FunctionalInterface

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FunctionalInterface

函数式接口

lambda表达式

Lambda：In programming languages such as Lisp,Python and Ruby lambda is an operator used to denote anonymous functions or closures,following the usage of lambda calculus.

为何需要Lambda表达式

  • 在java中，我们无法将函数作为参数传递给一个方法，也无法声明返回一个函数的方法
  • 在JavaScript中，函数参数是一个函数，返回值是另一个函数的情况非常常见的；JavaScript是一门非常典型的函数式语言

Lambda表达式作用

  • Lambda表达式为Java添加了缺失的函数式编程特性，使我们能将函数当成一等公民看待
  • 在将将函数当成一等公民的语言中，Lambda表达式的类型是函数。但在java中，Lambda表达式是对象，他们必须依附于一类特别的对象类型————函数式接口（functional interface）
  • 传递行为，而不仅仅是值

关于函数式接口

  • 如果一个接口只有一个抽象方法，那么该接口就是一个函数式接口。
  • 如果我们在某个接口上声明了FunctionalInterface注解，那么编译器会按照函数式接口的定义来要求该接口。
  • 如果某个接口只有一个抽象方法，但我们没有给该接口声明FunctionalInterface注解，那么编译器依旧会将该接口看作是函数式接口

常用函数式接口

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/**
 * 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);
    ...
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/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> {
    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);
    ...
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/**
 * Represents a function that accepts two arguments and produces a result.
 * This is the two-arity specialization of {@link Function}.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object, Object)}.
 *
 * @param <T> the type of the first argument to the function
 * @param <U> the type of the second argument to the function
 * @param <R> the type of the result of the function
 *
 * @see Function
 * @since 1.8
 */
@FunctionalInterface
public interface BiFunction<T, U, R> {
    /**
     * Applies this function to the given arguments.
     *
     * @param t the first function argument
     * @param u the second function argument
     * @return the function result
     */
    R apply(T t, U u);
    ...
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/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {
    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);
    
    ...
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/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {
    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}