Java · Lesson 13 of 15

Lambdas and Streams

Functional interfaces, lambdas, method references and the Stream pipeline.

Intermediate
16 min read
3 objectives

Before this lessonLesson 12: Generics

What you will learn

Write lambdas
Filter, map and collect
Use Optional safely

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A lambda is a short anonymous function. It can be used wherever Java expects a functional interface: an interface with a single abstract method, such as Runnable, Comparator or Function.

Lambdas and method references

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>(List.of("Grace", "Al", "Linus"));
        names.sort((a, b) -> a.length() - b.length());
        System.out.println(names);
        names.forEach(System.out::println);   // method reference
    }
}

Output

[Al, Grace, Linus]
Al
Grace
Linus

Stream pipelines

A stream takes data through steps: a source, zero or more intermediate operations (filter, map, sorted) and one terminal operation (collect, count, sum) that actually runs it. The source list is never modified.

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

public class Main {
    public static void main(String[] args) {
        List<Integer> nums = List.of(1, 2, 3, 4, 5, 6);
        List<Integer> evenSquares = nums.stream()
            .filter(n -> n % 2 == 0)
            .map(n -> n * n)
            .collect(Collectors.toList());
        System.out.println(evenSquares);
        System.out.println(nums.stream().mapToInt(Integer::intValue).sum());
        System.out.println(nums.stream().anyMatch(n -> n > 5));
    }
}

Output

[4, 16, 36]
21
true

Grouping

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

public class Main {
    public static void main(String[] args) {
        List<String> words = List.of("apple", "avocado", "banana", "blueberry", "cherry");
        Map<Character, List<String>> byLetter = words.stream()
            .collect(Collectors.groupingBy(w -> w.charAt(0)));
        System.out.println(byLetter);
    }
}

Output

{a=[apple, avocado], b=[banana, blueberry], c=[cherry]}

Optional

Optional<T> makes "there may be no value" explicit, so callers cannot forget the empty case.

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Optional<String> first = List.of("x", "yy").stream().filter(s -> s.length() > 5).findFirst();
        System.out.println(first.orElse("none"));
        System.out.println(first.isPresent());
    }
}

Output

none
false

Up next · Lesson 14Files and I/ORead and write files with java.nio and handle resources with try-with-resources.