How to Iterate over a Set/HashSet without an Itera

Question

How can I iterate over a Set/HashSet without the following?

Iterator iter = set.iterator();
while (iter.hasNext()) {
    System.out.println(iter.next());
}

Answer 1

Converting your set into an array may also help you for iterating over the elements:

Object[] array = set.toArray();

for(int i=0; i<array.length; i++)
   Object o = array[i];

Answer 2

Enumeration(?):

Enumeration e = new Vector(set).elements();
while (e.hasMoreElements())
    {
        System.out.println(e.nextElement());
    }

Another way (java.util.Collections.enumeration()):

for (Enumeration e1 = Collections.enumeration(set); e1.hasMoreElements();)
    {
        System.out.println(e1.nextElement());
    }

Java 8:

set.forEach(element -> System.out.println(element));

or

set.stream().forEach((elem) -> {
    System.out.println(elem);
});

Answer 3

You can use functional operation for a more neat code

Set<String> set = new HashSet<String>();

set.forEach((s) -> {
     System.out.println(s);
});

Answer 4

You can use an enhanced for loop:

Set<String> set = new HashSet<String>();

//populate set

for (String s : set) {
    System.out.println(s);
}

Or with Java 8:

set.forEach(System.out::println);

Answer 5

There are at least six additional ways to iterate over a set. The following are known to me:

Method 1

// Obsolete Collection
Enumeration e = new Vector(movies).elements();
while (e.hasMoreElements()) {
  System.out.println(e.nextElement());
}

Method 2

for (String movie : movies) {
  System.out.println(movie);
}

Method 3

String[] movieArray = movies.toArray(new String[movies.size()]);
for (int i = 0; i < movieArray.length; i++) {
  System.out.println(movieArray[i]);
}

Method 4

// Supported in Java 8 and above
movies.stream().forEach((movie) -> {
  System.out.println(movie);
});

Method 5

// Supported in Java 8 and above
movies.stream().forEach(movie -> System.out.println(movie));

Method 6

// Supported in Java 8 and above
movies.stream().forEach(System.out::println);

This is the HashSet which I used for my examples:

Set<String> movies = new HashSet<>();
movies.add("Avatar");
movies.add("The Lord of the Rings");
movies.add("Titanic");

Answer 6

Here are few tips on how to iterate a Set along with their performances:

public class IterateSet {

    public static void main(String[] args) {

        //example Set
        Set<String> set = new HashSet<>();

        set.add("Jack");
        set.add("John");
        set.add("Joe");
        set.add("Josh");

        long startTime = System.nanoTime();
        long endTime = System.nanoTime();

        //using iterator
        System.out.println("Using Iterator");
        startTime = System.nanoTime();
        Iterator<String> setIterator = set.iterator();
        while(setIterator.hasNext()){
            System.out.println(setIterator.next());
        }
        endTime = System.nanoTime();
        long durationIterator = (endTime - startTime);


        //using lambda
        System.out.println("Using Lambda");
        startTime = System.nanoTime();
        set.forEach((s) -> System.out.println(s));
        endTime = System.nanoTime();
        long durationLambda = (endTime - startTime);


        //using Stream API
        System.out.println("Using Stream API");
        startTime = System.nanoTime();
        set.stream().forEach((s) -> System.out.println(s));
        endTime = System.nanoTime();
        long durationStreamAPI = (endTime - startTime);


        //using Split Iterator (not recommended)
        System.out.println("Using Split Iterator");
        startTime = System.nanoTime();
        Spliterator<String> splitIterator = set.spliterator();
        splitIterator.forEachRemaining((s) -> System.out.println(s));
        endTime = System.nanoTime();
        long durationSplitIterator = (endTime - startTime);


        //time calculations
        System.out.println("Iterator Duration:" + durationIterator);
        System.out.println("Lamda Duration:" + durationLambda);
        System.out.println("Stream API:" + durationStreamAPI);
        System.out.println("Split Iterator:"+ durationSplitIterator);
    }
}

The code is self explanatory.

The result of the durations are:

Iterator Duration: 495287
Lambda Duration: 50207470
Stream Api:       2427392
Split Iterator:    567294

We can see the Lambda takes the longest while Iterator is the fastest.