java8 Funciton和BiFunction都是功能性接口，Function包含一个入参（T），一个出参（R），输入输出可以是不不同的类型，BiFunction包含两个入参T和R，一个出参R。

1、Function<T, R>

下面这个例子展示了一个输入参数<T> String类型，然后输出一个<R> Integer类型

package com.cattles.function;

import java.util.function.Function;

/**
 * @author cattle -  稻草鸟人
 * @date 2020/3/1 下午5:40
 */
public class Java8Function1 {

    public static void main(String[] args) {
        Function<String, Integer> func = x -> x.length();
        Integer apply = func.apply("cattle");
        System.out.println(apply);
    }
}

输出：

6

2、Function<T, R> addThen和compose

package com.cattles.function;

import java.util.function.Function;

/**
 * @author cattle -  稻草鸟人
 * @date 2020/3/1 下午5:45
 */
public class Java8Function2 {

    public static void main(String[] args) {
        Function<String, Integer> func1 = x -> x.length();
        Function<Integer, Integer> func2 = x -> x * 2;
        Integer ret = func1.andThen(func2).apply("cattle");
        System.out.println(ret);

        Integer ret2 = func2.compose(func1).apply("cattle");
        System.out.println(ret2);
    }
}

输出:

12
12

问题：思考addThen和compose的区别

3、Function复杂一些的例子

package com.cattles.function;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Base64;
import java.util.List;
import java.util.function.Function;

/**
 * @author cattle -  稻草鸟人
 * @date 2020/3/1 下午10:43
 */
public class Java8Function3 {

    public static void main(String[] args) {
        Java8Function3 obj = new Java8Function3();
        List<String> list = Arrays.asList("java", "python", "C#", "PHP", "javascript");
        List<String> ret = obj.list(list, obj::base64);
        ret.forEach(System.out::println);
    }

    public <T, R> List<R> list(List<T> list, Function<T, R> function) {
        List<R> ret = new ArrayList<>();
        for (T t : list) {
            ret.add(function.apply(t));
        }
        return ret;
    }

    public String base64(String st) {
        return Base64.getEncoder().encodeToString(st.getBytes());
    }
}

输出:

amF2YQ==
cHl0aG9u
QyM=
UEhQ
amF2YXNjcmlwdA==

4、BiFunction<T, U, R>

package com.cattles.function;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.BiFunction;

/**
 * @author cattle -  稻草鸟人
 * @date 2020/3/1 下午10:54
 */
public class Java8BiFunction1 {

    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> func = (T, U) -> T * U;
        System.out.println(func.apply(3, 4));

        Java8BiFunction1 obj = new Java8BiFunction1();
        List<String> list1 = Arrays.asList("a", "b", "c");
        List<Integer> list2 = Arrays.asList(1, 2, 3);
        List<String> result = obj.listCombiner(list1, list2, (a, b) -> a + b);
        System.out.println(result);
    }

    private <T, U, R> List<R> listCombiner(
            List<T> list1, List<U> list2, BiFunction<T, U, R> combiner) {
        List<R> result = new ArrayList<>();
        for (int i = 0; i < list1.size(); i++) {
            result.add(combiner.apply(list1.get(i), list2.get(i)));
        }
        return result;
    }
}

输出：

12
[a1, b2, c3]

5、BiFunction<T, U, R> addThen

package com.cattles.function;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.BiFunction;
import java.util.function.Function;

/**
 * @author cattle -  稻草鸟人
 * @date 2020/3/1 下午11:02
 */
public class Java8BiFunction2 {

    public static void main(String[] args) {
        BiFunction<String, String, String> func1 = (T, U) -> T + U;
        Function<String, String> func2 = x -> x + " Java";
        System.out.println(func1.andThen(func2).apply("I", " Love"));

        List<String> list1 = Arrays.asList("a", "b", "c");
        List<String> list2 = Arrays.asList("1", "2", "3");
        Java8BiFunction2 obj = new Java8BiFunction2();
        List<String> result = obj.listCombiner(list1, list2, String::concat);
        System.out.println(result);
    }

    private <T, U, R> List<R> listCombiner(
            List<T> list1, List<U> list2, BiFunction<T, U, R> combiner) {
        List<R> result = new ArrayList<>();
        for (int i = 0; i < list1.size(); i++) {
            result.add(combiner.apply(list1.get(i), list2.get(i)));
        }
        return result;
    }
}

输出：

I Love Java
[a1, b2, c3]