Java Stream流程常用操作總結

package com.tonis.demo;

import com.tonis.demo.entity.User;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class StreamTest {
    public static void main(String[] args) {
        // stream實例例化
        // 數組
        String[] str = {"a", "b","c"};
        Stream<String> stream1 = Arrays.stream(str);
        Stream<String> stream4 = Stream.of(str);
        // list集合
        List<String> list = Arrays.asList("a", "b", "c");
        Stream<String> stream2 = list.stream();
        // set集合
        Set<String> set = new HashSet();
        Stream<String> stream3 = set.stream();

        //
        List<User> userList = new ArrayList<>();
        userList.add(new User(1,"陳晨", 20));
        userList.add(new User(2,"張曉霞", 15));
        userList.add(new User(3,"李書童", 40));

        // 常用api
        // foreach遍歷
        list.forEach(System.out::print);  //print abc
        list.stream().forEach(System.out::print); // print abc
        // filter
        list.stream().filter(e->e.equals("b")).forEach(System.out::print); // print b
        // sorted 默認升序
        Stream.of(1,9,6,2,7).sorted().forEach(System.out::print); //print 12679
        Stream.of(1,9,6,2,7).sorted((a,b)-> b-a).forEach(System.out::print); // print 97621
        userList.stream().sorted(Comparator.comparing(User::getAge)).forEach(u-> System.out.print(u.getName()+"\t"));
        userList.stream().sorted(Comparator.comparing(User::getAge).reversed()).forEach(u-> System.out.print(u.getName()+"\t"));
        // skip limit 從1開始
        // 取第一元素
        userList.stream().limit(1).forEach(e-> System.out.println(e.getName()));
        // 取第二個元素
        userList.stream().skip(1).limit(1).forEach(e-> System.out.println(e.getName()));
        // map
        List<String> userNameList = userList.stream().map(User::getName).collect(Collectors.toList());
        // map to 聚合操作
        // 聚合操作時有空數據需先使用filte過濾，否則會報空指針異常
        System.out.println(userList.stream().mapToInt(User::getAge).sum());
        IntSummaryStatistics summaryStatistics = userList.stream().mapToInt(User::getAge).summaryStatistics();
        System.out.println(summaryStatistics.getCount());
        System.out.println(summaryStatistics.getMax());
        System.out.println(summaryStatistics.getMin());
        System.out.println(summaryStatistics.getAverage());
        System.out.println(summaryStatistics.getSum());
        // reduce 歸約聚合
        Integer sum = userList.stream().map(User::getAge).reduce(0, (a,b)->a+b);
        // out print:    reduce規約：75
        System.out.println("reduce歸約："+sum);
        Integer sum2 = userList.stream().map(User::getAge).reduce((a,b)->a+b).get();
        // out print:    reduce歸約：75
        System.out.println("reduce歸約："+sum2);
        // flat map 合併集合
        List<List<String>> listOfLists = Arrays.asList(
                Arrays.asList("1", "2"),
                Arrays.asList("5", "6"),
                Arrays.asList("3", "4")
        );
        IntStream ss = listOfLists.stream().flatMapToInt(e-> e.stream().mapToInt(Integer::new));
        ss.forEach(System.out::print);
        // distinct
        // 對象按hashCode和equals去重
        userList.add(new User(3,"李書童", 40));
        userList.stream().distinct().collect(Collectors.toList());
        // peek
        System.out.println("test peek method:");
        /**
        * 1、對流中元素有操作時，peek方法纔會被調用
        * 2、peek方法主要用於調式，便於debug時查看流中的元素
        */
        // 不打印任何信息
        Stream.of(1,9,6,2,7).peek(System.out::print);
        // out print: 9 6 2 7
        Stream.of(1,9,6,2,7).filter(e- 
          >e!=1).peek(System.out::println).collect(Collectors.toList());
        System.out.println("test peek method: end");

        // 項目中常用的場景
        // list轉array
        System.out.println(Arrays.toString(Arrays.asList(1,2,3).toArray()));
        // list轉string
        // Collectors.joining() 默認空白拼接   結果：陳晨張曉霞李書童李書童
        System.out.println(userList.stream().map(User::getName).collect(Collectors.joining()));
        // 指定拼接串 結果：陳晨-張曉霞-李書童-李書童
        System.out.println(userList.stream().map(User::getName).collect(Collectors.joining("-")));
        // 自定義屬性去重
        userList.stream().filter(distinctByKey(b -> b.getName())).forEach(e-> System.out.println(e.getName()));
        userList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getName))), ArrayList::new)).forEach(e-> System.out.println(e.getName()));
        userList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(u->u.getName()+u.getAge()))), ArrayList::new)).forEach(e-> System.out.println(e.getName()));
        // 分組
        Map<Integer, List<User>> groupMap =  userList.stream().collect(Collectors.groupingBy(User::getAge));
        System.out.println(groupMap);
        // 集合操作   交集、並集、差集
        // https://blog.csdn.net/weixin_41835612/article/details/83713891
    }

    static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}