java8 新特性3 Stream的操作案例2

Posted 2022-11-28 健康平安的活着

一 案例

数据：

public class StudentDb 
  public  static List<Student> stuList=new ArrayList<>();
    static
        stuList.add(new Student("beijing",23,90.34,"2021-10-04"));
        stuList.add(new Student("tianjin",34,30.34,"2021-09-04"));
        stuList.add(new Student("shanghai",56,45.24,"2022-10-04"));
        stuList.add(new Student("tianjin",89,70.89,"2021-06-03"));
        stuList.add(new Student("shenzhen",95,95.89,"2022-10-03"));
        stuList.add(new Student(45,89.89,"2022-07-03"));
    

1.1 查询出生为2021年，且按分数进行排序

        List<Student> studentList=StudentDb.stuList;
        studentList.stream()
                .filter( (e)->  return e.getBirthday().split("-")[0].equals("2022");)
                .sorted((t1,t2)-> Double.compare(t1.getScore(),t2.getScore())).
        forEach((x)->System.out.println(x););
         System.out.println("==========");

1.2 求出有多少个不同的学生名字

 studentList.stream().map((e)->return e.getName();).distinct().forEach((e)->System.out.println("e:"+e););

1.3 将姓名进行拼接出一个字符串

        System.out.println("===============");
      String s=  studentList.stream().map((x)->return x.getName()==null?"":x.getName();).reduce(",",String::concat);
      System.out.println("s:"+s);

1.4 查询姓名是否包含shenzhen

        boolean f=studentList.stream().anyMatch((e)->return e.getName().equals("shenzhen"););
        System.out.println("f:"+f);

1.5 获取学生的总分数和

        System.out.println("===============");
       Optional<Double> total= studentList.stream().map((x)->return x.getScore();).reduce(Double::sum);
       System.out.println("total:"+ total.get());

1.6 求5大聚合函数

       Optional<Double> max=   studentList.stream().map((e)->return e.getScore();).max(Double::compare);
        System.out.println("max:"+ max.get());
        Optional<Double> min=   studentList.stream().map((e)->return e.getScore();).min(Double::compare);
        Optional<Double> min2=   studentList.stream().map((e)->return e.getScore();).min((x1,x2)->return Double.compare(x1,x2););
        System.out.println("min:"+ min.get()+" min2:"+min2.get());
       long count=   studentList.stream().map((e)->return e.getScore();).count();
        System.out.println("count:"+count);
       //7.5大聚合函数
        DoubleSummaryStatistics lss = studentList.stream().collect(Collectors.summarizingDouble((x)->return x.getScore();));
        System.out.println("avg:"+lss.getAverage()+" max:"+lss.getMax()+" min:"+lss.getMin()+" sum:"+lss.getSum()+" count:"+lss.getCount());

  Optional<Double> op = studentList.stream().map(Student::getScore).reduce(Double::sum);
        System.out.println("op:"+op.get());

1.7 获取元素不为null个数据

              Instant start=Instant.now();
Long gshu= studentList.stream().filter((x)->return x.getName()!=null;).count(); ;
      System.out.println("geshu:"+gshu);
      //9.统计个数
        Optional<Integer> countx =studentList.stream().map((e) -> 1).reduce(Integer::sum);
        System.out.println(countx.get());
        Instant end=Instant.now();
        System.out.println("耗时:"+ Duration.between(start,end).toMillis());

1.8  将旧集合中某个属性放到新集合中

        List<String> nameList=new ArrayList<>();
        studentList.stream().forEach((e)->nameList.add(e.getName()););
        System.out.println("namelist:"+nameList);
        List<String> list = studentList.stream().map(Student::getName).collect(Collectors.toList());
        list.forEach(System.out::println);
        System.out.println("----------------------------------");

1.9  将list属性放到map集合中

        Map<String,Object> rMap=studentList.stream().collect(Collectors.toMap(x->x.getName(),x->x.getScore(), (entity1, entity2)-> 
            return entity2;
        ));
        rMap.forEach((k,v)->System.out.println("k:"+k+" v:"+v););

1.10  使用reduce求sum

     Optional<Double> op = studentList.stream().map(Student::getScore).reduce(Double::sum);
        System.out.println("op:"+op.get());

1.11 将map的key，value进行分别存储

  //13.将map的key，value进行分别存储
        Map<String,Integer> map = new HashMap<String, Integer>();
        map.put("ss",34);
        map.put("rt",56);
        List<String> keyList=new ArrayList<>();
        List<Integer> valueList=new ArrayList<>();
        map.keySet().stream().forEach((x)-> keyList.add(x););
        map.values().stream().forEach((x)-> valueList.add(x););
        System.out.println("kk:"+keyList+"  vv:"+valueList);