Whats the right way to implement a custom expression evaluator in Java?

Question

I have a map that contains a number of properties e.g. "a", "b", "c" ...

I want to define a template where i can evaluate an expression such as,

"a" && "b" && !"c" to mean the following,

true if

"a" is in the map, "b" is in the map but "c" is not in the map 

false otherwise

Whats a way to implement this in Java? Does JUEL help?

Edit:

To make this clear I need to create a configurable language where you could define any kind of expressions in a configuration file that will need to be evaluated at runtime.

For e.g. I need my java code to parse a file that could contain any expressions such as,

"a" && "b"

!("a" && "d")

I don't know what expressions will need to be evaluated at compile time. Hope this makes the requirement more clear.

Answer 1

Whats a way to implement this in Java?

I would use something like Javacc or Antlr:

• read the tutorials for the tool, and study the examples (especially if you have never learned / been taught about grammars and parser generators before)
• write a grammar for your simple language
• add "actions" to the grammar to evaluate the expressions on the fly
• generate the Java classes

You will have a learning curve, but this should be an extremely simple / non-problematic grammar to implement.

Does JUEL help?

Probably not. For a start, anything based on JUEL would accept full EL syntax.

Answer 2

The basic techniques are recursive descent, Dijkstra Shunting-yard algorithm, or parser generation via any of quite a number of systems. If you only need to handle parentheses, 'not', 'and', and 'or', I wouldn't go beyond recursive descent myself.