基于Java的表达式评估器

Question

I need an Expression Evaluator that can allow me to evaluate an expression such as follows:

(ItemWeight + PackageWeight) * 2

So, given the following Input:

ItemWeight = new Weight(2.0, LBS);
PackageWeight = new Weight(0.2, LBS);

Output would be: Weight(4.4, LBS)

public class Weight {
   private final float value;
   private final Unit unit;

   public float getValue() { return value; }
   public Unit getUnit() { return unit; }

   public enum Unit {
       LB, KG, GRAMS;
   }
}

Similarly, I'd like to add/subtract two Amount objects (where amount is made up of a value and currency symbol).

Note: It is OK in my use case to assume that two values that do not have the same unit cannot be added/subtracted, etc

I read about MVEL, but it didn't seem like MVEL would be able to handle arithmetic expressions involving POJOs. Other options that came to mind are Rhino and Commons EL.

What would be a good library that I can use (and if needed extend) for solving this problem?

Thanks!

Answer 1

Java isn't C++; you can't overload operators.

It might not be quite as visually appealing to do it using a fluent interface, but I would say it's easier than what you're proposing.

This is interesting, because you're going to have to think about more than just EL. The idea is far more general than your Weight class. It's more like QuantityWithUnits . Here are a few of the questions you'll have to answer:

1. How will you prohibit addition and subtraction operations on objects with differing units?
2. How will you account for creating new units when you divide and multiply?
3. Will you allow scalar multiplication and division?
4. Will you disallow addition of scalars to quantities with units? Or will you silently create new objects with like units behind the scenes?
5. Will you have other common physics operations like powers?
6. How will you work with systems of units and conversions?

You may think "I'm too clever for all that; I just want to do something 'simple and practical'", but eventually you'll have to answer all these.