Help on Object (casting)

Question

hi there :) ...
Let us assume the following line:
Operation op = new Intersection( new Union ( new Set(A ), new Set(B)),
new Union ( new Set(C ), new Set(D)));
Let us also assume that there exist Operation, Union, Intersection and Set classes. Then, as you can see, the previous line defines an Operation object, op, which consists of a conjunction of disjunctions.
Right, what I am wondering is how one can be able to define an create objects via String Sentences like the following example:
Operation setOperation;
setOperation = new SetSentence(" new Intersection( new Union ( new Set(A ), new Set(B)),
new Union ( new Set(C ), new Set(D))); ");
This is similar to Sql Statements on Java, such statements are defined and created via String sentences, so how one can do that?
Thank you in advance for your help
Jose

Answer 1

Operation op = new Intersection( new Union ( new Set(A), new Set(B)),
new Union ( new Set(C ), new Set(D))); Basically, given a String, you want to 'transform' or 'compile' that String into a sequence of your
predefined operations, like Union, Set, Intersection etc. right? If so, you can define a simple
'set-language'; the expression above can be represented by the String -- "(A+B)*(C+D)" where
the '+' operator represents the Union operationand the '*' operator represents the Intersection
operation. The identifiers A, B, C and D represent their corresponding 'Set(...)' operations.
Parsing an infix expression like the above is not very difficult, but it can be simplified even further.
The above expression can be written in postfix format as follows -- "A B + C D + *". As you can
see, no precedence rules nor parentheses are needed anymore. This String can be parsed
from left to right, using an operand stack --
A: push a new Set(A) on the stack
B: push a new Set(B) on the stack
+: pop two elements x and y from the stack and create a new Union(x, y), push it back on the stack
C: push a new Set(C) on the stack
D: push a new Set(D) on the stack
+: pop two elements x and y from the stack and create a new Union(x, y), push it back on the stack
*: pop two elements x and y from the stack and create a new Intersection(x, y); push it back on the stack,
When the entire string has been parsed (and evaluated as above), the top (and single) element on
the stack is the result of the expression. If, during evaluation, not enough elements are present on
the stack or, after evaluation, the stack doesn't contain one single element, the String contained a
syntax error.
kind regards,
Jos