Iterating over the Elements of a Collection

Using the for-each Pattern

Your simplest choice to iterate over the elements of a collection is to use the for-each pattern.

Collection<String> strings = List.of("one", "two", "three");

for (String element: strings) {
    System.out.println(string);
}

Running this code produces the following result:

one
two
three

This pattern is very efficient, as long as you only need to read the elements of your collection. The Iterator pattern allows to remove some of the elements of your collection while you are iterating over them. If you need to do that, then you want to use the Iterator pattern.

Using an Iterator on a Collection

Iterating over the elements of a collection uses a special object, an instance of the Iterator interface. You can get an Iterator object from any extension of the Collection interface. The iterator() method is defined on the Iterable interface, extended by the Collection interface, and further extended by all the interfaces of the collection hierarchy.

Iterating over the elements of a collection using this object is a two-steps process.

1. First you need to check if there are more elements to be visited with the hasNext() method
2. Then you can advance to the next element with the next() method.

If you call the next() method but there are no more elements in the collection, you will get a NoSuchElementException. Calling hasNext() is not mandatory, it is there to help you to make sure that there is indeed a next element.

Here is the pattern:

Collection<String> strings = List.of("one", "two", "three", "four");
for (Iterator<String> iterator = strings.iterator(); iterator.hasNext();) {
    String element = iterator.next();
    if (element.length() == 3) {
        System.out.println(element);
    }
}

This code produces the following result:

one
two

The Iterator interface has a third method: remove(). Calling this method removes the current element from the collection. There are cases though where this method is not supported, it will throw an UnsupportedOperationException. Quite obviously, calling remove() on an immutable collection cannot work, so this is one of the cases. The implementation of Iterator you get from ArrayList, LinkedList and HashSet all support this remove operation.

Updating a Collection While Iterating over It

If you happen to modify the content of a collection while iterating over it, you may get a ConcurrentModificationException. Getting this exception may be a little confusing, because this exception is also used in concurrent programming. In the context of the Collections Framework, you may get it without touching multithreaded programming.

The following code throws a ConcurrentModificationException.

Collection<String> strings = new ArrayList<>();
strings.add("one");
strings.add("two");

Iterator<String> iterator = strings.iterator();
while (iterator.hasNext()) {

    String element = iterator.next();
    strings.remove(element);
}

If what you need is to remove the elements of a collection that satisfy a given criteria, you may use the removeIf() method.

Implementing the Iterable Interface

Now that you saw what an iterator is in the Collection Framework, you can create a simple implementation of the Iterable interface.

Suppose you need to create a Range class that models a range of integers between two limits. All you need to do is iterate from the first integer to the last one.

You can implement the Iterable interface with a record, a feature introduced in Java SE 16:

record Range(int start, int end) implements Iterable<Integer> {

    @Override
    public Iterator<Integer> iterator() {
        return new Iterator<>() {
            private int index = start;
            
            @Override
            public boolean hasNext() {
                return index < end;
            }

            @Override
            public Integer next() {
                if (index > end) {
                    throw new NoSuchElementException("" + index);
                }
                int currentIndex = index;
                index++;
                return currentIndex;
            }
        };
    }
}

You can do the same with a plain class, in case your application does not support Java SE 16 yet. Note that the code of the implementation of Iterator is exactly the same.

class Range implements Iterable<Integer> {

    private final int start;
    private final int end;
    
    public Range(int start, int end) {
        this.start = start;
        this.end = end;
    }
    
    @Override
    public Iterator<Integer> iterator() {
        return new Iterator<>() {
            private int index = start;
            
            @Override
            public boolean hasNext() {
                return index < end;
            }

            @Override
            public Integer next() {
                if (index > end) {
                    throw new NoSuchElementException("" + index);
                }
                int currentIndex = index;
                index++;
                return currentIndex;
            }
        };
    }
}

In both cases, you can use an instance of Range in a for-each statement, since it implements Iterable:

for (int i : new Range1(0, 5)) {
    System.out.println("i = " + i);
}

Running this code gives you the following result:

i = 0
i = 1
i = 2
i = 3
i = 4