JAVA 8 函数式接口 - Functional Interface

什么是函数式接口（Functional Interface）

其实之前在讲Lambda表达式的时候提到过，所谓的函数式接口，当然首先是一个接口，然后就是在这个接口里面只能有一个抽象方法。

这种类型的接口也称为SAM接口，即Single Abstract Method interfaces。

函数式接口用途

它们主要用在Lambda表达式和方法引用（实际上也可认为是Lambda表达式）上。

如定义了一个函数式接口如下：

    @FunctionalInterface
    interface GreetingService
    {
        void sayMessage(String message);
    }

那么就可以使用Lambda表达式来表示该接口的一个实现(注：JAVA 8 之前一般是用匿名类实现的)：

GreetingService greetService1 = message -> System.out.println("Hello " + message);

关于@FunctionalInterface注解

Java 8为函数式接口引入了一个新注解@FunctionalInterface，主要用于编译级错误检查，加上该注解，当你写的接口不符合函数式接口定义的时候，编译器会报错。

正确例子，没有报错：

    @FunctionalInterface
    interface GreetingService
    {
        void sayMessage(String message);
    }

错误例子，接口中包含了两个抽象方法，违反了函数式接口的定义，Eclipse报错提示其不是函数式接口。

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" 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提醒：加不加@FunctionalInterface对于接口是不是函数式接口没有影响，该注解知识提醒编译器去检查该接口是否仅包含一个抽象方法

函数式接口里允许定义默认方法

函数式接口里是可以包含默认方法，因为默认方法不是抽象方法，其有一个默认实现，所以是符合函数式接口的定义的；

如下代码不会报错：

    @FunctionalInterface
    interface GreetingService
    {
        void sayMessage(String message);
 
        default void doSomeMoreWork1()
        {
            // Method body
        }
 
        default void doSomeMoreWork2()
        {
            // Method body
        }
    }

函数式接口里允许定义静态方法

函数式接口里是可以包含静态方法，因为静态方法不能是抽象方法，是一个已经实现了的方法，所以是符合函数式接口的定义的；

如下代码不会报错：

    @FunctionalInterface
    interface GreetingService
    {
        void sayMessage(String message);
        static void printHello(){
            System.out.println("Hello");
        }
    }

函数式接口里允许定义java.lang.Object里的public方法

函数式接口里是可以包含Object里的public方法，这些方法对于函数式接口来说，不被当成是抽象方法（虽然它们是抽象方法）；因为任何一个函数式接口的实现，默认都继承了Object类，包含了来自java.lang.Object里对这些抽象方法的实现；

如下代码不会报错：

    @FunctionalInterface
    interface GreetingService
    {
        void sayMessage(String message);
 
        @Override
        boolean equals(Object obj);
    }

JDK中的函数式接口举例

java.lang.Runnable,

java.awt.event.ActionListener,

java.util.Comparator,

java.util.concurrent.Callable

java.util.function包下的接口，如Consumer、Predicate、Supplier等

参考资料

https://sanaulla.info/2013/03/21/introduction-to-functional-interfaces-a-concept-recreated-in-java-8/

http://howtodoinjava.com/java-8/functional-interface-tutorial/