1. Overview

In this article I will write about converting an Iterator to Collection. Let us begin.

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2. Content

Below are few ways for converting Iterator to a Collection.

2.1. Prior to Java 8

Assume that if you call a method which returns you an Iterator and now you want to use this Iterator to pull data from underlying collection and convert it to a new Collection. Prior to Java 8 we would do something like this:

Iterator<String> iterator = doSomething();
List<String> result = new ArrayList<>();
while(iterator.hasNext()) {
    result.add(iterator.next());
}

2.2. Using forEachRemaining()

Is there a better way to do this task? Yes we can do that. Iterator interface has a new method called forEachRemaining added in Java 8. Let us use that method.

Iterator<String> iterator = doSomething();
List<String> result = new ArrayList<>();
iterator.forEachRemaining(new Consumer<String>() {
    @Override
        public void accept(String element) {
            result.add(element);
    }
});

2.3. Using Lambda Expression

Nice. Can we do better than this? As Consumer interface is a functional interface we can use lambda expression to replace the above code as follows:

Iterator<String> iterator = doSomething();
List<String> result = new ArrayList<>();
iterator.forEachRemaining(element -> result.add(element));

2.4. Using Method Reference

Excellent. Can we still do better? We can replace the lambda expression with method reference.

Iterator<String> iterator = doSomething();
List<String> result = new ArrayList<>();
iterator.forEachRemaining(result::add);

Awesome. So we used a new Java 8 method of Iterator interface to convert Iterator to Collection.

3. Conclusion

In this article, we explored 4 different ways to convert Iterator to Collection.