Lambda Expressions

Write concise anonymous function expressions using Java lambdas to replace verbose anonymous class syntax.

Overview

A lambda expression is a short, anonymous function that can be passed as an argument or stored in a variable. Introduced in Java 8, lambdas make code shorter and more readable, especially with the Collections API and streams. A lambda can be used wherever a functional interface is expected. A functional interface is any interface with exactly one abstract method (like Runnable, Comparator, or Predicate). Syntax: (parameters) -> expression or (parameters) -> { statements; }

Key Points

•Syntax: (params) -> expression or (params) -> { body; return value; }
•Parameter types can be omitted when the compiler can infer them
•Single-parameter lambdas can omit parentheses: x -> x * 2
•No-parameter lambdas still need empty parentheses: () -> System.out.println("hi")
•Lambdas can only be used where a functional interface is expected
•Lambdas capture effectively-final local variables from the enclosing scope
•this inside a lambda refers to the enclosing class, not the lambda itself
•Method references (::) are shorthand for certain lambda patterns (covered in J053)

Examples

Lambda as Runnable and Comparator

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        // Runnable with lambda (no parameters, no return)
        Runnable greet = () -> System.out.println("Hello from lambda!");
        greet.run();

        // Comparator with lambda
        List<String> names = new ArrayList<>(List.of("Charlie","Alice","Bob","Diana"));
        names.sort((a, b) -> a.compareTo(b));
        System.out.println("Sorted: " + names);

        // Sort by length
        names.sort((a, b) -> Integer.compare(a.length(), b.length()));
        System.out.println("By length: " + names);
    }
}

names.sort((a, b) -> ...) replaces the verbose new Comparator<String>() { public int compare(String a, String b) { ... } } anonymous class.

Lambda with forEach

import java.util.List;

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

        // Print each number
        numbers.forEach(n -> System.out.print(n + " "));
        System.out.println();

        // Print only evens
        numbers.forEach(n -> {
            if (n % 2 == 0) System.out.print(n + " ");
        });
        System.out.println();

        // Multi-line body needs braces and explicit return if returning a value
        List<String> words = List.of("hello", "world", "java");
        words.forEach(w -> {
            String upper = w.toUpperCase();
            System.out.println(upper);
        });
    }
}

Single-expression lambdas omit braces and return. Multi-statement lambdas use braces.

Capturing variables from the enclosing scope

import java.util.List;

public class Main {
    public static void main(String[] args) {
        String prefix = "Item: "; // effectively final
        int multiplier = 3;       // effectively final

        List<String> items = List.of("apple", "banana", "cherry");
        items.forEach(item -> System.out.println(prefix + item));

        List<Integer> nums = List.of(1, 2, 3);
        nums.forEach(n -> System.out.println(n * multiplier));
    }
}

Lambdas can use local variables from the enclosing method if those variables are effectively final (never reassigned after their initial assignment).

Common Mistakes

Trying to modify a captured local variable inside a lambda. It must be effectively final; use an array or a field instead if you need mutation
Using a lambda where the target type is not a functional interface. An interface must have exactly one abstract method
Forgetting braces and return when the lambda body has multiple statements

Note

The simplest lambda form is the expression lambda: x -> x * 2. Reserve the block form (x -> { ... }) for cases with multiple statements. Keeping lambdas short makes them readable.

Lambda Expressions

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What is a functional interface?