Built-in functional interfaces (java.util.function)

The six core interfaces, their method names, and when to reach for each.

The standard toolkit

java.util.function ships a family of ready-made functional interfaces so you don't have to invent your own. The six you must know for OCP:

Interface	Signature	Use
`Supplier<T>`	`T get()`	produce a T from nothing
`Consumer<T>`	`void accept(T t)`	consume a T, return nothing
`Function<T, R>`	`R apply(T t)`	map T to R
`Predicate<T>`	`boolean test(T t)`	test T, return a boolean
`UnaryOperator<T>`	`T apply(T t)`	specialised Function where T == R
`BinaryOperator<T>`	`T apply(T t1, T t2)`	specialised BiFunction where all types match

BiFunction<T, U, R> is the two-input generalisation of Function: R apply(T t, U u).

Primitive specialisations avoid boxing overhead — IntFunction<R>, IntConsumer, IntPredicate, IntSupplier, ToIntFunction<T>, IntUnaryOperator, and so on for Long and Double. Prefer them in hot paths.

Supplier<String>   greet   = () -> "hello";
Consumer<String>   printer = System.out::println;
Function<String, Integer> len = String::length;
Predicate<String>  blank   = String::isBlank;
UnaryOperator<String> upper = String::toUpperCase;
BinaryOperator<Integer> max = Integer::max;