Lambda basics

Learn lambda expressions with () -> ....

Lambda expression

A lambda expression is a short way to write a small function in one line. The syntax is (parameters) -> body. With no parameters use (); with one parameter the parentheses are optional.

Example: Runnable r = () -> System.out.println("Hello");. This lambda is assigned to a Runnable; when r.run() is called the body executes. A lambda can be assigned to any single-method interface (functional interface); Java infers the type from the method signature.

Functional interface

A lambda can be assigned only to a functional interface, which is an interface with exactly one abstract method. Runnable is an example: it has a single run() method. Java matches the lambda body to that one method.

Interfaces with multiple abstract methods cannot accept a lambda. The @FunctionalInterface annotation warns the compiler; adding a second abstract method becomes a compile error. This keeps lambda assignments safe.

Exercise Write a Runnable lambda

Define Runnable r = () -> System.out.println("Hello");. Then call r.run(); to execute the lambda. A single line should print Hello.

Show example solution

public class Main {
    public static void main(String[] args) {
        Runnable r = () -> System.out.println("Hello");
        r.run();
    }
}

Test yourself

What is the basic syntax of a lambda expression?
- function(parameters) { body }
- (parameters) -> body
- def parameters: body
- lambda body end
Show answer

(parameters) -> body — A lambda is written as (parameters) -> body.

Complete the no-arg lambda

Runnable r = ____ -> System.out.println("Hello");

Show answer

Runnable r = () -> System.out.println("Hello");

Match the lambda parts

()

No parameters
->

Arrow token
body

Code to run
Runnable

Single-method interface

Next lesson: Parameterized lambda

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