51工具盒子你好，我是猿java。
Lambda官方说明 {#Lambda官方说明}
上一篇文章我们分析了lambda是什么，里面提到一个点：lambda必须依赖于函数式接口，这篇博文我们就来一起分析下什么是函数式接口
首选我们看一个List遍历的例子
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 | public class ListTest { public static void main(String[] args) { List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6); //for method for(int i = 0; i < list.size(); i++){ System.out.println("i=" + i); } // lambda method list.forEach(System.out::println); } // forEach method default void forEach(Consumer<? super T> action) { Objects.requireNonNull(action); for (T t : this) { action.accept(t); } } } /** * Represents an operation that accepts a single input argument and returns no * result. Unlike most other functional interfaces, {@code Consumer} is expected * to operate via side-effects. * * <p>This is a <a href="package-summary.html">functional interface</a> * whose functional method is {@link #accept(Object)}. * * @param <T> the type of the input to the operation * * @since 1.8 */ @FunctionalInterface public interface Consumer<T> { /** * Performs this operation on the given argument. * * @param t the input argument */ void accept(T t); /** * Returns a composed {@code Consumer} that performs, in sequence, this * operation followed by the {@code after} operation. If performing either * operation throws an exception, it is relayed to the caller of the * composed operation. If performing this operation throws an exception, * the {@code after} operation will not be performed. * * @param after the operation to perform after this operation * @return a composed {@code Consumer} that performs in sequence this * operation followed by the {@code after} operation * @throws NullPointerException if {@code after} is null */ default Consumer<T> andThen(Consumer<? super T> after) { Objects.requireNonNull(after); return (T t) -> { accept(t); after.accept(t); }; } } |
从上面的代码可以看出，从jdk8开始，list增加了forEach()方法进行遍历，forEach()接收一个Consumer类型的参数，进入Consumer可以看出该类有一个@FunctionalInterface
的注解，这个标识Consumer是一个函数式接口，接下来我们进入FunctionalInterface这个注解
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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 | /** * 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 * @jls 9.6.4.9 @FunctionalInterface * @since 1.8 */ @Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface FunctionalInterface {} |
从源码中我们可以抽取FunctionalInterface注解的几个重要信息：
- FunctionalInterface注解是从jdk1.8 开始引入
- FunctionalInterface注解是一种信息性注释类型，用于指示接口类型声明旨在成为 Java 语言规范定义的功能接口
- 函数式接口有且仅有一个抽象方法，因为所有的类默认继承java.lang.Object类，所以从Object类继承下来的抽象方法不会计入该接口的抽象方法总数
- 函数式接口的实例有3种创建方式：lambda表达式、方法引用或构造函数引用
- FunctionalInterface注解只能用于接口类型，而不能用于注释类型、枚举或类，如果此注解用于非接口，则编译器会生成错误信息
- 只要接口符合函数式接口的特征，不论接口是否加了FunctionalInterface注解，编译器都会把这个接口当做函数式接口处理
