CODEWITHSUNDEEP
javadatabase
user1723326
user1723326

Reputation: 27

How to define a class with variable properties?

I'm making a database program. I want the user to be able to define their own columns, as many as they want. How would I then define each record in its class file?(Since the properties would be different user to user)

It's part of a school assignment-it's going to hold different scores and the likes for the teacher for different students they can add, but they will also be able to add a new assignment, test(a column) .

Upvotes: 1

Views: 306

Answers (5)

Reflex2java
Reflex2java

Reputation: 11

You actually dont need a class. Basically, in the Java world all classes needs to be compiled before they can be executed. So dynamically adding fields to a class based on property is not a good approach. Rather, I would recommend you to use collections. This can be achieved like: 

List<Map<String,Object>> data;

You can have the Map like column name as key and value of the column value. The keys can be configured in properties and data can be stored in any order.

Upvotes: 1

Damian Leszczyński - Vash
Damian Leszczyński - Vash

Reputation: 30865

In this type of task the main problem is how to store the configuration. You do not need to create a class for each structure modification.

You should divide your task into three.

  1. How to extend table definition ?
  2. How to run query against such table ?
  3. How to display result from query result ?

A1: How to extend a table ?

If you want to extend a table with column you should have some data to do that.

So you should get from the user:

  • Table name
  • Column name
  • Column type

Having those data you can easily, create an ALTER statement to add column.

A2: How to execute query on a dynamic table ?

We have two option here, clever or normal.

  • Normal is to store the user input into some table
  • Clever is to use database table informations.

Ist up to you how you will retrive those data what is importart is how would you store them in yor application. I suggest you to create some object that will map the table.

The result set

TABLE_A | COLUMN_1 | String
TABLE_A | COLUMN_2 | Integer
TABLE_A | COLUMN_3 | Boolean

Simplyfied code

UserTable userTable = new UserTable("TABLE_A");
  userTable.addColumn(new UserTableColumn("COLUMN_1",String.class));
  userTable.addColumn(new UserTableColumn("COLUMN_2",Integer.class));
  userTable.addColumn(new UserTableColumn("COLUMN_3",Boolean.class));

Having such structure you are abre to easily create a query against such table.

A3: How to manipulate data in dynamic table ?

This is the tricky part. There exists various of ways to do this. The less complicated is to create an object that have a key-value structure. where key is the column name and value is the value. What is important is to define the primary-key, the indentyfier that allow you in simple way to establish that this is new row so you should insert it or old one that need only update. 

public class TableRow {

 private Map<UserTableColumn,Object> rowData = new HashMap<UserTableColumn,Object>();

 public Object getValue(UserTableColumn column);
 public void setValue(UserTableColumn column, Object value);

}

I hope this short explanation meets your needs.

Good luck !

Upvotes: 0

Massimiliano Peluso
Massimiliano Peluso

Reputation: 26737

you need "something" that generates the classes against your Database table.

Have a look at Hibernate

There are different ways to do that:

http://www.hibernate.org/subprojects/tools.html

http://www.wikihow.com/Generate-Hibernate-Pojo-Classes-from-DB-Tables

Upvotes: 0

jn1kk
jn1kk

Reputation: 5102

You need this kind of database design, so you do not need to add new "columns" (class properties) or anything:

TEACHERS (Class called Teacher)
________
TeacherId
Name


STUDENTS (Class called Student)
________
StudentId
Name


ASSIGNMENTS (Class called Assignment)
___________
AssignmentId
TeacherId (REF)
Name


GRADES (Class called Grade)
______
AssignmentId (REF)
StudentId (REF)
Grade

You should be able to translate to a Java class design using Maps.

I am specifically giving DB design ("pseudocode") and not Java code since this is homework.

FYI: REF stands for reference (foreign key in DB language).

Upvotes: 2

artbristol
artbristol

Reputation: 32407

If you want to add and remove columns to a database while it's running, that usually indicates your design is wrong. Your 'columns' should probably correspond to rows in a single table.

Upvotes: 1

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