1. An interface that contains only one abstract method is called a functional interface.
  2. Objects of this interface can be created using Lambda expressions.
  3. We can use the @functionalinterface annotation on any FunctionalInterface to check if it is a FunctionalInterface.

1, JAVA built-in four core functional interfaces

Functional interface The parameter types The return type methods
Consumer<T> T void void accept(T t)
Supplier<T> There is no T T get()
Function<T,R> T R R apply(T t)
Predicate<T> T boolean boolean test(T t)

2. Other functional interfaces

Functional interface The parameter types The return type methods
BiFunction<T,U,R> T,U R R apply(T t, U u)
UnaryOperator<T>

(Function subinterface)		 T T T apply(T t)
BinaryOperator<T>

(BiFunction subinterface)		 T,T T T apply(T t1, T t2)
BiConsumer<T,U> T,U void void accept(T t, U u)

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