Stream流的常用方法大全

短命女 2024-03-31 13:43 23阅读 0赞

**目录**

前言

1. forEach遍历

2. filter过滤

3. distinct去重

4. limit截取

5. skip跳过

6.排序sorted

7.最值max,min

8. 统计reduce

9. List结构转换Map结构

10. List对象转List

11. List对象转List

总结

--------------------

## 前言 ##

毕业入职工作后开始接触Stream流，觉得挺方便的，尽可能全面的记录一下Stream流的方法。

> ## 1. forEach遍历 ##

forEach:该方法接收一个Consumer接口函数，将每一个流元素交给该函数处理  
forEach方法：用来遍历流中的数据  
 注：是一个终结方法，遍历之后就不能继续调用Stream流中的其他方法

public class Stream_ForEach {
        public static void main(String[] args) {
        //获取一个Stream流
        Stream<String>stream= Stream.of("张三","李四","王五","赵六");
            //使用Stream流的方法forEach对stream流中的数据遍历
            stream.forEach((String name)->{
                System.out.println(name);
            });
        }
    }

> ### 2. filter过滤 ###

filter：用于对Stream流中的数据进行过滤  
filter(Predicate<? super T>predicate)  
filter方法的参数Predicate是一个函数式接口，可以使用lambda表达式  
Predicate中的抽象方法  
boolean test（T t）

public class Stream_filter {
        public static void main(String[] args) {
            //创建一个Stream流
            Stream<String> stream = Stream.of("张三", "李四", "王五", "赵六1", "刘老七");
    
            //单条件过滤
            Stream<String> stream1 = stream.filter((String name) -> {
                return name.startsWith("刘");
            });
    
            //多条件过滤
            List<String> stream2 = stream.filter((String name) -> {
                if(name.length()>=3 && name.equals("刘老七")){
                    return true;
                }
                    return false;
            }).collect(Collectors.toList);
    
            //遍历stream1
            stream1.forEach((name)-> System.out.println(name));
            
            //输出stream2
            System.out.println(stream2)
            
        }
    }

> ## 3. distinct去重 ##

public class Stream_distinct {
        public static void main(String[] args) {
            //创建一个Stream流
            Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
            //去重
            List<String> stream1 = stream().distinct().collect(Collectors.toList());
            //输出stream1
            System.out.println(stream1)
        }
    }

> ## 4. limit截取 ##

limit:用于截取流中的元素  
limit可以对流进行截取，只取用前n个  
limit（long maxSize）;  
参数是一个long型，如果集合当前长度大于参数则进行截取，否则不进行操作  
limit是一个延迟方法，可以继续使用Stream流方法

public class Stream_limit {
        public static void main(String[] args) {
            //创建一个Stream流
            Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
            //去重
            List<String> list = stream().distinct().collect(Collectors.toList());
            //截取去重后的前2个元素
            list = list.stream().limit(2).collect(Collectors.toList();
            //输出stream1
            System.out.println(list)
        }
    }

> ## 5. skip跳过 ##

skip方法：用于跳过元素  
skip(long n)  
如果流的当前长度大于n，则跳过前n个，否则将会得到一个长度为0的空流

public class Stream_skip {
        public static void main(String[] args) {
            //创建一个Stream流
            Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
            //去重
            List<String> list = stream().distinct().collect(Collectors.toList());
            //跳过去重后的前2个元素
            list = list.stream().skip(2).collect(Collectors.toList());
            //输出stream1
            System.out.println(list)
        }
    }

> ## 6.排序sorted ##

public class Stream_sorted {
        public static void main(String[] args) {
            List<Test> list = new ArrayList<>();
            list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
    
            //根据年龄从大到小排序
            list = list.stream()
                       .sorted(Comparator.comparing(Test::getAge).reversed())
                       .collect(Collectors.toList());
            list.forEach(System.out::println);
    
    
        }
    }

> ## 7.最值max,min ##

public class Stream_max_min {
        public static void main(String[] args) {
            List<Test> list = new ArrayList<>();
            list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
            
            //获取年龄最大的人
            Test maxPeople = list.stream().max(Comparator.companing(Test::getAge)).get();
          
            //获取年龄最小的人
            Test minPeople = list.stream().min(Comparator.companing(Test::getAge)).get();
    
        }
    }

> ## 8. 统计reduce ##

public class Stream_reduce {
        public static void main(String[] args) {
            List<Test> testList = new ArrayList<Test>();
            testList.add(new Test("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            testList.add(new Test("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            testList.add(new Test("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));
    
            //统计年龄总和
            int totalAge =testList.stream().mapToInt(Test::getAge).sum();
    
            //统计工资总和
            BigDecimal totalSalary = testList.stream().map(Test::getSalary)
    
            //统计工资乘以各自系数的总和(向上保留两位)
            BigDecimal totalRatioSalary = testList.stream()
                                                  .map(s->s.getSalary()
                                                  .multiply(s.getRatio())
                                                  .setScale(2,BigDecimal.ROUND_HALF_UP))
                                                  .reduce(BigDecimal.ZERO,BigDecimal::add);
        }
    }

> ## **9. List结构转换Map结构** ##

public class Stream_List_Map {
        public static void main(String[] args) {
            List<Test> testList = new ArrayList<Test>();
            testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
            //根据姓名转map,map的key为name
            Map<String, Test> nameMap= testList.stream().collect(Collectors.toMap(Test::getName, Test -> Test);
            System.out.println(map);
        }
    }

> ## 10. List<Object>对象转List<String> ##

public class Stream_object_string {
        public static void main(String[] args) {
            List<Test> testList = new ArrayList<Test>();
            testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
            //获取姓名集合
            List<String> nameList = testList.stream().map(Test::getName()).collect(Collectors.toList());
            System.out.println("value:"+nameList);
        }
    }

> ## 11. List<Object>对象转List<Object> ##

public class Stream_object_object {
        public static void main(String[] args) {
            List<People> peopleList = new ArrayList<People>();
            peopleList.add(new People("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
            peopleList.add(new People("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
            peopleList.add(new People("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
            //对象转对象
            List<Student> studentList = peopleList.stream().map(s->{
                return new Student(s.getName(),s.getAge());
            }).collect(Collectors.toList());
            System.out.println("value:"+studentList);
        }
    }

> ## 总结 ##

略