Java1.8-Lambda

Posted 2020-09-05 dearYou

写的很全很好的一个博客： http://blog.csdn.net/ioriogami/article/details/12782141

 

什么是函数接口： http://www.lambdafaq.org/what-is-a-functional-interface/ 肤浅的理解就是：函数注解的接口是这样一个类型，这个类型的作用相当于一个函数

单看这两个博客理解起来有些吃力，看看java1.8中对FounctionalInterface的解释：

/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.lang;

import java.lang.annotation.*;

/**
 * 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.
 * ：functionalInterface注解用来指明一个接口类型是functional interface，这句话好像没什么用
 * 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.
 *：一个函数注解借口准确的说，只有一个抽象方法，且这个方法不能是重写的Object的public方法（不计入does not count toward）
 * <p>Note that instances of functional interfaces can be created with
 * lambda expressions, method references, or constructor references.
 *：一个函数注解借口可以使lambda表达式、方法的引用、或者构造函数的引用
 * <p>If a type is annotated with this annotation type, compilers are
 * required to generate an error message unless:
 *：使用这个注解的时候，为了使编译器不报错，至少要满足：
 * <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>
 *：type是借口而不是注解、枚举、类
 * <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.
 *：这里描述没用使用functionalInterface注解却被认作是函数注解接口的情况：meeting the definition of a functional interface即满足了函数接口的定义
 * @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 {}

简单的例子：

Runnable接口早就有了，FunctionalInterface是1.8才出的，Runnable接口只有一个抽象函数，满足FounctionalInterface的定义，属于上面提到的特殊情况，在1.8中加上了@FunctionalInterface注解

Thread th=new Thread(()->System.out.println("hello"));
        th.start();