Adobe中的FuntionalInterface注释类型(FuntionalInterface Annotation Type in Java)

 我正在阅读关于java注释的内容，并出现了一个新的疑问。 
 
 在文档中解释了FunctionalInterface Annotation Type： 
 
 
 
 具有一个抽象方法声明的接口称为功能接口。编译器验证所有使用@FunctionalInterface注释的接口，接口实际上只包含一个抽象方法。 如果使用此批注注释的接口不是功能接口，则会生成编译时错误。 在类，注释类型和枚举上使用此注释也是一个编译时错误。 FunctionalInterface注释类型是标记接口。 
 
 
 我做了一些测试，我不需要用这个注释类型标记我的界面。 然后，我的问题是：如果每个具有一个方法的接口始终是一个功能接口，为什么我需要这个注释？ 
 
 示例代码 
 
// @FunctionalInterface
interface Wizard {
    int spell(String power);
}

class TestLambda {
    public static void main(String[] args) {
        Wizard gandalf = str -> str.length();
        int power = gandalf.spell(args[0]);
        System.out.println("The spell length is: " + power+ " points");
    }
}

I was reading about java annotation and a new doubt appears. 
 
In the documentation explain the FunctionalInterface Annotation Type:
 
 
 
An interface with one abstract method declaration is known as a functional interface.The compiler verifies all interfaces annotated with a @FunctionalInterface that the interfaces really contain one and only one abstract method. A compile-time error is generated if the interfaces annotated with this annotation are not functional interfaces. It is also a compile-time error to use this annotation on classes, annotation types, and enums. The FunctionalInterface annotation type is a marker interface.
 
 
I did some test and I did not need to mark my interface with this annotation type. Then, my question is: Why do I need this annotation if every interface with one method is always a functional interface?
 
Exampe Code
 
// @FunctionalInterface
interface Wizard {
    int spell(String power);
}

class TestLambda {
    public static void main(String[] args) {
        Wizard gandalf = str -> str.length();
        int power = gandalf.spell(args[0]);
        System.out.println("The spell length is: " + power+ " points");
    }
}

原文：https://stackoverflow.com/questions/29954781