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Java 8 Features > Stream API > Lambda Expressions

Lambda Expressions

Author: Venkata Sudhakar

Lambda expressions are a key feature introduced in Java 8 that allow you to write concise functional-style code. They enable you to treat functionality as a method argument and write inline implementations of functional interfaces.

Lambda Expression Syntax:

(parameters) -> expression or (parameters) -> { statements; }

The below example shows various ways to use Lambda Expressions in Java 8.

import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;

public class LambdaExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David", "Eve");

// 1. Lambda with forEach
        System.out.println("All names:");
        names.forEach(name -> System.out.println("  " + name));

// 2. Lambda as Predicate (filter)
        Predicate<String> longName = name -> name.length() > 4;
        System.out.println("\nNames longer than 4 chars:");
        names.stream().filter(longName).forEach(System.out::println);

// 3. Lambda as Function (transform)
        Function<String, String> toUpper = String::toUpperCase;
        System.out.println("\nUppercase names:");
        names.stream().map(toUpper).forEach(System.out::println);

// 4. Lambda as Comparator
        List<String> sorted = names.stream()
            .sorted((a, b) -> a.length() - b.length())
            .collect(java.util.stream.Collectors.toList());
        System.out.println("\nSorted by length: " + sorted);

// 5. Lambda with multiple parameters
        Consumer<String> greet = name -> System.out.println("Hello, " + name + "!");
        greet.accept("World");
    }
}

It gives the following output,

All names:
  Alice
  Bob
  Charlie
  David
  Eve

Names longer than 4 chars:
Alice
Charlie
David

Uppercase names:
ALICE
BOB
CHARLIE
DAVID
EVE

Sorted by length: [Bob, Eve, Alice, David, Charlie]
Hello, World!

Functional Interfaces used with Lambda:

Predicate<T> - Takes T, returns boolean. Used for filtering.

Function<T,R> - Takes T, returns R. Used for transformation.

Consumer<T> - Takes T, returns void. Used for side effects.

Supplier<T> - Takes nothing, returns T. Used for lazy creation.

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