Home/ Questions/Q 4064292
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-20T15:50:11+00:00

I am aware that by default Java does not have the so-called eval (what

  • 0

I am aware that by default Java does not have the so-called eval (what I pronounce as “evil”) method. This sounds like a bad thing—knowing you do not have something which so many others do. But even worse seems being notified that you can’t have it.

My question is: What is solid reasoning behind it? I mean, Google’ing this just returns a massive amount of old data and bogus reasons—even if there is an answer that I’m looking for, I can’t filter it from people who are just throwing generic tag-words around.

I’m not interested in answers that are telling me how to get around that; I can do that myself:

Using Bean Scripting Framework (BSF)

File sample.py (in py folder) contents:

def factorial(n): 
    return reduce(lambda x, y:x * y, range(1, n + 1))

And Java code:

ScriptEngine engine = new ScriptEngineManager().getEngineByName("jython");
engine.eval(new FileReader("py" + java.io.File.separator + "sample.py"));
System.out.println(engine.eval("factorial(932)"));

Using designed bridges like JLink

example

This is equivalent to:

String expr = "N[Integrate[E^(2 y^5)/(2 x^3), {x, 4, 7}, {y, 2, 3}]]";
System.out.println(MM.Eval(expr));
//Output: 1.5187560850359461*^206 + 4.2210685420287355*^190*I

Other methods

  • Using Dijkstras shunting-yard algorithm or alike and writing an expression evaluator from scratch.
  • Using complex regex and string manipulations with delegates and HashMultimaps.
  • Using Java Expressions Library
  • Using Java Expression Language
  • Using JRE compliant scripting language like BeanShell.
  • Using the Java Assembler and approach below or direct bytecode manipulation like Javaassist.
  • Using the Java Compiler API and reflections.
  • Using Runtime.getRuntime().exec as root
Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    “eval” is only available in scripting languages, because it uses the same interpreter that runs the rest of the code; in such languages the feature is free and well integrated, as in scripting environment it makes little difference if you run a string or a “real” function.

    In copiled languages, adding “eval” would mean bundling the whole compiler – which would defy the purpose of compiling. No compiled language I know (even dynamic ones, like ActionScrip3) has eval.

    Incidentally, the easiest way to eval in Java is the one you forgot to mention: JRE 1.6 comes with Javascript engine, so you can eval any Javascript in two lines of code. You could even argue that the presuposition of your question is false. Java 1.6 bundles a very advanced expression evaluator.

    • 0