Using Java 8 Stream API for Data Processing with PO Objects

Java 8 added the Stream API, providing a declarative way to process collections similar to SQL queries. It treats a collection as a flow of elements that can be filtered, sorted, mapped, and aggregated through a pipeline of operations.

The article uses a UserPo class with name and score fields to illustrate each Stream operation.

PO Code

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

filter

Filters elements that satisfy a predicate; for example, counting students whose score is not null.

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

map

Transforms each element to another form, such as extracting all scores or joining all names.

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

String nameString = list.stream()
    .map(p -> p.getName())
    .collect(Collectors.joining(","));

sorted

Sorts the stream; the example sorts students by score in descending order.

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

forEach

Applies an action to each element; here it adds 10 points to every student's score.

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

collect

Aggregates elements, e.g., grouping students by score.

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

statistics

Computes summary statistics such as max, min, sum, and average.

DoubleSummaryStatistics stats = filteredList.stream()
    .mapToDouble(p -> p.getScore())
    .summaryStatistics();

parallelStream

Uses multiple threads to process the stream for better performance, with the same filter example.

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

Full Example

The complete runnable example combines all the above operations, demonstrating how to create a list of UserPo objects, apply filters, mappings, sorting, grouping, statistics, and parallel processing.

package com.cmx.tcn.stream;

import java.util.*;
import java.util.stream.Collectors;

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));
        // ... more data ...
        long count = list.stream().filter(p -> null != p.getScore()).count();
        System.out.println("参加考试的学生人数：" + count);
        // other operations as described above
    }
}

The article concludes with a reminder that most Stream operations (except forEach when it mutates data) do not modify the original collection.