Java Collectors

[CG]jupoulton

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What's a Collector?

A Collector is a Java construct to use in conjunction with streams in order to turn a stream into a whatever-you-like.

Built-in Collectors exist and include:

toList() Everything ends up in a list
toSet() Everything ends up in a set
joining(String s) Everything ends up in a string, separated by s
And many more along the same lines.

Where are they ? in java.util.stream.Collectors;.

Using a built-in Collector

Basic use of a Collector

// {
//toList is a static function which returns a brand new Collector
import static java.util.stream.Collectors.toList;
public class Builtin {
    public static List<Integer> filterOutSmallValues(Integer[] arrayOfInts, int minimumValue) {
        List<Integer> list = null;
        
        // Here is a nice little stream
        Stream<Integer> stream = Stream.of(arrayOfInts).filter(i -> i >= minimumValue);
        // We want to return a list, so we need to COLLECT the values of the stream
        //list = stream.collect(/* A collector goes here. */);
        
        // We return the result as a list
        return list;
    }
}

Writing your own Collector

There are many reasons why you might want to write your own Collector. Improving the readibility of a core part of your code is one of them.

Let's take a look at how to do this with an example.

Let's say you need to convert a string of space-separated integers to a list of Point instances where every two integers are X and Y coordinates.

There are a few methods to @Override when writing a new subclass of Collector. Look for every occurence of Fix me! to solve this exercice.

Writing a toPointList Collector

// {
public class Custom {
    /**
     * @param manyInts
     *            is a String containing pairs of integers such as: "4 5 6 3 2 10"
     * @return a list of Points using each pair of ints as coordinates: Point(x=4, y=5), Point(x=3, y=2), Point(x=2, y=10)
     */
    public static List<Point> coordStringToPoints(String manyInts) {
        return
        //First, let's turn the space-separated integers into a stream
        // we can manipulate
        Stream.of(manyInts.split(" "))
                // Next, we parse each int
                .map(Integer::valueOf)
                // Then we collect the ints
                .collect(toPointList());
    }
    // It's a good idea to use a factory function to return a new Collector
    public static PointListCollector toPointList() {
        return new PointListCollector();
    }
}

Going further

Want to learn more about Collectors now you've toyed with them a little?

Check out the official Java doc.

Check out some more advanced usage of Collectors by browsing StackOverflow

Open Source Your Knowledge: become a Contributor and help others learn. Create New Content

Open Source Your Knowledge, Become a Contributor

What's a Collector?

Using a built-in Collector

Writing your own Collector

Going further

Avoiding Null Anti Patterns

Java Behavior Driven Development

Beginner Java Concepts

Kotlin basics, part 1