Java 8：函数式接口 Function：1个参数, 有返回值

#Java 8

本文关键词：Java 8， 函数式接口，lambda 。

介绍

全路径: java.util.function.Function 。

Java 8 中引入。

源码：

/**
 * 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);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

示例

代码示例1

import java.util.function.Function;

public class Main {

    public static void main(String[] args) {
        Function<Long, String> function = aLong -> {
            return "数字是: " + aLong;
        };

        String result = function.apply(123L);
        System.out.println(result);
    }

}

执行结果：

数字是: 123

代码示例2

import java.util.function.Function;

public class Main {

    public static void main(String[] args) {
        Function<Long, Long> function = aLong -> {
            System.out.println("数字加2");
            return aLong + 2;
        };

        Function<Long, Long> function2 = function.compose(aLong -> {
            System.out.println("数字加10");
            return aLong + 10;
        });

        Function<Long, Long> function3 = function2.andThen(aLong -> {
            System.out.println("数字加100");
            return aLong + 100;
        });

        Long result = function3.apply(123L);
        System.out.println("执行结果: " + result);
    }

}

执行结果：

数字加10
数字加2
数字加100
执行结果: 235

( 本文完 )