Java8 Stream分组并排序
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Customer {
private String name;
private TypeEnum type;
private int age;
}
@Getter
public enum TypeEnum {
/**
* 第一
*/
First("甲类", 2),
Second("乙类", 12),
Three("丙类",0),
Four("丁类",3),
;
private String desc;
/**
* 优先级
*/
private int priority;
TypeEnum(String desc, int priority) {
this.desc = desc;
this.priority = priority;
}
}
public class GroupByTest {
@Test
public void test() {
List<Customer> customers = new ArrayList<>();
// 按照type 分组，组内 age 排序
customers.add(new Customer("张三", TypeEnum.First, 18));
customers.add(new Customer("赵六", TypeEnum.Second, 25));
customers.add(new Customer("李四", TypeEnum.Second, 22));
customers.add(new Customer("张一", TypeEnum.Three, 31));
customers.add(new Customer("王五", TypeEnum.Four, 45));
customers.add(new Customer("张二", TypeEnum.Four, 40));
// 如此分组，treeMap 默认排序会按照 TypeEnum 枚举顺序
TreeMap<TypeEnum, List<Customer>> treeMap = customers.stream().collect(Collectors.groupingBy(Customer::getType, TreeMap::new, Collectors.toList()));
System.out.println("默认排序：" + treeMap);
// 如果 treeMap 想要自定义排序，则可以这么做
TreeMap<TypeEnum, List<Customer>> treeMap2 = customers.stream().collect(Collectors.groupingBy(Customer::getType, GroupByTest::getCustomSortTreeMap, Collectors.toList()));
for (Map.Entry<TypeEnum, List<Customer>> entry : treeMap2.entrySet()) {
entry.getValue().sort(Comparator.comparing(Customer::getAge));
}
System.out.println("自定义排序:" + treeMap2);
// 按指定字段分组
Map<TypeEnum, List<Integer>> map3 = customers.stream().collect(Collectors.groupingBy(Customer::getType,
Collectors.mapping(Customer::getAge, Collectors.toList())));
System.out.println("按指定字段分组:" + map3);
TreeMap<TypeEnum, List<Customer>> treeMap4 = customers.stream().sorted(Comparator.comparing(Customer::getAge, Comparator.nullsLast(Integer::compareTo)))
.collect(Collectors.groupingBy(Customer::getType, TreeMap::new, Collectors.toList()));
System.out.println("排序后分组:" + treeMap4);
}
public static TreeMap<TypeEnum, List<Customer>> getCustomSortTreeMap() {
return new TreeMap<>(Comparator.comparingInt(TypeEnum::getPriority));
}