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