Traverse through a HashMap in Java

HashMap is a part of the java collections framework. It internally uses the hashing technique.

This post contains different ways of traversing through a HashMap, which is given below:

1. Using an Iterator: Iterator is an interface in java.util package which is used to iterate through a collection.
   • hm.entrySet() is used to retrieve all the key-value pairs called Map.Entries and stores internally into a set.
   • hm.entrySet().iterator() returns an iterator which acts as a cursor and points at the first element of the set and moves on till the end.
   • hmIterator.hasNext() checks for the next element in the set and returns a boolean
   • hmIterator.next() returns the next element(Map.Entry) from the set.
   • mapElement.getKey() returns the key of the associated Map.Entry
   • mapElement.getValue() return the value of the associated Map.Entry

   Example:

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   // Java program to traverse through // a hashmap using iterator    import java.util.*;    class GfG {     public static void main(String[] args)     {         // Consider the hashmap contains         // student name and their marks         HashMap<String, Integer> hm =                      new HashMap<String, Integer>();            // Adding mappings to HashMap         hm.put("GeeksforGeeks", 54);         hm.put("A computer portal", 80);         hm.put("For geeks", 82);            // Printing the HashMap         System.out.println("Created hashmap is" + hm);            // Getting an iterator         Iterator hmIterator = hm.entrySet().iterator();            // Iterate through the hashmap         // and add some bonus marks for every student         System.out.println("HashMap after adding bonus marks:");            while (hmIterator.hasNext()) {             Map.Entry mapElement = (Map.Entry)hmIterator.next();             int marks = ((int)mapElement.getValue() + 10);             System.out.println(mapElement.getKey() + " : " + marks);         }     } }

   Output:

   Created hashmap is{GeeksforGeeks=54, A computer portal=80, For geeks=82}
   HashMap after adding bonus marks:
   GeeksforGeeks : 64
   A computer portal : 90
   For geeks : 92
   

3. Using for-each loop:filter_none

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   // Java program for traversing // through a hashmap using for-each loop    import java.util.*;    class GfG {     public static void main(String[] args)     {            // Consider the hashmap containing         // student name and their marks         HashMap<String, Integer> hm =                       new HashMap<String, Integer>();            // Adding mappings to HashMap         hm.put("GeeksforGeeks", 54);         hm.put("A computer portal", 80);         hm.put("For geeks", 82);            // Printing the HashMap         System.out.println("Created hashmap is" + hm);            // Loop through the hashmap         System.out.println("HashMap after adding bonus marks:");            // Using for-each loop         for (Map.Entry mapElement : hm.entrySet()) {             String key = (String)mapElement.getKey();                // Add some bonus marks             // to all the students and print it             int value = ((int)mapElement.getValue() + 10);                System.out.println(key + " : " + value);         }     } }

   Output:

   Created hashmap is{GeeksforGeeks=54, A computer portal=80, For geeks=82}
   HashMap after adding bonus marks:
   GeeksforGeeks : 64
   A computer portal : 90
   For geeks : 92
   

4. Using forEach() method: forEach() is a method of hashmap which is introduced in java 8. It is used to iterate through the hashmap and also reduces the number of lines of code.filter_none

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   // Java program for traversing // through a hashmap using for-each loop    import java.util.*;    class GfG {     public static void main(String[] args)     {            // Consider the hashmap containing         // student name and their marks         HashMap<String, Integer> hm =                        new HashMap<String, Integer>();            // Adding mappings to HashMap         hm.put("GeeksforGeeks", 54);         hm.put("A computer portal", 80);         hm.put("For geeks", 82);            // Printing the HashMap         System.out.println("Created hashmap is" + hm);            // Loop through the hashmap         // and add bonus marks         System.out.println("HashMap after adding bonus marks:");            // Using Hashmap.forEach()         hm.forEach((k, v) -> System.out.println(k + " : " + (v + 10)));     } }

   Output:

   Created hashmap is{GeeksforGeeks=54, A computer portal=80, For geeks=82}
   HashMap after adding bonus marks:
   GeeksforGeeks : 64
   A computer portal : 90
   For geeks : 92