Using Java 8 Stream API for Collection Processing

Java 8 added the Stream API, a high‑level abstraction that lets developers process collections in a declarative, SQL‑like manner. A stream represents a sequence of elements flowing through a pipeline where intermediate operations (filter, map, sorted, etc.) transform the data and a terminal operation produces a result.

The tutorial uses a simple UserPo POJO (with name and score fields) to illustrate each operation:

filter : selects elements that satisfy a predicate, e.g., list.stream().filter(p -> p.getScore() != null).count();

map : transforms each element, e.g., extracting scores with list.stream().map(UserPo::getScore).collect(Collectors.toList()); or joining names with Collectors.joining(",") .

sorted : orders elements, for example descending by score using .sorted(Comparator.comparing(UserPo::getScore).reversed()) .

forEach : performs an action on each element, such as adding 10 points to every student's score.

collect : aggregates results, demonstrated with groupingBy(UserPo::getScore) to produce a map of scores to student lists, as well as collecting to a list or a comma‑separated string.

statistics : obtains summary statistics (max, min, sum, average) via DoubleSummaryStatistics stats = stream.mapToDouble(UserPo::getScore).summaryStatistics(); .

parallelStream : runs the pipeline in parallel to leverage multiple CPU cores, shown with a parallel count operation.

The article also includes the full source code for the UserPo class and a StreamTest class that creates a list of UserPo objects and applies the above operations, printing intermediate and final results.

public class UserPo {
    private String name;
    private Double score;
    // constructors, getters, setters omitted for brevity
    @Override
    public String toString() {
        return "UserPo{name='" + name + "', score=" + score + "}";
    }
}

public class StreamTest {
    public static void main(String[] args) {
        List
list = new ArrayList<>();
        list.add(new UserPo("小一", 10.0));
        list.add(new UserPo("小五", 50.0));
        // ... more data
        long count = list.stream().filter(p -> p.getScore() != null).count();
        System.out.println("参加考试的学生人数：" + count);
        // other operations: map, sorted, forEach, collect, statistics, parallelStream
    }
}