3 ways to loop over Set or HashSet in Java? Examples

Since Set interface or HashSet class doesn't provide a get() method to retrieve elements, the only way to take out elements from a Set is to iterate over it by using Iterator, or loop over Set using advanced for loop of Java 5. You can get the iterator by calling the iterator() method of Set interface. This method returns an iterator over the elements in the sets but they are returned in no particular order, as Set doesn't guarantee any order. Though individual Set implementations e.g. LinkedHashSet or TreeSet can impose ordering and in such iterator will return elements on that order. You can only traverse in one direction using iterator i.e. from first to last elements, there is no backward traversing allowed as was the case with List interface and ListIterator. Similarly, if you use advanced for loop, then also you can traverse in only one direction.

In this tutorial, you will learn both ways to traverse over Set or HashSet in Java i.e. by using both Iterator and enhanced for loop. Btw, Java 8 also introduced a new to loop over set, that is by using the forEach() method if you are lucky to be running on Java 8 then you can use that as well.


Iterating over Set using Iterator


Here are the steps to traverse over as Set using Iterator in Java:
  1. Obtain the iterator by calling the iterator() method.
  2. You can use while or for loop along with hasNext(), which return true if there are more elements in the Set.
  3. Call the next() method to obtain the next elements from Set.

Iterator<String> itr = setOfStocks.iterator();

// traversing over HashSet
System.out.println("Traversing over Set using Iterator");
while(itr.hasNext()){
  System.out.println(itr.next());
}



Loop over HashSet using for loop

There are no particular steps to loop over Set using enhanced for loop, as you just need to use the Set as per loop construct as shown below:

for(String stock : setOfStocks){
   System.out.println(stock);
}

This will print all the stocks from the setOfStocks into the console.


Traversing over HashSet using forEach() method

This method is only available from Java 8 onwards. This forEach() method belongs to Iterable interface and since Set implements that you can use this to traverse over Set as shown below:

public class HashSetLooperJava8{

    public static void main(String args[]) {

        // Creating and initializing an HashSet for iteration
        Set<String> setOfBooks = new HashSet<>();
        setOfBooks.add("Effective Java");
        setOfBooks.add("Clean Code");
        setOfBooks.add("Refactoring");
        setOfBooks.add("Head First Java");
        setOfBooks.add("Clean Coder");
        
        // iterating over HashSet using forEach() method in Java 8
        setOfBooks.forEach(System.out::println);

    }

}

Output:
Clean Code
Effective Java
Head First Java
Clean Coder
Refactoring

There is another forEach() method defined in the Stream class from new JDK 8 Stream API, See 10 ways to use forEach in Java 8 for more examples.


Here is the summary of all three ways to iterate over HashSet or any Set in Java:
3 example to iterate over HashSet in Java


Java Program to loop over HashSet in Java

Here is a sample program to show you how you can loop over a Set in Java using both Iterator and advanced for loop. Only difference in two ways is that you can remove elements while iterating using Iterator's remove() method but you cannot remove elements while looping over HashSet using enhanced for loop, even though Set provides a remove() method. Doing so will result in ConcurrentModificationException in Java.

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class HashSetDemo {

    public static void main(String args[]) {

        // Creating and initializing an HashSet for iteration
        Set<String> setOfStocks = new HashSet<>();
        setOfStocks.add("INFY");
        setOfStocks.add("BABA");
        setOfStocks.add("GOOG");
        setOfStocks.add("MSFT");
        setOfStocks.add("AMZN");

        System.out.println("Set: " + setOfStocks);

        // Example 1 - iterating over HashSet using Iterator
        // Obtaining the Iterator
        Iterator<String> itr = setOfStocks.iterator();

        // traversing over HashSet
        System.out.println("Traversing over Set using Iterator");
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }

        // Example 2 - looping over HashSet using for loop
        System.out.println("Looping over HashSet using advanced for loop");
        for (String stock : setOfStocks) {
            System.out.println(stock);
        }
    }
}

Output:
Set: [AMZN, INFY, MSFT, GOOG, BABA]
Traversing over Set using Iterator
AMZN
INFY
MSFT
GOOG
BABA
Looping over HashSet using advanced for loop
AMZN
INFY
MSFT
GOOG
BABA


That's all about how to traverse over HashSet or Set in Java. You can use the same technique to loop over other Set implementations like LinkedHashSet, TreeSet, EnumSet, CopyOnWriteHashSet or ConcurrentSkipListSet in Java.

All these implementations provide the iterator() method which returns the elements in the order this class individually guarantee e.g. LinkedHashSet guarantees insertion order and TreeSet guarantees the sorted order imposed by Comparable or Comparator provided while creating TreeSet instance.

You can choose between Iterator and advanced for loop to traverse over Set depending upon whether you want to remove the elements during iterator or not. The iterator allows you to remove elements but enhanced for loop doesn't allow that.

Further Reading
Head First Java, 2nd Edition
Core Java, Volume 1 9th Edition



2 comments :

Anonymous said...

which one should one prefer?

Javin Paul said...

@Anonymous, use Iterator if you want to remove elements, otherwise enhanced for loop looks more readable. Use forEach() method if you are writing code in Java 8.

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