ using keySet()\n    System.out.println(\"Keys: \" + languages.keySet());\n\n    \/\/ return set view of values\n    \/\/ using values()\n    System.out.println(\"Values: \" + languages.values());\n\n    \/\/ return set view of key\/value pairs\n    \/\/ using entrySet()\n    System.out.println(\"Key\/Value mappings: \" + languages.entrySet());\n  }\n}<\/code><\/pre>\n\n\n\n
      Output<\/strong><\/p>\n\n\n\n
      HashMap: {1=Java, 2=Python, 3=JavaScript}\nKeys: [1, 2, 3]\nValues: [Java, Python, JavaScript]\nKey\/Value mappings: [1=Java, 2=Python, 3=JavaScript]<\/samp><\/code><\/pre>\n\n\n\n
      In the above example, we have created a hashmap named languages<\/var>. Here, we are accessing the keys<\/strong>, values<\/strong>, and key\/value<\/strong> mappings from the hashmap.<\/p>\n\n\n\n
      Recommended Readings<\/strong><\/p>\n\n\n\n
      • Java HashMap get()<\/li>
      • Java Hashmap getOrDefault()<\/li>
      • Java HashMap keySet()<\/li>
      • Java HashMap values()<\/li>
      • Java HashMap entrySet()<\/li><\/ul>\n\n\n\n
        \n\n\n\n
        3. Change HashMap Value<\/h3>\n\n\n\n
        We can use the replace()<\/code> method to change the value associated with a key in a hashmap. For example,<\/p>\n\n\n\n
        import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    HashMap<Integer, String> languages = new HashMap<>();\n    languages.put(1, \"Java\");\n    languages.put(2, \"Python\");\n    languages.put(3, \"JavaScript\");\n    System.out.println(\"Original HashMap: \" + languages);\n\n    \/\/ change element with key 2\n    languages.replace(2, \"C++\");\n    System.out.println(\"HashMap using replace(): \" + languages);\n  }\n}<\/code><\/pre>\n\n\n\n
        Output<\/strong><\/p>\n\n\n\n
        Original HashMap: {1=Java, 2=Python, 3=JavaScript}\nHashMap using replace(): {1=Java, 2=C++, 3=JavaScript}<\/samp><\/code><\/pre>\n\n\n\n
        In the above example, we have created a hashmap named languages<\/var>. Notice the expression,<\/p>\n\n\n\n
        languages.replace(2, \"C++\");<\/code><\/pre>\n\n\n\n
        Here, we are changing the value referred to by key 2<\/strong> with the new value C++<\/var>.<\/p>\n\n\n\n
        The HashMap<\/code> class also provides some variations of the replace()<\/code> method. To learn more, visit<\/p>\n\n\n\n
        • Java HashMap replace()<\/li>
        • Java HashMap replaceAll()<\/li><\/ul>\n\n\n\n
          \n\n\n\n
          4. Remove HashMap Elements<\/h3>\n\n\n\n
          To remove elements from a hashmap, we can use the remove() method. For example,<\/p>\n\n\n\n
          import java.util.HashMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    HashMap<Integer, String> languages = new HashMap<>();\n    languages.put(1, \"Java\");\n    languages.put(2, \"Python\");\n    languages.put(3, \"JavaScript\");\n    System.out.println(\"HashMap: \" + languages);\n\n    \/\/ remove element associated with key 2\n    String value = languages.remove(2);\n    System.out.println(\"Removed value: \" + value);\n\n    System.out.println(\"Updated HashMap: \" + languages);\n  }\n}<\/code><\/pre>\n\n\n\n
          Output<\/strong><\/p>\n\n\n\n
          HashMap: {1=Java, 2=Python, 3=JavaScript}\nRemoved value: Python\nUpdated HashMap: {1=Java, 3=JavaScript}<\/samp><\/code><\/pre>\n\n\n\n
          Here, the remove()<\/code> method takes the key<\/strong> as its parameter. It then returns the value<\/strong> associated with the key<\/strong> and removes the entry<\/strong>.<\/p>\n\n\n\n
          We can also remove the entry only under certain conditions. For example,<\/p>\n\n\n\n
          remove(2, \"C++\");<\/code><\/pre>\n\n\n\n
          Here, the remove()<\/code> method only removes the entry<\/strong> if the key 2<\/strong> is associated with the value C++<\/strong>. Since 2<\/strong> is not associated with C++<\/strong>, it doesn’t remove the entry.<\/p>\n\n\n\n
          <\/p>\n\n\n\n
          \n\n\n\n
          Other Methods of HashMap<\/h2>\n\n\n\n
          Method<\/th>Description<\/th><\/tr>
          clear()<\/a><\/td>removes all mappings from the HashMap<\/code><\/td><\/tr>
          compute()<\/a><\/td>computes a new value for the specified key<\/td><\/tr>
          computeIfAbsent()<\/a><\/td>computes value if a mapping for the key is not present<\/td><\/tr>
          computeIfPresent()<\/a><\/td>computes a value for mapping if the key is present<\/td><\/tr>
          merge()<\/a><\/td>merges the specified mapping to the HashMap<\/code><\/td><\/tr>
          clone()<\/a><\/td>makes the copy of the HashMap<\/code><\/td><\/tr>
          containsKey()<\/a><\/td>checks if the specified key is present in Hashmap<\/td><\/tr>
          containsValue()<\/a><\/td>checks if Hashmap<\/code> contains the specified value<\/td><\/tr>
          size()<\/a><\/td>returns the number of items in HashMap<\/code><\/td><\/tr>
          isEmpty()<\/a><\/td>checks if the Hashmap<\/code> is empty<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
