java jdk1.8 使用stream流进行list 分组归类

爱被打了一巴掌 2023-05-30 10:59 27阅读 0赞

import com.alibaba.fastjson.JSON;
    import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.Collectors;
    
    
    public class Foo{
        private String name;
        private String type;
        private Double typeValue;
        private Integer count;
    
        public Foo(String name, String type, Double typeValue, Integer count) {
            this.name = name;
            this.type = type;
            this.typeValue = typeValue;
            this.count = count;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public String getType() {
            return type;
        }
    
        public void setType(String type) {
            this.type = type;
        }
    
        public Double getTypeValue() {
            return typeValue;
        }
    
        public void setTypeValue(Double typeValue) {
            this.typeValue = typeValue;
        }
    
        public Integer getCount() {
            return count;
        }
    
        public void setCount(Integer count) {
            this.count = count;
        }
    
        @Override
        public String toString() {
            return "Foo{" +
                    "name='" + name + '\'' +
                    ", type='" + type + '\'' +
                    ", typeValue=" + typeValue +
                    ", count=" + count +
                    '}';
        }
    
        public static void main(String[] args) {
                List<Foo> fooList = new ArrayList<Foo>();
            fooList.add(new Foo("A","san",1.0,2))  ;
            fooList.add( new Foo("A","nas",13.0,1))  ;
            fooList.add(new Foo("B","san",112.0,3))  ;
            fooList.add(new Foo("C","san",43.0,5))  ;
            fooList.add(new Foo("B","nas",77.0,7))  ;
    
            List<List<Foo>> groupList = new ArrayList<>();
            fooList.stream()
                    .collect(Collectors.groupingBy(Foo::getName,Collectors.toList()))
                    .forEach((name,fooListByName)->{
                        groupList.add(fooListByName);
                    });
            System.err.print(groupList.get(0));     //一个索引一个栏目
    //        System.err.println(groupList);
            for (int i = 0; i<groupList.size(); i++){
    //            System.out.print(groupList.get(0));
            }
    //        System.out.println(JSON.toJSONString(groupList));
        }
    }

[Foo {
    	name = 'A', type = 'san', typeValue = 1.0, count = 2
    }, Foo {
    	name = 'A', type = 'nas', typeValue = 13.0, count = 1
    }]

**List为多列进行分组如下**

Map<String, List<DTO>> result = list
                    .stream()
                    .collect(Collectors.groupingBy(e ->
                            StrUtil.concat(true,
                                    StrUtil.toString(分组列1),
                                    StrUtil.toString(分组列2),
                                    StrUtil.toString(分组列3),
                                    StrUtil.toString(分组列4)
                            )));
    
      //以优先级来分组,先按分组列1，在按分组列2，在按分组列3，在按分组列4来进行分组
     //返回的是分组后的Key-Value,一个Key对应好一个分组好的List