Why Lambdas?

Before Java 8, passing behaviour around meant writing verbose anonymous inner classes. Lambdas let you treat a single-method action as a value — shorter, clearer, and a better fit for collections + streams.

Lambda Syntax

(parameters) -> { body }

• Zero parameters — () -> System.out.println("Hi") • One parameter — x -> x * x (parens optional) • Many parameters — (a, b) -> a + b • Block body — (a, b) -> { int r = a + b; return r; }

Functional Interface

An interface with exactly one abstract method (SAM — Single Abstract Method). Optionally annotated with @FunctionalInterface so the compiler enforces the rule.

Java's java.util.function package ships the most common shapes:

Method References

A short-hand for a lambda that just calls an existing method.

list.forEach(System.out::println);     // Class::staticMethod  OR  instance::method
names.stream().map(String::toUpperCase);
list.sort(String::compareToIgnoreCase);

Variable Capture

A lambda can read final or effectively-final local variables. It cannot mutate them — that keeps closures predictable in multi-threaded code.