compareTo（）方法：a.compareTo(b),如果a<b return -1,如果 a>b return 1   在下列的排序中，如果a<b,则返回-1，正常递增排序，如果a>b，返回1，则a和b的位置替换下。

public class EqualsAndHashCode {
    private List<Student>list;
    @Test
    public void testCompare(){
        System.out.println("list = " + list);
        list.sort((Student o1,Student o2)->{
            /**规则：返回值<0  o1,o2....正常递增顺序
             * 当o1==null时，o2!=0时，如果我希望null在前面，则值<0
             * 当o1!=null,o2==null,时，null在前面，则让值>0,反之亦然*/
             /**正常递增顺序写法1*/
             // o1==null?(o2==null?0:-1):(o2==null?1:o1.getAge()-o2.getAge());
             // o1==null?(o2==null?0:Integer.MIN_VALUE):(o2==null?Integer.MAX_VALUE:o1.getAge().compareTo(o2.getAge()))
              if(o1==o2) return 0;
              if(o1==null){
                  return -1;
              }else if(o2==null){
                  return 1;
              }else {
                  return o1.getAge()-o2.getAge();
              }
        });
        System.out.println("排序后===========================排序后");
        System.out.println(list);

    }

    @Before
    public void init(){
        list=new LinkedList<>();
        list.add(null);
        list.add(new Student(0,"第零",new User("baiuser")));
        list.add(new Student(1,"第一",new User("baiuser")));
        list.add(null);
        list.add(new Student(2,"第二",new User("zhaowen")));
        list.add(new Student(4,"第三",new User("wangmancang")));
        list.add(new Student(3,"第四",new User("wangmancang")));
        list.add(null);
    }
}
@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
class Student{
    private Integer age;
    private String name;
    private User user;
}
@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
class User{
    private String userName;
}

数据输出：

list = [null, Student(age=0, name=第零, user=User(userName=baiuser)), Student(age=1, name=第一, user=User(userName=baiuser)), null, Student(age=2, name=第二, user=User(userName=zhaowen)), Student(age=4, name=第三, user=User(userName=wangmancang)), Student(age=3, name=第四, user=User(userName=wangmancang)), null]
排序后===========================排序后
[null, null, null, Student(age=0, name=第零, user=User(userName=baiuser)), Student(age=1, name=第一, user=User(userName=baiuser)), Student(age=2, name=第二, user=User(userName=zhaowen)), Student(age=3, name=第四, user=User(userName=wangmancang)), Student(age=4, name=第三, user=User(userName=wangmancang))]

方法二：

使用 Collections.sort方法排序，缺陷，当this对象为null时，会报空指针异常

public class EqualsAndHashCode {
    private List<Student>list;
    @Test
    public void testCompare(){
        Collections.sort(list);
        System.out.println(list);
    }

    @Before
    public void init(){
        list=new LinkedList<>();
        list.add(null);
        list.add(new Student(0,"第零",new User("baiuser")));
        list.add(new Student(1,"第一",new User("baiuser")));
        list.add(new Student(2,"第二",new User("zhaowen")));
        list.add(new Student(4,"第三",new User("wangmancang")));
        list.add(new Student(3,"第四",new User("wangmancang")));
       // list.add(null);
    }
}
@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
class Student implements Comparable<Student>{
    private Integer age;
    private String name;
    private User user;

    @Override
    public int compareTo(Student o) {
        if(this==o){
            return 0;
        }else if(null==this){
            return -1;
        }else if(null==o){
            return 1;
        }else {
           // System.out.println();
            return age-o.age;
        }

    }
}
@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
class User{
    private String userName;
}

解决方法：在方法中直接指定比较器，compare方法可以去掉compareTo（）方法的弊端

 @Test
    public void testCompare(){
        Collections.sort(list,(Student o1,Student o2)->{
            return o1==null?(o2==null?0:-1):(o2==null?1:o1.getAge()-o2.getAge());
        });
        System.out.println(list);
    }