Java Streams API Cheat Sheet — Quick Reference with Daily Use Examples |

Java Streams API Cheat Sheet — Quick Reference with Daily Use Examples

Java Streams API Cheat Sheet — Quick Reference with Daily Use Examples

Introduction

The Java Streams API allows developers to process collections in a functional and elegant way. Whether it’s filtering, transforming, or collecting results, streams can make your code more readable and concise. This cheat sheet is your quick reference for daily use — packed with simple explanations and ready-to-use code examples.

1️⃣ Creating Streams

// From a collection
List<String> list = List.of("a", "b", "c");
Stream<String> stream = list.stream();

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

// Using Stream.of()
Stream<String> streamOf = Stream.of("one", "two", "three");

2️⃣ Intermediate Operations

Operation	Example	What it does
filter()	`stream.filter(x -> x.startsWith("A"))`	Filters elements based on a condition
map()	`stream.map(String::toUpperCase)`	Transforms each element
flatMap()	`list.stream().flatMap(List::stream)`	Flattens nested collections
distinct()	`stream.distinct()`	Removes duplicates
sorted()	`stream.sorted()`	Sorts elements
limit()	`stream.limit(5)`	Limits the number of elements
skip()	`stream.skip(3)`	Skips the first 3 elements

3️⃣ Terminal Operations

Operation	Example	What it does
collect()	`stream.collect(Collectors.toList())`	Collects elements into a collection
count()	`stream.count()`	Counts elements
forEach()	`stream.forEach(System.out::println)`	Performs an action on each element
reduce()	`stream.reduce("", String::concat)`	Reduces stream to a single value
anyMatch()	`stream.anyMatch(x -> x.length() > 3)`	Checks if any match the condition
allMatch()	`stream.allMatch(x -> x.length() > 1)`	Checks if all match the condition
noneMatch()	`stream.noneMatch(x -> x.isEmpty())`	Checks if none match the condition

4️⃣ Collectors Quick Reference

Operation	Example	Purpose
To List	`collect(Collectors.toList())`	Collect results into a List
Grouping By	`collect(Collectors.groupingBy(Person::getAge))`	Group elements by a property
Joining Strings	`collect(Collectors.joining(", "))`	Join strings with delimiter
Summing	`collect(Collectors.summingInt(Item::getQuantity))`	Sum values of a property

5️⃣ Common Use Cases

Find duplicates:

List<String> duplicates = list.stream()
    .collect(Collectors.groupingBy(e -> e, Collectors.counting()))
    .entrySet().stream()
    .filter(e -> e.getValue() > 1)
    .map(Map.Entry::getKey)
    .collect(Collectors.toList());

Count frequency of elements:

Map<String, Long> frequency = list.stream()
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

Sort by custom property:

list.stream()
    .sorted(Comparator.comparing(String::length))
    .forEach(System.out::println);

Group by first letter:

Map<Character, List<String>> grouped = list.stream()
    .collect(Collectors.groupingBy(s -> s.charAt(0)));

Spring Boot

Cheat Sheet