Traverse through a HashMap in Java

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

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

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 a 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.get_Value() return the value of the associated Map.Entry
  Example:

// Java program to traverse through
// a hashmap using iterator

import java.util.*;

class PAI {
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("PrutordotAi", 54);
hm.put("Coding Lab", 80);
hm.put("called prutor.ai", 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.get_Value() + 10);
System.out.println(mapElement.getKey() + " : " + marks);
}
}
}
result:
Created hashmap is{PrutordotAi=54, Coding Lab=80, called prutor.ai=82}
HashMap after adding bonus marks:
PrutordotAi : 64
Coding Lab : 90
called prutor.ai : 92
Using for-each loop:

// Java program for traversing
// through a hashmap using for-each loop

import java.util.*;

class PAI {
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("PrutordotAi", 54);
hm.put("Coding Lab", 80);
hm.put("called prutor.ai", 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.get_Value() + 10);

System.out.println(key + " : " + value);
}
}
}
result:
Created hashmap is{PrutordotAi=54, Coding Lab=80, called prutor.ai=82}
HashMap after adding bonus marks:
PrutordotAi : 64
Coding Lab : 90
called prutor.ai : 92
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.

// Java program for traversing
// through a hashmap using for-each loop

import java.util.*;

class PAI {
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("PrutordotAi", 54);
hm.put("Coding Lab", 80);
hm.put("called prutor.ai", 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)));
}
}
result:
Created hashmap is{PrutordotAi=54, Coding Lab=80, called prutor.ai=82}
HashMap after adding bonus marks:
PrutordotAi : 64
Coding Lab : 90
called prutor.ai : 92