Using Java 8 Stream API for Collection Processing – Concepts and Code Examples

Java 8 introduced the Stream API, providing a declarative way to process collections similar to SQL queries.

The Stream pipeline treats a collection as a flow of elements that can be filtered, sorted, mapped, and collected.

Example PO class UserPo is used to demonstrate operations.

filter – selects elements meeting a predicate, e.g., counting non‑null scores.

long count = list.stream()
    .filter(p -> p.getScore() != null)
    .count();

map – transforms each element, e.g., extracting scores or joining names.

List<Double> scoreList = list.stream()
    .map(UserPo::getScore)
    .collect(Collectors.toList());

String nameString = list.stream()
    .map(UserPo::getName)
    .collect(Collectors.joining(","));

sorted – orders elements, e.g., descending by score.

List<UserPo> sorted = list.stream()
    .filter(p -> p.getScore() != null)
    .sorted(Comparator.comparing(UserPo::getScore).reversed())
    .collect(Collectors.toList());

forEach – performs an action on each element, such as adding 10 points.

sorted.forEach(p -> p.setScore(p.getScore() + 10));

collect – aggregates results, demonstrated with grouping by score and converting to lists or strings.

Map<Double, List<UserPo>> byScore = list.stream()
    .filter(p -> p.getScore() != null)
    .collect(Collectors.groupingBy(UserPo::getScore));

statistics – obtains summary statistics of numeric streams.

DoubleSummaryStatistics stats = sorted.stream()
    .mapToDouble(UserPo::getScore)
    .summaryStatistics();

parallelStream – runs the pipeline in parallel to improve performance, with the same operations as above.

long parallelCount = list.parallelStream()
    .filter(p -> p.getScore() != null)
    .count();

Full example code for UserPo class and a main method ( StreamTest) is provided, showing creation of sample data and applying the described Stream operations.

public class UserPo {
    private String name;
    private Double score;
    // constructors, getters, setters, toString...
}

public class StreamTest {
    public static void main(String[] args) {
        List<UserPo> list = new ArrayList<>();
        list.add(new UserPo("小一", 10d));
        list.add(new UserPo("小五", 50d));
        list.add(new UserPo("小六", 60d));
        list.add(new UserPo("小6", 60d));
        list.add(new UserPo("小空", null));
        list.add(new UserPo("小九", 90d));
        // apply stream operations as shown above
    }
}