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Java Stream API

~1 min read · Java
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The Stream API in Java is used for functional-style operations on collections of data. It was introduced in Java 8 and provides a powerful way to process sequences of elements.

Key Features of Stream API

Declarative – Focuses on what to do, not how to do it.

Lazy Evaluation – Operations are performed only when necessary.

Functional Programming – Uses lambda expressions and method references.

Parallel Processing – Improves performance on large data sets.

Non-Mutable – Does not modify the original collection.

Creating Streams:

import java.util.List; import java.util.stream.Stream; public class Main { public static void main(String[] args) { List names = List.of("Alice", "Bob", "Charlie"); // Creating a stream Stream nameStream = names.stream(); nameStream.forEach((item)->{ System.out.println("item -> "+item); }); } }

Stream Operations:

There are two types of operations in Stream API:

  1. Intermediate Operations – Return another Stream (e.g., filter(), map(), sorted()).
  2. Terminal Operations – Produce a result (e.g., forEach(), collect(), reduce()).


Method

Description

filter(Predicate<T>)

Filters elements based on a condition.

collect(Collectors.toList())

Collects results into a list.

map(Function<T, R>)

Transforms elements.

sorted()

Sorts elements.

count()

Counts elements in the stream.

reduce()

Combines all elements into one.

anyMatch() / allMatch()

Checks conditions on elements.

Filtering Elements (filter()) and Collecting Elements (collect()) :

  • Filters elements based on a condition.
  • Converts the Stream into a List, Set, or Map.
import java.util.List; import java.util.stream.Collectors; public class Main { public static void main(String[] args) { List numbers = List.of(10, 15, 20, 25, 30); // Filter even numbers List evenNumbers = numbers.stream() .filter((n) -> { if(n % 2 == 0){ return true; }else{ return false; } }).collect(Collectors.toList()); System.out.println(evenNumbers); } }

Transforming Elements (map()):

Modifies each element in the stream.

import java.util.List; import java.util.stream.Collectors; public class Main { public static void main(String[] args) { List names = List.of("Alice", "Bob", "Charlie"); // Convert names to uppercase List upperCaseNames = names.stream() .map((item)->{ return item.toUpperCase(); }) .collect(Collectors.toList()); System.out.println(upperCaseNames); } }

Sorting Elements (sorted()):

Sorts elements in natural order or custom order.

import java.util.List; public class Main { public static void main(String[] args) { List names = List.of("Charlie", "Alice", "Bob"); // Sort names names.stream().sorted().forEach((item)->{ System.out.println(item); }); } }

Counting Elements (count()):

Counts the number of elements in the stream.

import java.util.List; public class Main { public static void main(String[] args) { List names = List.of("Alice", "Bob", "Charlie"); // Count elements long count = names.stream().count(); System.out.println("Total count: " + count); } }

Reducing Elements (reduce()):

Combines all elements into a single value.

import java.util.List; public class Main { public static void main(String[] args) { List numbers = List.of(1, 2, 3, 4, 5); // Sum all numbers int sum = numbers.stream() .reduce(0, (item,acc)->{ acc += item; return acc; }); System.out.println("Sum: " + sum); } }

Checking Conditions (anyMatch(), allMatch()):

import java.util.List; public class Main { public static void main(String[] args) { List numbers = List.of(10, 20, 30, 40, 50); // Check if any number is greater than 25 boolean anyGreaterThan25 = numbers.stream().anyMatch(n -> n > 25); boolean allGreaterThan25 = numbers.stream().allMatch(n -> n > 25); System.out.println("Any number > 25: " + anyGreaterThan25); System.out.println("All number > 25: " + allGreaterThan25); } }

Parallel Streams for Performance:

Java provides parallel streams for multi-threaded processing.

import java.util.List; public class Main { public static void main(String[] args) { List numbers = List.of(1, 2, 3, 4, 5); // Using parallel stream numbers.parallelStream().forEach(item-> System.out.println(item)); } }

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