Java 8 Stream API Tutorial with POJO Examples

Java 8 added a new abstraction called Stream, allowing data to be processed in a declarative, SQL‑like manner; streams treat collections as pipelines where operations like filter, sort, and map can be chained.

The tutorial defines a simple POJO

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

to illustrate real‑world usage instead of plain strings.

filter : Demonstrates counting students whose score is not null with

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

.

map : Shows extracting a list of scores and joining names into a comma‑separated string using

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

and

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

.

sorted : Sorts the filtered list by score in descending order via

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

.

forEach : Applies a custom action to each element, e.g., adding 10 points to every student's score with

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

. A note warns that only forEach mutates the original collection.

collect : Uses Collectors.groupingBy to group students by score, returns a list of scores, or joins names into a string, illustrating various collection results.

statistics : Computes summary statistics (max, min, sum, average) on scores via

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

.

parallelStream : Shows parallel processing with

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

, noting the need for careful evaluation of thread‑safety.

The article concludes with the complete source code for UserPo and a StreamTest class containing imports, data setup, and all demonstrated stream operations inside the main method.