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Java 8 Features > Stream API > Stream filter(), map() and reduce()

Stream filter(), map() and reduce()

Author: Venkata Sudhakar

Java 8 Streams provide a functional approach to processing collections. The three most commonly used operations are filter() for selecting elements, map() for transforming elements, and reduce() for aggregating elements into a single result.

Operation Overview:

1. filter(Predicate) - Keeps only elements matching the predicate.

2. map(Function) - Transforms each element using the function.

3. reduce(identity, BinaryOperator) - Combines all elements into a single result.

The below example shows how to use filter, map and reduce with Java 8 Streams.

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

public class StreamFilterMapReduceExample {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

// 1. filter() - keep only even numbers
        System.out.println("Even numbers:");
        numbers.stream()
            .filter(n -> n % 2 == 0)
            .forEach(System.out::println);

// 2. map() - square each number
        System.out.println("\nSquares:");
        numbers.stream()
            .map(n -> n * n)
            .forEach(System.out::println);

// 3. reduce() - sum all numbers
        int sum = numbers.stream()
            .reduce(0, Integer::sum);
        System.out.println("\nSum: " + sum);

// 4. Chaining: filter + map + reduce
        int sumOfSquaresOfEvens = numbers.stream()
            .filter(n -> n % 2 == 0)     // keep evens: 2,4,6,8,10
            .map(n -> n * n)              // square: 4,16,36,64,100
            .reduce(0, Integer::sum);     // sum: 220
        System.out.println("Sum of squares of even numbers: " + sumOfSquaresOfEvens);

// 5. Working with Objects
        List<String> words = Arrays.asList("hello", "world", "java", "stream", "api");
        Optional<String> longest = words.stream()
            .filter(w -> w.length() > 4)
            .map(String::toUpperCase)
            .reduce((a, b) -> a.length() >= b.length() ? a : b);
        System.out.println("Longest word > 4 chars (uppercase): " + longest.orElse("none"));
    }
}

It gives the following output,

Even numbers:
2
4
6
8
10

Squares:
1
4
9
16
25
36
49
64
81
100

Sum: 55
Sum of squares of even numbers: 220
Longest word > 4 chars (uppercase): STREAM

Terminal vs Intermediate Operations:

Intermediate (lazy, return Stream): filter(), map(), flatMap(), sorted(), distinct(), limit(), peek().

Terminal (eager, trigger processing): forEach(), collect(), reduce(), count(), findFirst(), anyMatch(), toArray().

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