java 集合按照属性进行排序对于属性为空进行处理

1、实体类如下：

import lombok.Data;
import lombok.NoArgsConstructor;
import java.time.LocalDate;

@Data
@NoArgsConstructor
public class Student {

   private String name;

   private Integer age;
   
   private LocalDate date;
   
   public Student(String name, Integer age) {
      this.name = name;
      this.age = age;
   }
   
   public Student(String name, Integer age, LocalDate date) {
      this.name = name;
      this.age = age;
      this.date = date;
   }
   
   @Override
   public String toString() {
      return "Student{" +
            "name='" + name + ''' +
            ", age=" + age +
            ", date=" + date +
            '}';
   }
}

2、构造集合

public static List<Student> initData() {
   List<Student> students =CollUtil.newArrayList();
   students.add(new Student("1",21));
   students.add(new Student("2",null));
   students.add(new Student("3",null));
   students.add(new Student("4",24));
   students.add(new Student("5",null));
   students.add(new Student("6",25));
   students.add(new Student("7",null));
   return students;
}

3、对于student 集合按照 年龄 age 进行排序

3.1 使用java8流进行排序

List<Student> students = initData();
students.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
System.out.println(students);

输出如下：提示空指针异常

4、解决属性为空时异常

4.1 传递排序规则，使 null 排在后面

// null 排在后面
students = students.stream().sorted(Comparator.comparing(Student::getAge, (a,b)->{
   if (a == null) {
      return (b == null) ? 0 : 1;
   } else if (b == null) {
      return  -1;
   } else {
      return a - b ;
   }
   
})).collect(Collectors.toList());

System.out.println(students);

4.2 使用 Comparator.nullsLast

// null 排在后面
students = students.stream().sorted(Comparator.comparing(Student::getAge,
      Comparator.nullsLast(Integer::compareTo))).collect(Collectors.toList());

System.out.println(students);

4.3 使用 Hutool CollUtil.sortByProperty

//默认空排在了前面
students = CollUtil.sortByProperty(students,"age");
System.out.println(students);

4.4 使用Hutool CompareUtil工具

CollUtil.sort(students,(s1,s2) -> CompareUtil.compare(s1.getAge(),s2.getAge(),false));
System.out.println(students);