Java stream 分组操作

小灰灰 2024-03-23 13:13 43阅读 0赞

#### 根据单/多字段分组 ####

单字段分组可以直接使用指定字段，多字段分组则采用拼接Key的形式

单字段:

//单字段分组
    Map<String, List<User>> singleAttr = list.stream().collect(
                    Collectors.groupingBy(t -> t.getNativePlace()));
    //还可以写成简化版
    //Map<String, List<User>> collect = list.stream().collect(
    // Collectors.groupingBy(User::getNativePlace));

多字段：

//多字段分组
    Map<String, List<User>> multiAttr = list.stream().collect(
                    Collectors.groupingBy(t -> t.getNativePlace()+"_"+t.getAge()));

多层级：

//多级分组
    Map<String, Map<Integer, List<User>>> multiLevel = list.stream().collect(
            Collectors.groupingBy(
                    t -> t.getNativePlace(),
                    Collectors.groupingBy(t -> t.getAge())
            )
    );

#### 测试 ####

package test;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    import java.util.Optional;
    import java.util.stream.Collectors;
    
    public class Test {
        public static void main(String[] args) {
            ArrayList<User> list=new ArrayList<User>();
            list.add(new User(11,"a"));
            list.add(new User(12,"a"));
            list.add(new User(14,"c"));
            list.add(new User(14,"c"));
            //单字段分组
            Map<String, List<User>> singleAttr = list.stream().collect(Collectors.groupingBy(t -> t.getNativePlace()));
            //还可以写成简化版
            //Map<String, List<User>> collect = list.stream().collect(Collectors.groupingBy(User::getNativePlace));
            //多字段分组
            Map<String, List<User>> multiAttr = list.stream().collect(Collectors.groupingBy(t -> t.getNativePlace()+"_"+t.getAge()));
            //多级分组
            Map<String, Map<Integer, List<User>>> multiLevel = list.stream().collect(
                    Collectors.groupingBy(
                            t -> t.getNativePlace(),
                            Collectors.groupingBy(t -> t.getAge())
                    )
            );
        }
    }
    
    class User{
        private int age;
        private String nativePlace;
    
        public String getNativePlace() {
            return nativePlace;
        }
    
        public void setNativePlace(String nativePlace) {
            this.nativePlace = nativePlace;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        public User(int age,String nativePlace) {
            this.age = age;
            this.nativePlace=nativePlace;
        }
    }