java8

💡 函数式编程

一、 Function 源码：

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.util.function;
import java.util.Objects;
/**
 * 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;
    }
}

1.1 ）apply(T t)；方法：

翻译过来就是：将给定的参数应用到这个函数上,传入的参数类型为T返回类型为R

public class TestFunction {
    public static void main(String[] args) {
        System.out.println(testFunction(10, integer -> integer * 2));
    }
    public static int testFunction(int i, Function<Integer, Integer> function) {
        return function.apply(i);
    }
}
--  输出：20

1.2） compose(Function<? super V, ? extends T> before)

翻译过来就是：接收一个function类型的函数作为参数

public class TestFunctionofcompose {
    public static void main(String[] args) {
        System.out.println(testFunction(10, i -> i * 2 + 9, j -> j * j));
    }
    public static int testFunction(int i, Function<Integer, Integer> function1, Function<Integer, Integer> function2 ) {
        return function1.compose(function2).apply(i);
    }
}
-- 输出: 209

1.3) andThen(Function<? super R, ? extends V> after)
翻译过来就是：就是现将传过来的参数执行apply(T t)方法,之后把apply(T t)里面返回的结果再去执行第二个Function函数

https://blog.csdn.net/qq_27416233/article/details/84832013