Stream Basics (map, filter, forEach)

Syntax

collection.stream().filter(...).map(...).collect(...)

Examples

import java.util.List;

List<String> names = List.of("Fola", "Zain", "Jamal");

names.stream()
     .forEach(name -> System.out.println("Hello, " + name));

import java.util.List;

List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

List<Integer> evens = numbers.stream()
    .filter(n -> n % 2 == 0)
    .toList();

System.out.println(evens);  // [2, 4, 6, 8, 10]

import java.util.List;

List<String> names = List.of("fola", "zain", "jamal");

List<String> capitalized = names.stream()
    .map(String::toUpperCase)
    .toList();

System.out.println(capitalized);  // [FOLA, ZAIN, JAMAL]

import java.util.List;

List<String> words = List.of("apple", "hi", "banana", "ok", "cherry");

List<String> result = words.stream()
    .filter(w -> w.length() > 2)
    .map(String::toUpperCase)
    .sorted()
    .toList();

System.out.println(result);  // [APPLE, BANANA, CHERRY]

Best practices

• Chain intermediate operations (filter, map, sorted) in the order that reduces the data as early as possible for better performance
• Remember streams are lazy - intermediate operations do nothing until a terminal operation (like collect or forEach) is called
• Use method references (String::toUpperCase) instead of an equivalent lambda (s -> s.toUpperCase()) when the lambda just calls one existing method
• Remember a stream can only be consumed once - calling a terminal operation twice on the same stream throws an IllegalStateException

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation