java8-stream-lambda-表达式常用方法总结的-优快云博客

本文链接：https://blog.youkuaiyun.com/mijklgps/article/details/131638498
该代码示例展示了如何使用Java8的StreamAPI进行数据过滤、映射、扁平化、聚合、排序、转换以及收集到各种集合等操作。文章通过创建和操作Data对象列表，演示了StreamAPI的基本用法，包括filter、map、flatMap、sum、average、count、sorted、reduce、toCollection和toMap等方法。
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package me.ele.newretail.ecology.partner;

import com.alibaba.fastjson.JSON;
import lombok.experimental.Accessors;

import java.util.*;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * @author 雨火
 */
public class TestStream {

    // 测试数据准备
    static List<Data> testList1 = Stream.of(new Data().setId(1L).setName("名称1").setAge(11), new Data().setId(2L).setName("名称2").setAge(11), new Data().setId(3L).setName("名称3").setAge(33)).collect(Collectors.toList());
    static List<Data> testList2 = Stream.of(new Data().setId(4L).setName("名称4").setAge(44)).collect(Collectors.toList());
    static List<List<Data>> listList = Stream.of(testList1, testList2).collect(Collectors.toList());

    @lombok.Data
    @Accessors(chain = true)
    private static class Data {
        private Long id;
        private String name;
        private Integer age;
    }

    public static void main(String[] args) {
        baseApi();
        toCollection();
        toMap();
        groupingBy();
        collectingAndThen();
    }

    private static void baseApi() {
        // filter 过滤ID=1的元素
        List<Data> filter = testList1.stream().filter(e -> e.getId().equals(1L)).collect(Collectors.toList());
        System.out.printf("filter:%s%n", JSON.toJSONString(filter));

        // map 将testList1中所有的元素的id取出来, 重新放进一个新的list
        List<Long> map = testList1.stream().map(Data::getId).collect(Collectors.toList());
        System.out.printf("map:%s%n", JSON.toJSONString(map));

        // 将testList1和testList2中的所有元素取出来, 重新放在一个list里面
        List<Data> flatMap = listList.stream().flatMap(Collection::stream).collect(Collectors.toList());
        System.out.printf("flatMap:%s%n", JSON.toJSONString(flatMap));

        // testList1的年龄相加
        int sum1 = testList1.stream().mapToInt(Data::getAge).sum();
        System.out.printf("testList1的年龄相加:%s%n", JSON.toJSONString(sum1));
        // testList1年龄的平均值
        double v = testList1.stream().mapToInt(Data::getAge).average().orElse(0);
        System.out.printf("testList1年龄的平均值:%s%n", JSON.toJSONString(v));
        // 计算个数
        long count = testList1.stream().mapToInt(Data::getAge).count();
        System.out.printf("计算个数:%s%n", JSON.toJSONString(count));
        // 拆箱和包箱
        List<Integer> collect = testList1.stream().mapToInt(Data::getAge).boxed().collect(Collectors.toList());
        System.out.printf("拆箱和包箱:%s%n", JSON.toJSONString(collect));

        // listList中的所有元素的年龄相加
        long sum = listList.stream().flatMapToLong(e -> e.stream().mapToLong(Data::getAge)).sum();
        System.out.printf("listList中的所有元素的年龄相加:%s%n", JSON.toJSONString(sum));

        // 排序, 按照年龄排序
        List<Data> collect1 = testList1.stream().sorted(Comparator.comparing(Data::getAge)).collect(Collectors.toList());
        System.out.printf("按照年龄排序:%s%n", JSON.toJSONString(collect1));
        List<Data> collect2 = testList1.stream().sorted(Comparator.comparing(Data::getAge).reversed()).collect(Collectors.toList());
        System.out.printf("按照年龄排序倒叙:%s%n", JSON.toJSONString(collect2));
        // 按照年龄正序, 在按照名字倒叙
        List<Data> collect3 = testList1.stream().sorted(Comparator.comparing(Data::getAge).thenComparing(Data::getName).reversed()).collect(Collectors.toList());
        System.out.printf("按照年龄正序, 在按照名字倒叙:%s%n", JSON.toJSONString(collect3));
        // 按照年龄正序, 在按照名字倒叙
        List<Data> collect4 = testList1.stream().sorted(((o1, o2) -> {
            if (o1.getAge().compareTo(o2.getAge()) > 0) {
                return 1;
            } else if (o1.getAge().compareTo(o2.getAge()) == 0) {
                if (o1.getName().compareTo(o2.getName()) == 0) {
                    return 0;
                } else if (o1.getName().compareTo(o2.getName()) <= 0) {
                    return 1;
                } else {
                    return -1;
                }
            } else {
                return -1;
            }
        })).collect(Collectors.toList());
        System.out.printf("按照年龄正序, 在按照名字倒叙:%s%n", JSON.toJSONString(collect4));

        // testList1 将元素的每个字段都累加起来
        Data data = testList1.stream().reduce((o1, o2) -> {
            //
            return new Data().setId(o1.getId() + o2.getId()).setName(o1.getName() + o2.getName()).setAge(o1.getAge() + o2.getAge());
        }).orElse(null);
        System.out.printf("将元素的每个字段都累加起来:%s%n", JSON.toJSONString(data));

    }

    private static void toCollection() {
        List<Data> collect = testList1.stream().collect(Collectors.toList());
        System.out.printf("toList:%s%n", JSON.toJSONString(collect));
        ArrayList<Data> collect1 = testList1.stream().collect(Collectors.toCollection(() -> new ArrayList<>(testList1.size())));
        System.out.printf("ArrayList:%s%n", JSON.toJSONString(collect1));
        LinkedList<Data> collect2 = testList1.stream().collect(Collectors.toCollection(LinkedList::new));
        System.out.printf("LinkedList:%s%n", JSON.toJSONString(collect2));
        HashSet<Data> collect3 = testList1.stream().collect(Collectors.toCollection(() -> new HashSet<>()));
        System.out.printf("HashSet:%s%n", JSON.toJSONString(collect3));
        LinkedHashSet<Data> collect4 = testList1.stream().collect(Collectors.toCollection(() -> new LinkedHashSet<>()));
        System.out.printf("LinkedHashSet:%s%n", JSON.toJSONString(collect4));

        List<Data> collect5 = testList1.stream().collect(Collectors.toList());
        System.out.printf("toList:%s%n", JSON.toJSONString(collect5));
        Set<Data> collect6 = testList1.stream().collect(Collectors.toSet());
        System.out.printf("toSet:%s%n", JSON.toJSONString(collect6));
    }

    private static void toMap() {
        Map<Long, Data> collect = testList1.stream().collect(Collectors.toMap(Data::getId, e -> e));
        System.out.printf("toMap:%s%n", JSON.toJSONString(collect));
        Map<Long, Data> collect1 = testList1.stream().collect(Collectors.toMap(Data::getId, e -> e, (p, s) -> p));
        System.out.printf("collect1:%s%n", JSON.toJSONString(collect1));
        HashMap<Long, Data> collect2 = testList1.stream().collect(Collectors.toMap(Data::getId, e -> e, (p, s) -> p, () -> new HashMap<>((int) (testList1.size() / 0.75))));
        System.out.printf("collect2:%s%n", JSON.toJSONString(collect2));
    }

    private static void groupingBy() {
        Map<Long, List<Data>> collect = testList1.stream().collect(Collectors.groupingBy(Data::getId));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect));
        Map<Long, List<Data>> collect1 = testList1.stream().collect(Collectors.groupingBy(Data::getId, Collectors.toList()));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect1));
        Map<Long, LinkedList<Data>> collect2 = testList1.stream().collect(Collectors.groupingBy(Data::getId, Collectors.toCollection(LinkedList::new)));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect2));
        HashMap<Long, LinkedList<Data>> collect3 = testList1.stream().collect(Collectors.groupingBy(Data::getId, HashMap::new, Collectors.toCollection(LinkedList::new)));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect3));
        HashMap<Long, LinkedList<Data>> collect4 = testList1.stream().collect(Collectors.groupingBy(Data::getId, () -> new HashMap<>(64), Collectors.toCollection(LinkedList::new)));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect4));
        HashMap<Long, LinkedList<Data>> collect5 = testList1.stream().collect(Collectors.groupingBy(Data::getId, () -> new LinkedHashMap<>(64), Collectors.toCollection(LinkedList::new)));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect5));
        ConcurrentMap<Long, List<Data>> collect6 = testList1.stream().collect(Collectors.groupingByConcurrent(Data::getId));
        System.out.printf("groupingBy:%s%n", JSON.toJSONString(collect6));
    }

    private static void collectingAndThen() {
        List<Long> collect = testList1.stream().collect(Collectors.collectingAndThen(Collectors.toList(), e -> e.stream().map(Data::getId).collect(Collectors.toList())));
        System.out.printf("collectingAndThen:%s%n", JSON.toJSONString(collect));
    }

}