Understanding Java 8 Stream API with Practical PO Examples

Java 8 introduced the Stream API, which allows declarative processing of collections similar to SQL queries.

The stream treats the source collection as a pipeline where operations such as filter, sorted, map, and collect can be applied.

For developers familiar with Linux pipelines, the concept is analogous to the pipe operator.

Using a PO class UserPo (with fields name and score ), the article demonstrates common stream operations:

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

filter : selects elements that satisfy a predicate, e.g., counting students with a non‑null score.

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

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

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

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

sorted : orders elements, for example by score in descending order.

List
sortedList = 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 to every student's score.

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

collect : aggregates results, demonstrated with grouping by score and converting to lists.

Map
> groupByScore = list.stream()
    .filter(p -> p.getScore() != null)
    .collect(Collectors.groupingBy(UserPo::getScore));

statistics : obtains summary statistics like max, min, sum, and average.

DoubleSummaryStatistics stats = list.stream()
    .filter(p -> p.getScore() != null)
    .mapToDouble(UserPo::getScore)
    .summaryStatistics();

parallelStream : runs stream operations in parallel to improve performance, with a note on its lack of thread‑local context.

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

The article concludes with the full source code of UserPo and a StreamTest class that puts all the above operations together.

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