什么是函数式接口 @FunctionalInterface

源码定义

/**
 * An informative annotation type used to indicate that an interface
 * type declaration is intended to be a <i>functional interface</i> as
 * defined by the Java Language Specification.
 *
 * Conceptually, a functional interface has exactly one abstract
 * method.  Since {@linkplain java.lang.reflect.Method#isDefault()
 * default methods} have an implementation, they are not abstract.  If
 * an interface declares an abstract method overriding one of the
 * public methods of {@code java.lang.Object}, that also does
 * <em>not</em> count toward the interface's abstract method count
 * since any implementation of the interface will have an
 * implementation from {@code java.lang.Object} or elsewhere.
 *
 * <p>Note that instances of functional interfaces can be created with
 * lambda expressions, method references, or constructor references.
 *
 * <p>If a type is annotated with this annotation type, compilers are
 * required to generate an error message unless:
 *
 * <ul>
 * <li> The type is an interface type and not an annotation type, enum, or class.
 * <li> The annotated type satisfies the requirements of a functional interface.
 * </ul>
 *
 * <p>However, the compiler will treat any interface meeting the
 * definition of a functional interface as a functional interface
 * regardless of whether or not a {@code FunctionalInterface} 
 * annotation is present on the interface declaration.
 *
 * @jls 4.3.2. The Class Object
 * @jls 9.8 Functional Interfaces
 * @jls 9.4.3 Interface Method Body
 * @since 1.8
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}

翻译

1. 如果一个接口只有一个抽象方法，那么该接口就是函数式接口。
2. 如果一个接口里面有一个抽象方法，该方法重写了Object的方法，那么不会向接口的抽象方法加1，该接口的抽象方法还是一个，仍然满足函数式接口的定义，那么该接口也会定义为函数式接口。
3. 如果我们在某个接口上声明了FunctionalInterface注解，那么编译器就会按照函数式接口的定义来要求该接口，不满足就会报错。
4. 如果某一个接口只有一个抽象方法，但是我们并没有在该接口上声明FunctionalInterface注解，那么编译器依旧会将该接口看作函数式接口。

示例

正确定义

• 一般定义

package com.demo.fuctionalface;

@FunctionalInterface
public interface InterfaceDemo {
    void say();
}

• 不添加@FunctionalInterface注解编译器也会将此接口当成函数式接口，建议添加

package com.demo.fuctionalface;

public interface InterfaceDemo {
    void say();
}

• 重写了Object中的方法，仍然是函数式接口

package com.demo.fuctionalface;

public interface InterfaceDemo {
    void say();
    @Override
    String toString();
}

重点解释一下为什么第三种也是函数式接口:

• 首先 Object类是所有java的父类,所以InterfaceDemo的实现类，直接或者间接的继承Object类，也就是说也继承了Object的toString方法，实现在Object内。只需要实现say()方法即可。子类只需要实现一个抽象方法，符合函数式接口的定义。

错误定义

• 编译器就会报错

package com.demo.fuctionalface;

@FunctionalInterface
public interface InterfaceDemo {
    void say();
    String MyString();
}

java8新特性 - 什么是函数式接口 @FunctionalInterface？

原文：https://www.cnblogs.com/karlMa/p/11304441.html