java Class hashCode() function , Whether the comparison class is the same .
Want to see hashCode() Function comparison class , You can see the catalogue directly 3.

1. The problem background

Recently, I'm writing a database final assignment , Make a Web project . What I do is an online classroom reservation project . Based on the ruoyi framework 、echarts Written .
There are two tables —— Classroom schedule 、 Classroom appointment form .
There are the following entities :
classroom classroom.class

// Classroom number  
private String classroom_id  ;
// Capacity 
private Int classroom_accept_number;

Classroom reservation classroomAppointment.class

// Classroom number  
  private String classId;
// Time of appointment  yyyy-MM-dd
  private Date appointmentDataTime;
// Appointment session  
  private String appointmentUserClass;

notes ： A classroom in a day There are five time periods to choose ——{“12”, “34”, “56”, “78”, “90”}.appointmentUserClass = “12” Hour represents appointment 12 lesson

If The classroom number is 100 My classroom 12 section 2022 year 1 month 1 Number Made an appointment , When the user queries 2022 year 1 month 1 Number The number is 100 classroom when , Users can only see “34”, “56”, “78”, “90” You can make an appointment .

2. How to get rid of it Information that has been reserved

My idea of booking a classroom is , According to the user's choice Time 、 Classroom number , Find out the classroom information , Add to classroomAppointment.class, Plus Section , Store in List< classroomAppointment > in , such 2022 year 1 month 1 Number The classroom number is 100 Of the following ：

[[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 12
],
[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 34
],
[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 56
],[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 78
],
[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 90
]]

Then query the reservation table , The information in the classroom number in this date .

[ 
classId=100
appointmentDataTime = 2022-01-01
appointmentUserClass = 12
]

3. rewrite hashCode() function

 @Override
    public int hashCode() {
    
        int result = appointmentDataTime != null?appointmentDataTime.hashCode():0;
            result = 31*result+(appointmentUserClass !=null ?appointmentUserClass.hashCode():0);
        return result;
    }

Only two classes need to be judged appointmentDataTime And appointmentUserClass Whether these two attributes are the same , To determine whether the two classes are the same .