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Different ways to iterate HashMap in Java

How to Iterate hashmap in Java

In this article, we are going to see how we can iterate hashmap in java. Let's have a look at different ways of iteration.
The below created HashMap is going to be iterate in this post: 
// Map to store Roll no. of students with their name
Map<Integer, String> hashMap = new HashMap<>();
hashMap.put(101, "Shubham");
hashMap.put(102, "Jatin");
hashMap.put(103, "Nikhil");
hashMap.put(104, "Priyanka");
hashMap.put(105, "Kajal");

How to iterate HashMap in Java using forEach() method

In this section, we see forEach() method to iterate hashmap in java, java provides forEach() method which takes an argument of BiConsumer, a BiConsumer is a consumer which takes 2 arguments and consume them. 
(key, value) -> System.out.println(key + ":" + value)
When we invoked the forEach() method onto a map, then the BiConsumer provided as the argument will be called and execute for each and every entry or key-value pair stored in the hashmap. For example: 
hashMap.forEach(
    (key, value) -> System.out.println(key + ":" + value)
);

Collections within HashMap in Java

To iterate hashmap in java, we can also fetch a collection view from the hashmap.A Java Hashmap provides 3 collections over a hashmap namely Entry Set, Key Set and Value Set. Let see how to iterate hashmap in java using each of the collection views one-by-one.

How to Iterate HashMap in Java using Entry Set

To iterate hashmap in java using Entry Set, we first need to understand Map.Entry<K,V> interface of java.util package. An instance or object of this interface shows a key-value pair stored in a hashmap. By using entrySet() method of hashmap, we can fetch the Entry Set i.e., set of all entries or key-value pairs and iterate using the entry set. 
Set<Entry<Integer, String>> entrySet = hashMap.entrySet();
Using forEach() method : 
entrySet.forEach(
     entry -> System.out.println(entry.getKey() + ":" + entry.getValue())
);
Using for-Each loop : 
for (Entry<Integer, String> entry : entrySet) {
     System.out.println(entry.getKey() + ":" + entry.getValue());
}
Using iterators : 
Iterator<Entry<Integer, String>> iterator = entrySet.iterator();
while (iterator.hasNext()) {
     Entry<Integer, String> entry = iterator.next();
     System.out.println(entry.getKey() + ":" + entry.getValue());
}

How to Iterate HashMap in Java using Key Set

In this section, we see the approach of iterate hashmap in java using Key Set. The keySet() method is used to fetch the Key Set i.e, the set of all keys stored in hashmap. Then we can use keys to iterate over the hashmap. 
Set<Integer> keySet = hashMap.keySet();
Using forEach() method : 
keySet.forEach(
     key -> System.out.println(key +":"+hashMap.get(key))
);
Using for-Each loop : 
for(Integer key : keySet) {
     System.out.println(key +":"+hashMap.get(key));   
}
Using iterators : 
Iterator<Integer> iterator = keySet.iterator();
while(iterator.hasNext()) {
     Integer key = iterator.next();
     System.out.println(key +":"+hashMap.get(key));
}

How to Iterate HashMap in Java using Value Set

In this section, we see the approach of iterate hashmap in java using Value Set, this approach can be used where we only need to use the values of the hashmap not the entire map with keys and values. The values() method is used to fetch the Value Set i.e, the list of all values stored in the hashmap. 
Collection<String> values = hashMap.values();
Using forEach() method : 
values.forEach(value -> System.out.println(value));
// Or By Method Reference
values.forEach(System.out::println);
Using for-Each loop : 
for (String value : values) {
     System.out.println(value);
}
Using iterators : 
Iterator<String> iterator = values.iterator();
while (iterator.hasNext()) {
     String value = iterator.next();
     System.out.println(value);
}

So, these are some methods or approaches represents how we can iterate hashmap in java. Of-course there are also exists a lot of other methods for iteration, but these are the few that are easy to use and comes in usual coding practices.

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