Java 8 Streams: How to call once the Collection.stream() method and retrieve an array of several aggregate values with different fields

Question

I\'m starting with the Stream API in Java 8.

Here is my Person object I use:

public class Person {

    private String firstName;
    private String last         


        

Answer 1

Here is the collector

public class PersonStatistics {
    private long firstNameCounter;
    private int maxAge = Integer.MIN_VALUE;
    private double minHeight = Double.MAX_VALUE;
    private double totalWeight;
    private long total;
    private final Predicate firstNameFilter;

    public PersonStatistics(Predicate firstNameFilter) {
        Objects.requireNonNull(firstNameFilter);
        this.firstNameFilter = firstNameFilter;
    }

    public void accept(Person p) {
        if (this.firstNameFilter.test(p)) {
            firstNameCounter++;
        }

        this.maxAge = Math.max(p.getAge(), maxAge);
        this.minHeight = Math.min(p.getHeight(), minHeight);
        this.totalWeight += p.getWeight();
        this.total++;
    }

    public PersonStatistics combine(PersonStatistics personStatistics) {
        this.firstNameCounter += personStatistics.firstNameCounter;
        this.maxAge = Math.max(personStatistics.maxAge, maxAge);
        this.minHeight = Math.min(personStatistics.minHeight, minHeight);
        this.totalWeight += personStatistics.totalWeight;
        this.total += personStatistics.total;

        return this;
    }

    public Object[] toStatArray() {
        return new Object[]{firstNameCounter, maxAge, minHeight, total == 0 ? 0 : totalWeight / total};
    }
}

You can use this collector as follows

public class PersonMain {
    public static void main(String[] args) {
        List personsList = new ArrayList<>();

        personsList.add(new Person("John", "Doe", 25, 180, 80));
        personsList.add(new Person("Jane", "Doe", 30, 169, 60));
        personsList.add(new Person("John", "Smith", 35, 174, 70));
        personsList.add(new Person("John", "T", 45, 179, 99));

        Object[] objects = personsList.stream().collect(Collector.of(
                () -> new PersonStatistics(p -> p.getFirstName().equals("John")),
                PersonStatistics::accept,
                PersonStatistics::combine,
                PersonStatistics::toStatArray));
        System.out.println(Arrays.toString(objects));
    }
}