java8Stream实现多聚合

水深无声 2022-05-16 06:11 332阅读 0赞

package com.kexin.aggregate;
    
    import java.util.*;
    import java.util.function.BiConsumer;
    import java.util.function.BinaryOperator;
    import java.util.function.Function;
    import java.util.function.Supplier;
    import java.util.stream.Collector;
    
    /**
     * @Author KeXin
     * @Date 2018/7/25 上午9:49
     **/
    public class AggregateCollector1 implements Collector<Student,List<Map<String,Double>>,List<Map<String,Double>>> {
    
    
        @Override
        public Supplier<List<Map<String, Double>>> supplier() {
            return () -> {
                List<Map<String,Double>> list = combiner().apply(new ArrayList<>(),null);
                return list;
            };
        }
    
        @Override
        public BiConsumer<List<Map<String, Double>>, Student> accumulator() {
            return (list, student) -> {
    
                Map<String,Double> map_sum = new HashMap<>();
                Map<String,Double> map_ave = new HashMap<>();
                Map<String,Double> map_count = new HashMap<>();
    
                if (list.size()==0) {
                    list.add(map_sum);
                    list.add(map_ave);
                    list.add(map_count);
                }
                map_sum = list.get(0);
                map_sum.put("sum",map_sum.getOrDefault("sum",0.0)+student.getGrade());
                list.set(0,map_sum);
    
                map_count = list.get(2);
                map_count.put("count",map_count.getOrDefault("count",0.0)+1);
                list.set(2,map_count);
    
                map_ave = list.get(1);
                map_ave.put("ave",map_sum.getOrDefault("sum",0.0)/map_count.get("count"));
                list.set(1,map_ave);
            };
        }
    
        @Override
        public BinaryOperator<List<Map<String, Double>>> combiner() {
            return (left, right) -> {
                if (left != null) {
                    return left;
                }
                return right;
            };
        }
    
        @Override
        public Function<List<Map<String, Double>>, List<Map<String, Double>>> finisher() {
            return res -> res;
        }
    
        @Override
        public Set<Characteristics> characteristics() {
            return Collections.emptySet();
        }
    }

package com.kexin.aggregate;
    import java.util.*;
    import java.util.stream.Collectors;
    
    /**
     * @Author KeXin
     * 使用java8 stream实现groupBy的多聚合函数
     * 继承Collector接口，自定义collector
     * @Date 2018/7/24 下午7:22
     **/
    public class Aggregate {
    
        public static void main(String[] args) {
            List<Student> studentList = new ArrayList<>();
            studentList.add(new Student("1","1",10));
            studentList.add(new Student("3","1",10));
            studentList.add(new Student("5","1",60));
    
            studentList.add(new Student("2","2",20));
            studentList.add(new Student("6","2",20));
    
            studentList.add(new Student("4","3",20));
    
            //单个
            System.out.println("---------单个聚合函数---------");
            System.out.println(studentList.stream().collect(Collectors.groupingBy(Student::getGlass,Collectors.counting())));
    
            //两个
            System.out.println("---------collectingAndThen两个聚合函数（stream次数多）---------");
            Map<String, AbstractMap.SimpleEntry<Double, Double>> map = studentList.stream()
                    .collect(Collectors.groupingBy(Student::getGlass,
                            Collectors.collectingAndThen(Collectors.toList(), list -> {
                                double sum = list.stream().mapToInt(Student::getGrade).sum();
                                double average = list.stream().collect(
                                        Collectors.averagingDouble(Student::getGrade));
                                return new AbstractMap.SimpleEntry<>(sum, average);
                            })));
    
            System.out.println(map);
    
            System.out.println("---------map1---------");
            //多个
            Map<String,List> map1 = studentList.stream()
                    .collect(Collectors.groupingBy(Student::getGlass,
                            Collectors.collectingAndThen(Collectors.toList(), list -> {
                                double sum = list.stream().mapToInt(Student::getGrade).sum();
                                double average = list.stream().collect(
                                        Collectors.averagingDouble(Student::getGrade));
                                double count = list.stream().count();
    
                                return Arrays.asList(sum,average,count);
                            }
                            )));
            System.out.println(map1);
    
    
            // 自定义collector，collector内指定groupBy元素
            System.out.println("---------map2---------");
            Map<String,List<Map<String,Double>>> map2 = studentList.stream()
                    .collect(new AggregateCollector());
            System.out.println(map2);
    
            // 自定义collector
            System.out.println("---------map3---------");
            Map<String, List<Map<String, Double>>> map3 = studentList.stream()
                    .collect(Collectors.groupingBy(Student::getGlass,new AggregateCollector1()));
            System.out.println(map3);
            System.out.println("---------取数据样例---------");
            System.out.println("1班："+map3.get("1"));
            System.out.println("1班总分："+map3.get("1").get(0).get("sum"));
        }
    
    }