Question

evaluating expression in string

Asked by: jaxrpc

hi,

i am trying to evaluate a given if() statement. but the problem is the given if() is a String.
Example.

String someIF = "if((a>b)||(c==d)&&(z==1))";

how can i evaluate someIF and extracting the different variables needed for evaluation?

thanks

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Asked On
2008-08-21 at 21:05:17ID23669231
Tags

java

Topics

Parsers

,

Java Programming Language

,

Java Standard Edition

Participating Experts
3
Points
500
Comments
5

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Answers

 

by: PrasathAnnamalaiPosted on 2008-08-21 at 21:30:08ID: 22286698

you can write like this

String someIF = (a>b)||(c==d)&&(z==1)? "success": "Failure";

public class AnotherTest {
	
	public static void main(String[] args) {
		
		
		 int a=30;
		 int b=25;
		 int c=20;
		 int d=30;
		 int z=1;
		String someIF = (a>b)||(c==d)&&(z==1)? "success": "Failure";
		
		System.out.println("someIF :" +someIF);
		
		
	}
 
}

                                              
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by: jaxrpcPosted on 2008-08-21 at 21:41:46ID: 22286788

but my "(a>b)||(c==d)&&(z==1)" is an expression in string format.
i found another way of doing at

http://forums.sun.com/thread.jspa?threadID=5144807&messageID=9536652

 

by: softwarepearls_comPosted on 2008-08-22 at 01:19:04ID: 22287766

The solution depends a lot on the exact grammar of the allowed expressions. If your expressions are allowed to be real Java expressions, then you're looking at a very non-trivial requirement.

On the other hand, if your expressions are only allowed to look like the expression in your example, then you could write your own custom parser for composite boolean expression consisting of simple ones involving no more than variables and constants (note: no sub-expressions, or method calls!).

Check out StringTokenizer.. a very old class that has been reused many times to do the latter.

 

by: softwarepearls_comPosted on 2008-08-22 at 01:25:55ID: 22287794

Another approach that would be possible in Java 1.6+ environments is to wrap the expression in a compilable method harnass, itself wrapped by a class body.. then use the compilation facilities to compile this generated class. Then.. you load this class (dynamic loading), passing your variables' values to your class/method, and have Java itself evaluate the expression.

So your synthetic class would have to look something lke

public class BooleanExpressionEvaluator {

  public static boolean eval(int a, int b, int c, int d, int z) {
     return (a>b) || (c==d) && (z==1);
  }
}

This approach would be a fair bit of work too.. but it would support much more complex expressions.

 

by: mbodewesPosted on 2008-08-23 at 05:18:08ID: 22296633

I've got the strong feeling you are a script programmer trying to do some Java :) Welcome to the world without eval("code"). You can circumvent things like these using the already proposed solution, or by using things like bsh, which provide a scripting platform. Just creating a expression from a string and using it directly within a method body is not an option in Java. You might also want to check out the latest scripting support in Java (where you can use Java Script within Java applications).

20120131-EE-VQP-002

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