Java - Dynamic Comparison with Primitive Data Types

Question

Friends,

We are writing a framework for a validation...

We do have a config file like below...

<root>
<property name="Premium">
    <xmlTag>//Message/Request/Product/Benefit/Premium/amount</xmlTag>
    <valueType>float</valueType>
    <validation condition=">" value="0">Premium Amount cannot be less than Zero.</validation>
</property>

I get the XML Value using XPath and convert it to float by <valueType> element value...

No, I do have value="0" also been converted to float.

Now, I do have to apply the condition which has been specified as condition=">".

I don't want to do this on IF ELSEIF....ELSE loop.

Is there any other way to convert "<" in to an operator < or use compare operator on a String?

In this way, my code will be simple and useful for future more operators.

=============================================================================

Thanks all for the suggestions and answers...

I have decided to use the BeanShell's bsh.Interpreter. It does the work for me...

sample code for you all...

        System.out.println(new bsh.Interpreter().eval("1 < 0"));
        System.out.println(new bsh.Interpreter().eval("1 > 0"));
        System.out.println(new bsh.Interpreter().eval("1 >= 0"));
        System.out.println(new bsh.Interpreter().eval("0 >= 0"));
        System.out.println(new bsh.Interpreter().eval("1 != 0"));
        System.out.println(new bsh.Interpreter().eval("0 != 0"));
        System.out.println(new bsh.Interpreter().eval("1 == 0"));
        System.out.println(new bsh.Interpreter().eval("0 == 0"));

returned me true/false.

Thanks & Good luck...

You don't have to use `if-then-else` chain, you can use a hash map. But there is no shortcuts to do it dynamically, so you are in for a lot of coding. — Sergey Kalinichenko, Apr 18 '12 at 11:16
Do you need '<=' as well? (Or any other more-than-one-char operators) — Angelo Fuchs, Apr 18 '12 at 11:18
Have a look at this question as well: http://stackoverflow.com/questions/1432245/java-parse-a-mathematical-expression-given-as-a-string-and-return-a-number and also this one: http://stackoverflow.com/questions/3832323/java-string-and-mathematical-expression-evaluators — Angelo Fuchs, Apr 18 '12 at 11:21
Yes, I would like to cover all comparisons possible like <, <=, >, >=, ==, !=...etc — SANSSAN, Apr 18 '12 at 11:27
Thanks all for the quick reply and many possible ways.... Thanks Angelo for suggesting nice framework... `System.out.println(new bsh.Interpreter().eval("1 < 0")); System.out.println(new bsh.Interpreter().eval("1 > 0")); System.out.println(new bsh.Interpreter().eval("1 >= 0")); System.out.println(new bsh.Interpreter().eval("0 >= 0")); System.out.println(new bsh.Interpreter().eval("1 != 0")); System.out.println(new bsh.Interpreter().eval("0 == 0"));` [BeanShell's](http://www.beanshell.org/download.html) bsh.Interpreter did the magic — SANSSAN, Apr 18 '12 at 11:44

Angelo Fuchs · Answer 1 · 2012-04-18T11:20:32.803

2

You can use a switch statement

char operator = ...;
switch(operator) {
   case '<': return value1 < value2;
   case '=': return value1 == value2;
}

edited Apr 18 '12 at 11:20

answered Apr 18 '12 at 11:15

Angelo Fuchs

9,825
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You introduced an assignment in the last `case` by mistake; use `==` instead. – Costi Ciudatu Apr 18 '12 at 11:21

Wojciech Owczarczyk · Answer 2 · 2012-04-18T11:39:39.177

I would recommend using an expression language such as Java EL or even better Apache Commons Jexl, since it is much easier to integrate. Here is a code sample taken from JEXL website:

    // Assuming we have a JexlEngine instance initialized in our class named 'jexl':
    // Create an expression object for our calculation
    String calculateTax = "((G1 + G2 + G3) * 0.1) + G4";
    Expression e = jexl.createExpression( calculateTax );

    // populate the context
    JexlContext context = new MapContext();
    context.set("G1", businessObject.getTotalSales());
    context.set("G2", taxManager.getTaxCredit(businessObject.getYear()));
    context.set("G3", businessObject.getIntercompanyPayments());
    context.set("G4", -taxManager.getAllowances());
    // ...

    // work it out
    Float result = (Float)e.evaluate(context);

In your particular example you could change your validation XML to something like:

<property name="Premium">
    <xmlTag>//Message/Request/Product/Benefit/Premium/amount</xmlTag>
    <valueType>float</valueType>
    <validation expression="Premium> 0">Premium Amount cannot be less than Zero.</validation>
</property>

and then build up your own JEXL context:

JexlContext context = new MapContext();
context.set("PREMIUM", <Premium value fetched from XML>);

In my opinion this is the most scalable solution as it allows you to build complex validation expressions in just one line of code.

Costi Ciudatu · Answer 3 · 2012-04-18T11:38:25.747

0

Wrap the primitive value into the corresponding wrapper:

Float f = new Float(floatValue)

Then you can use the provided compareTo() method polymorphically.

EDIT: You can also have a look at full-featured implementations for expression parsing; besides others that have already been mentioned here, I would add Spring EL.

edited Apr 18 '12 at 11:38

answered Apr 18 '12 at 11:15

Costi Ciudatu

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how does this give the OP the possibility to use the String "<" as comparison method? – Angelo Fuchs Apr 18 '12 at 11:17
the `<` test should translate into `a.compareTo(b) < 0`. The String `compareTo` will perform the lexicographic test, while the numeric wrappers will do what you expect; however, you can treat them all as `Comparable` instances. – Costi Ciudatu Apr 18 '12 at 11:19
Note also that all the <, <=, >, >=, ==, != operators can be represented by a table of results for `compareTo()` being < 0, ==0, or >= (so, for example, <= would just be defined as `true, true, false` and != as `true, false, true`) – Russell Zahniser Apr 18 '12 at 11:22
I fail to realize how that solves the ops problem of having too many IF-ELSE-ELSEIF hops for every operator. – Angelo Fuchs Apr 18 '12 at 11:23
1

Sorry, I think I misunderstood your question. You read the String "<" from the XML and want to use it as such in your code. That's not an option in Java. Also, in my opinion, even in languages that provide an `eval()` procedure you should avoid this. – Costi Ciudatu Apr 18 '12 at 11:23
To fully answer your question, you should keep a Map that associates each comparison "symbol" (`<`, `<=` etc.) with the corresponding implementation of some `Comparison` interface. Then you can do `comparisons.get("<=").check(a, b)`. Inside the `check()` method you can use `Comparable.compareTo` or some Comparator of yours for custom use cases. You can also use an `Enum` for this if you don't need to add new comparisons without recompiling. – Costi Ciudatu Apr 18 '12 at 11:27
@DownVoters: shouldn't down-votes be explained somehow ? I want to learn from my mistakes. :) – Costi Ciudatu Apr 18 '12 at 11:43

Java - Dynamic Comparison with Primitive Data Types

3 Answers3