Lambdas: compact objects with one job

A lambda is a function-shaped object

A lambda expression is a very concise way to produce an instance of a single-abstract-method interface — Runnable, Callable, Comparator, Consumer, Predicate, or your own @FunctionalInterface. The compiler handles the boilerplate; you supply the body.

Comparator<String> byLen = (a, b) -> Integer.compare(a.length(), b.length());

// Captured locals must be effectively final
String prefix = "user:";
Function<Integer, String> id = n -> prefix + n;

// Method references are the shorthand form
list.sort(Comparator.comparingInt(String::length));

Effective final — why the compiler cares

A lambda captures local variables. If those variables could change after the lambda was created, the lambda's behavior would depend on timing — a hostile foot-gun, especially once the lambda runs on another thread. Java rules out this bug class by requiring captured locals to be effectively final (assigned exactly once).