Question

I want to develop a java application where a user should be able to add calculation items depending on each other. Consider this scenario:

• Add an item (A) with a value lets say 50
• Add another item (B) which is item A*5
• Add item (C) which has a value of (A*50%)+B
• Delete item (A) fails as long it is used in other items.

I can store the equations as varchar in the database and parse it back in java when I recall using equation parser or something similar. But when I tried this, the problem was what If item A is deleted? The database wont know if the item is linked to another item if I use a varchar. I want the items to be linked to each other using foreign keys, how can I achieve that?

Sorry if this is a duplicate question, but I could not find a proper answer that solves my problems.

Answer 1

You can use the tree java structure to represent the expression. each node will represent an operator and 2 operands. The concrete values will be at the leaf level.
The calculation would be a simple in order walk on the tree.

In the DB you will need a representation of the expression (tree) pointing to all its components (tree nodes).

(Or use alternative expression tree DB representation)

           EXP1:
        OPERATOR:+
          /    \
         /      \
        /        \
   OPERAND:1   OPERAND:2

the DB

expression entries table:

| ID     | NAME    |  DEPENDS_ON_NODE_ID  |
| 1      | EXP1    |  1                   |
| 2      | EXP1    |  2                   |
| 2      | EXP1    |  3                   |

nodes table:

| ID     | NAME     |  TYPE         |  VALUE |
| 1      | NODE1    |  OPERATOR     |   +    |
| 2      | NODE1    |  OPERAND      |   1    |
| 3      | NODE1    |  OPERAND      |   2    |