Java — Streams

Creating streams

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

// From collection
List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
Stream<String> stream = names.stream();

// From array
Stream<Integer> arrStream = Arrays.stream(new Integer[]{1, 2, 3});

// From values
Stream<String> valStream = Stream.of("a", "b", "c");

// Infinite stream
Stream<Integer> infinite = Stream.iterate(0, n -> n + 2);

Intermediate operations

List<String> names = List.of("Alice", "Bob", "Charlie", "David");

// filter
names.stream().filter(n -> n.length() > 3).forEach(System.out::println);

// map
names.stream().map(String::toUpperCase).forEach(System.out::println);

// sorted
names.stream().sorted().forEach(System.out::println);

// distinct
Stream.of(1, 1, 2, 3, 3).distinct().forEach(System.out::println);

Terminal operations

List<Integer> nums = List.of(1, 2, 3, 4, 5);

// collect
List<Integer> evens = nums.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());

// reduce
int sum = nums.stream().reduce(0, Integer::sum);

// forEach
nums.stream().forEach(System.out::println);

// count
long count = nums.stream().filter(n -> n > 3).count();

// anyMatch, allMatch, noneMatch
boolean hasEven = nums.stream().anyMatch(n -> n % 2 == 0);

// findFirst
Optional<Integer> first = nums.stream().findFirst();

Collectors

import java.util.stream.Collectors;

// Joining
String joined = names.stream().collect(Collectors.joining(", "));

// Grouping
Map<Integer, List<String>> byLength = names.stream()
    .collect(Collectors.groupingBy(String::length));

// Partitioning
Map<Boolean, List<Integer>> partitioned = nums.stream()
    .collect(Collectors.partitioningBy(n -> n % 2 == 0));

Mini Practice

Write Java code that:

1. Creates a stream from a list
2. Chains filter, map, and reduce
3. Uses Collectors to join strings
4. Groups elements by a property

Up Next

In the next lesson, you'll learn about File Handling — reading and writing files.