stream流的常用操作

梁霖编程工具库 2023-05-16

325

reduce()操作

定义：前一个操作的结果会传递给下一个操作继续执行，即前一轮reduce操作的结果会作为参数传递给下一轮reduce的参数‍‍‍‍‍
使用场景：对集合求和，求集合的最大值，求集合的最小值。

public class Main {


    public static void main(String[] args) throws Exception {
    
        int sum = Stream.of(1, 2, 3, 4, 5).reduce((item1, item2) -> item1 + item2).get();
        System.out.println(sum);


//       int value =  Stream.of(1, 2, 3, 4, 5).reduce(new BinaryOperator<Integer>() {
//            @Override
//            public Integer apply(Integer integer, Integer integer2) {
//                return integer+integer2;
//            }
//        }).get();
//
//        System.out.println(value);




//        int value = Stream.of(1, 2, 3, 4, 5).reduce(100,(item1,item2)->item1+item2);
//        System.out.println(value);


        int value = Stream.of(1645, 234345, 32, 44434, 564534, 435, 34343542, 212)
                .reduce((item1, item2) -> item1 > item2 ? item1 : item2).get();
        System.out.println(value);




    }
}

groupingBy操作

定义：对一个流的划分不同的分组，并返回这个分组的key，类似于mysql中的分组概念。‍‍‍‍‍
适用场景：对于一个流进行多分类时使用，返回的是分类的key

public class Main {


    public static void main(String[] args) throws Exception {
        List<Student> students = Arrays.asList(new Student("广东", 23), new Student("广东", 24),
                new Student("广东", 23),
                new Student("北京", 22), new Student("北京", 20),
                new Student("北京", 20), new Student("海南", 25));
        //根据省份进行划分，注意怎么看返回类型！！！！
        Map<String,List<Student>> listMap =  students.stream().collect(Collectors.groupingBy(obj->obj.getProvince()));
        listMap.forEach((key,value)->{
            System.out.println("=======");
            System.out.println(key);
            value.forEach(obj->{
                System.out.println(obj.getAge());
            });


        });


    }
}




class Student {
    private int age;


    private String province;


    public Student(String province, int age) {
        this.province = province;
        this.age = age;
    }


    public Student() {
    }


    public int getAge() {
        return age;
    }


    public void setAge(int age) {
        this.age = age;
    }


    public String getProvince() {
        return province;
    }


    public void setProvince(String province) {
        this.province = province;
    }
}

partitionBy操作

定义：将一个流分为两个分组，注意只能partitionBy只能将一个流中的数据分为两个分组。
适用场景：对一个流的操作需要分为两类，并且者两类之间有明显的界限

/**
 * partitioningBy():将条件为true的分为一组集合，条件为false的分为一组集合
 * {false=[java, CSS3], true=[springboot, HTML5]}
 */
public class Main {


    public static void main(String[] args) throws Exception {


        List<String> list = Arrays.asList("java", "springboot", "HTML5", "CSS3");
        
        Map<Boolean, List<String>> result = list.stream().collect(Collectors.partitioningBy(obj -> obj.length() > 4));


        System.out.println(result);


    }
}

jion操作

定义：将stream流中的元素用指定的连接符连接起来并返回一个字符串

public class Main {


    public static void main(String[] args) throws Exception {


         List<String> list = Arrays.asList("springboot教程","springcloud教程","java教程","架构教程");
         String result1 = list.stream().collect(Collectors.joining());
         System.out.println(result1);


        String result2 = list.stream().collect(Collectors.joining("||"));
        System.out.println(result2);


        String result3 = list.stream().collect(Collectors.joining("||","[","]"));
        System.out.println(result3);




        List<String> list1 = Arrays.asList("springboot","mysql","html5","css3");
        System.out.println(list1);
        //快速创建一个带分隔符带string
        String result = Stream.of("springboot","mysql","html5","css3").collect(Collectors.joining(",","[","]"));
        System.out.println(result);


    }
}

summarizing操作

定义：一个方法把统计学上的功能都完成，包括求平均值，求最大值，求数量，求和等等
分类：summarizingInt、 summarizingLong 、summarizingDouble

public class Main {


    public static void main(String[] args) throws Exception {
        List<Student> students = Arrays.asList(new Student("广东", 23), new Student("广东", 24),
                new Student("广东", 23),
                new Student("北京", 22), new Student("北京", 20),
                new Student("北京", 20), new Student("海南", 25));


        IntSummaryStatistics summaryStatistics = students.stream().collect(Collectors.summarizingInt(Student::getAge));


        System.out.println("平均值"+summaryStatistics.getAverage());
        System.out.println("人数"+summaryStatistics.getCount());
        System.out.println("最大值"+summaryStatistics.getMax());
        System.out.println("最小值"+summaryStatistics.getMin());
        System.out.println("总和"+summaryStatistics.getSum());
        
    }
}




class Student {
    private int age;


    private String province;
    
    public Student(String province, int age) {
        this.province = province;
        this.age = age;
    }


    public Student() {
    }


    public int getAge() {
        return age;
    }


    public void setAge(int age) {
        this.age = age;
    }


    public String getProvince() {
        return province;
    }


    public void setProvince(String province) {
        this.province = province;
    }
}

流式操作的总结

普通集合通过stream()转为流
通过filter()/map()来将筛选/转换流上的每一个元素
通过collect()来收集流

比如收集统计信息/收集分类信息/仅仅只是收集为一个新的list或map或set

‍

string

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stream流的常用操作

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