Each of the following interfaces is annotated with @FunctionalInterface

Signature	Interface	Method
() → void	Runnable	void run()

() → T	Callable	T call() throws Exception
() → T	Supplier	T get()
() → int	IntSupplier	int getAsInt()

T → void	Consumer	void accept(T t)
int → void	IntConsumer	void accept(int i)

T → R	Function<T,R>	R apply(T t)
T → boolean	Predicate	boolean test(T t)
int → R	IntFunction	R apply(int i)
T → int	ToIntFunction	int applyAsInt(T t)

Method Reference

The operator :: allows referencing an existing method (static or not)

:: on type	BinOp add = Integer::sum; ToIntFunction<String> fun = String::length;
:: on instance	String hello = “hello”; IntSupplier supplier = hello::length;