          \n\n\n\n
          Iterate through a HashMap<\/h2>\n\n\n\n
          To iterate through each entry of the hashmap, we can use\u00a0Java for-each loop. We can iterate through\u00a0keys only<\/strong>,\u00a0vales only<\/strong>, and\u00a0key\/value mapping<\/strong>. For example,<\/p>\n\n\n\n
          import java.util.HashMap;\nimport java.util.Map.Entry;\n\nclass Main {\n  public static void main(String[] args) {\n\n    \/\/ create a HashMap\n    HashMap<Integer, String> languages = new HashMap<>();\n    languages.put(1, \"Java\");\n    languages.put(2, \"Python\");\n    languages.put(3, \"JavaScript\");\n    System.out.println(\"HashMap: \" + languages);\n\n    \/\/ iterate through keys only\n    System.out.print(\"Keys: \");\n    for (Integer key : languages.keySet()) {\n      System.out.print(key);\n      System.out.print(\", \");\n    }\n\n    \/\/ iterate through values only\n    System.out.print(\"\\nValues: \");\n    for (String value : languages.values()) {\n      System.out.print(value);\n      System.out.print(\", \");\n    }\n    \n    \/\/ iterate through key\/value entries\n    System.out.print(\"\\nEntries: \");\n    for (Entry<Integer, String> entry : languages.entrySet()) {\n      System.out.print(entry);\n      System.out.print(\", \");\n    }\n  }\n}<\/code><\/pre>\n\n\n\n
          Output<\/strong><\/p>\n\n\n\n
          HashMap: {1=Java, 2=Python, 3=JavaScript}\nKeys: 1, 2, 3,\nValues: Java, Python, JavaScript,        \nEntries: 1=Java, 2=Python, 3=JavaScript,<\/samp><\/code><\/pre>\n\n\n\n
          Note that we have used the Map.Entry<\/code> in the above example. It is the nested class of the Map<\/code> interface that returns a view (elements) of the map.<\/p>\n\n\n\n
          We first need to import the java.util.Map.Entry<\/code> package in order to use this class.<\/p>\n\n\n\n
          This nested class returns a view (elements) of the map.<\/p>\n\n\n\n
          \n\n\n\n
          Creating HashMap from Other Maps<\/h2>\n\n\n\n
          In Java, we can also create a hashmap from other maps. For example,<\/p>\n\n\n\n
          import java.util.HashMap;\nimport java.util.TreeMap;\n\nclass Main {\n  public static void main(String[] args) {\n\n    \/\/ create a treemap\n    TreeMap<String, Integer> evenNumbers = new TreeMap<>();\n    evenNumbers.put(\"Two\", 2);\n    evenNumbers.put(\"Four\", 4);\n    System.out.println(\"TreeMap: \" + evenNumbers);\n\n    \/\/ create hashmap from the treemap\n    HashMap<String, Integer> numbers = new HashMap<>(evenNumbers);\n    numbers.put(\"Three\", 3);\n    System.out.println(\"HashMap: \" + numbers);\n  }\n}<\/code><\/pre>\n\n\n\n
          Output<\/strong><\/p>\n\n\n\n
          TreeMap: {Four=4, Two=2}\nHashMap: {Two=2, Three=3, Four=4}<\/samp><\/code><\/pre>\n\n\n\n
          In the above example, we have created a TreeMap<\/code> named evenNumbers<\/code>. Notice the expression,<\/p>\n\n\n\n
          numbers = new HashMap<>(evenNumbers)<\/code><\/pre>\n\n\n\n
          Here, we are creating a\u00a0HashMap<\/code>\u00a0named numbers using the\u00a0TreeMap<\/code>. <\/p>\n\n\n\n
          Note<\/strong>: While creating a hashmap, we can include optional parameters: capacity<\/strong> and load factor<\/strong>. For example,<\/p>\n\n\n\n
          HashMap<K, V> numbers = new HashMap<>(8, 0.6f);<\/code><\/pre>\n\n\n\n
          Here,<\/p>\n\n\n\n
          • 8<\/strong> (capacity is 8) – This means it can store 8 entries.<\/li>
          • 0.6f<\/strong> (load factor is 0.6) – This means whenever our hash table is filled by 60%, the entries are moved to a new hash table double the size of the original hash table.<\/li><\/ul>\n\n\n\n
            If the optional parameters not used, then the default capacity<\/strong> will be 16<\/strong> and the default load factor<\/strong> will be 0.75<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":"
            In this tutorial, we will learn about the Java HashMap class and its various operations with the help of examples. The\u00a0HashMap\u00a0class of the Java collections framework provides the functionality of the\u00a0hash table data structure. It stores elements in key\/value pairs. Here, keys are unique identifiers used to associate each value on a map. The HashMap class implements the Map interface. Create a HashMap In order […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[398],"tags":[],"_links":{"self":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1272"}],"collection":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/comments?post=1272"}],"version-history":[{"count":0,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1272\/revisions"}],"wp:attachment":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/media?parent=1272"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/categories?post=1272"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/tags?post=1272"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}