java8 stream 的项目中的使用 经典案例!java新手必读

文章目录

  • 前言
  • 1. list T为一个对象,取出某一个字段的集合 (map)
  • 2. list T为一个对象,取出某一个字段,判断该字段值,进行分类 (filter)
  • 3. 根据 list 获取 list t对象 (map)
  • 4. 根据list T对象某个字段进行归类
  • 5 对 list 在的T进行替换,想对哪个字段替换就对哪个字段替换(peek)
  • 6 排序,对list某个字段进行排序,但是只对基本数据类型有效
  • 7 遍历List后,向某个字段进行赋值
  • 参考文章

前言

流的使用包括三件事:
数据源,集合
中间操作,流水线
终端操作,执行流水线,生成结果

Stream 不是集合元素,它不是数据结构并不保存数据,它是有关算法和计算的,它更像一个高级版本的 Iterator。

获取一个数据源(source)→ 数据转换 → 执行操作获取想要的结果

1. list T为一个对象,取出某一个字段的集合 (map)

        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("123456").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(2L).username("2哥").password("123456").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(3L).password("123456").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("123456").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        //list取出id列
        List<Long> collect = list.stream().map(DxUser::getId).collect(Collectors.toList());
        collect.forEach(System.out::println);

        //list取出username列
        List<String> collect1 = list.stream().map(DxUser::getUsername).collect(Collectors.toList());
        collect1.forEach(System.out::println);

        //1
        //2
        //3
        //4
        //1哥
        //2哥
        //null
        //4哥

2. list T为一个对象,取出某一个字段,判断该字段值,进行分类 (filter)

        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("123456").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(2L).username("2哥").password("123456").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(3L).password("123456").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("123456").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        //找出id大于2L的
        List<DxUser> collect = list.stream().filter(x -> x.getId() > 2).collect(Collectors.toList());

        collect.forEach(System.out::println);

        //大于2L 和 小于等于2进行 分组
        Map<Boolean, List<DxUser>> collect1 = list.stream().collect(Collectors.groupingBy(x -> x.getId() > 2));
        System.out.println("===大于2L==");
        collect1.get(true).forEach(System.out::println);
        System.out.println("===小于等于2L==");
        collect1.get(false).forEach(System.out::println);

3. 根据 list 获取 list t对象 (map)

        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("123456").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(2L).username("2哥").password("123456").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(3L).password("123456").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("123456").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        List<Long> ids = Arrays.asList(2L, 3L, 4L);

        List<DxUser> dxUsers = ids.stream().map((x) -> {
                
        //     for (DxUser dxUser : list) {
        //         if(dxUser.getId().equals(x)){
        //             return dxUser;
        //         }
        //     }
        //     return null;

                List<DxUser> dxUserList = list.stream().filter(y -> y.getId().equals(x)).collect(Collectors.toList());
                return dxUserList.get(0);

        } ).collect(Collectors.toList());

        dxUsers.forEach(System.out::println);

4. 根据list T对象某个字段进行归类

  • 4.1 有几类就分几类 (Collectors.groupingBy)
  • 4.2 分为两类,true为符合条件,false不符合条件 (Collectors.partitioningBy)
        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("123456").department("java开发").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(2L).username("2哥").password("123456").department("c#开发").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(3L).password("123456").department("c++开发").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("123456").department("java开发").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        //用部门进行分类,以上看出,分出三类
        Map<String, List<DxUser>> collect = list.stream().collect(Collectors.groupingBy(DxUser::getDepartment));

        collect.forEach((x,y)->{
            System.out.println("x: " + x);
            System.out.println("y: size:"+ y.size() + y);
        });

        //x: c++开发
        //y: size:1[DxUser(id=3, username=null, password=123456,department=c++开发)]
        //x: c#开发
        //y: size:1[DxUser(id=2, username=2哥, password=123456,department=c#开发)]
        //x: java开发
        //y: size:2[DxUser(id=1, username=1哥, password=123456,department=java开发), DxUser(id=4, username=4哥, password=123456,department=java开发)]
        System.out.println("==========");

        Map<Boolean, List<DxUser>> collect1 = list.stream().collect(Collectors.partitioningBy(x -> x.getDepartment().equals("java开发")));
        collect1.forEach((x,y)->{
            System.out.println("x: " + x);
            System.out.println("y: size:"+ y.size() + y);
        });

        //x: false
        //y: size:2[DxUser(id=2, username=2哥, password=123456 department=c#开发), DxUser(id=3, username=null, password=123456 department=c++开发)]
        //x: true
        //y: size:2[DxUser(id=1, username=1哥, password=123456 department=java开发), DxUser(id=4, username=4哥, password=123456 department=java开发)]
    

5 对 list 在的T进行替换,想对哪个字段替换就对哪个字段替换(peek)

