java8的函数式接口(Functional Interface

2019-11-22

@FunctionalInterface

An informative annotation type used to indicate that an interface type
declaration is intended to be a functional interface as defined by the Java Language Specification.
Conceptually, a functional interface has exactly one abstract method.
Since default methods have an implementation,
they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object,
that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere.
Note that instances of functional interfaces can be created with lambda expressions, method references, or constructor references.
If a type is annotated with this annotation type, compilers are required to generate an error message unless:
The type is an interface type and not an annotation type, enum, or class.
The annotated type satisfies the requirements of a functional interface.
However, the compiler will treat any interface meeting the definition of a functional interface as a functional interface regardless of whether or not a FunctionalInterface annotation is present on the interface declaration.

简单总结就是函数式接口（Functional Interface）就是只含有一个抽象方法的接口，而这个抽象方法不包括具有default的方法和Object的方法的抽象。因为它们都有一个实现。
函数式接口的实例可以由Lambda表达式、方法引用、构造器引用来创建。
eg:

@FunctionalInterface
public interface FunctionalInterfaceTest {

    String parseName(String name) ;
//具有default的抽象方法不算在函数式接口的抽象方法里
    default String see(){
        return "you're beautiful ";
    }
//Object的方法的抽象不算在函数式接口的抽象方法里
    String toString();
}