        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("123456").department("java开发").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(2L).username("2哥").password("123456").department("c#开发").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(3L).password("123456").department("c++开发").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("123456").department("java开发").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        //把字段进行替换。username+(?),loginname="登录名"
        List<Object> collect = list.stream().peek((x) -> {x.setUsername(x.getUsername() + "(?)");x.setLoginname("登录名");}).collect(Collectors.toList());
        collect.forEach(System.out::println);

6 排序,对list某个字段进行排序,但是只对基本数据类型有效

  • 6.1 正序 (sorted)
  • 6.2 倒序
  • 6.3 排序后再分组 (sorted,groupingBy)
        DxUser dxUserBuilder = DxUser.builder().id(1L).username("1哥").password("3").department("java开发").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(-2L).username("2哥").password("4").department("c#开发").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(13L).password("13").department("c++开发").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).username("4哥").password("-1").department("java开发").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        //按照id进行排序
        List<DxUser> collect = list.stream().sorted(Comparator.comparing(DxUser::getId)).collect(Collectors.toList());
        collect.forEach(System.out::println);
        System.out.println("-------------------");
        //倒序
        List<DxUser> collect1 = list.stream().sorted(Comparator.comparing(DxUser::getId).reversed()).collect(Collectors.toList());
        collect1.forEach(System.out::println);
        System.out.println("-------------------");
        //分组后再排序
        Map<String, List<DxUser>> collect2 = list.stream().sorted(Comparator.comparing(DxUser::getId).reversed()).collect(Collectors.groupingBy(DxUser::getDepartment));

        collect2.forEach((x,y)->{
            System.out.println("x: " + x);
            System.out.println("y: size:"+ y.size() + y);
        });

7 遍历List后,向某个字段进行赋值

        DxUser dxUserBuilder = DxUser.builder().id(1L).status(32).username("1哥").password("3").department("java开发").build();
        DxUser dxUserBuilder1 = DxUser.builder().id(-2L).status(12).username("2哥").password("4").department("c#开发").build();
        DxUser dxUserBuilder2 = DxUser.builder().id(13L).status(10).password("13").department("c++开发").build();
        DxUser dxUserBuilder3 = DxUser.builder().id(4L).status(33).username("4哥").password("-1").department("java开发").build();

        List<DxUser> list = new ArrayList<>();

        list.add(dxUserBuilder);
        list.add(dxUserBuilder1);
        list.add(dxUserBuilder2);
        list.add(dxUserBuilder3);

        DxRole dxRole = DxRole.builder().sn("加入成功").build();

        List<DxUser> collect = list.stream().peek(x -> {
            if ("13".equals(x.getPassword())) {
                x.setRole(dxRole);
            }
        }).collect(Collectors.toList());

        collect.forEach(System.out::println);

参考文章

https://www.cnblogs.com/owenma/p/12207330.html

Student s1 = new Student("aa", 10,1);
Student s2 = new Student("bb", 20,2);
Student s3 = new Student("cc", 10,3);
List<Student> list = Arrays.asList(s1, s2, s3);
  
//装成list
List<Integer> ageList = list.stream().map(Student::getAge).collect(Collectors.toList()); // [10, 20, 10]
  
//转成set
Set<Integer> ageSet = list.stream().map(Student::getAge).collect(Collectors.toSet()); // [20, 10]
  
//转成map,注:key不能相同,否则报错
Map<String, Integer> studentMap = list.stream().collect(Collectors.toMap(Student::getName, Student::getAge)); // {cc=10, bb=20, aa=10}
  
//字符串分隔符连接
String joinName = list.stream().map(Student::getName).collect(Collectors.joining(",", "(", ")")); // (aa,bb,cc)
  
//聚合操作
//1.学生总数
Long count = list.stream().collect(Collectors.counting()); // 3
//2.最大年龄 (最小的minBy同理)
Integer maxAge = list.stream().map(Student::getAge).collect(Collectors.maxBy(Integer::compare)).get(); // 20
//3.所有人的年龄
Integer sumAge = list.stream().collect(Collectors.summingInt(Student::getAge)); // 40
//4.平均年龄
Double averageAge = list.stream().collect(Collectors.averagingDouble(Student::getAge)); // 13.333333333333334
// 带上以上所有方法
DoubleSummaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(Student::getAge));
System.out.println("count:" + statistics.getCount() + ",max:" + statistics.getMax() + ",sum:" + statistics.getSum() + ",average:" + statistics.getAverage());
  
//分组
Map<Integer, List<Student>> ageMap = list.stream().collect(Collectors.groupingBy(Student::getAge));
//多重分组,先根据类型分再根据年龄分
Map<Integer, Map<Integer, List<Student>>> typeAgeMap = list.stream().collect(Collectors.groupingBy(Student::getType, Collectors.groupingBy(Student::getAge)));
  
//分区
//分成两部分,一部分大于10岁,一部分小于等于10岁
Map<Boolean, List<Student>> partMap = list.stream().collect(Collectors.partitioningBy(v -> v.getAge() > 10));
  
//规约
Integer allAge = list.stream().map(Student::getAge).collect(Collectors.reducing(Integer::sum)).get(); //40

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