Using a string as a variable clause in an if statement

Question

private String boundaries = "(x > 750) & (x < 900)";
if (boundaries) {
//Do stuff
}

I wanted to be able to use the string as the variable in the if clause, and then change the string independently without having to edit the code itself. The example idea doesn't work, I was wondering if there is a mistake in it or another way to do this without using strings.

Thank you.

http://stackoverflow.com/questions/935175/convert-string-to-code — Ogen, Jan 26 '14 at 11:17
What is exactly what you want to do? I mean, what problem are you trying to solve? It may be simpler to answer that instead of what you are asking. — Darkhogg, Jan 26 '14 at 11:20
@Darkhogg I wanted it for me to be able to input a comparative string, which would be used in the "if" statement. — user3144201, Jan 26 '14 at 11:22
As of java 1.6 you can use the `ScriptEngine`. http://stackoverflow.com/questions/20797012/condition-string-resolver-in-java-api/20797167#20797167 — René Link, Jan 26 '14 at 11:31

Gergely Králik · Accepted Answer · 2014-01-26T11:37:39.283

8

Java is not a dynamically compiled language, so if you want to evaaluate such an expression you can use Javascript from Java.

Try this code:

      String boundaries = "(x > 750) & (x < 900)";
      ScriptEngineManager manager = new ScriptEngineManager();
      ScriptEngine engine = manager.getEngineByName("javascript");

      engine.eval("var x = 810;"); // Define x here
      engine.eval("var bln = false;")
      engine.eval("if (" + boundaries + ") bln = true; ")
      if((boolean)engine.eval(bln)) {
          // Do stuff
      }

Hope this helps. Reference: Oracle: Java Scripting Programmer's Guide

edited Jan 26 '14 at 11:37

answered Jan 26 '14 at 11:24

Gergely Králik

440
4
14

+1 Do you need to define a binding to `x` anywhere? – Peter Lawrey Jan 26 '14 at 11:29
1

+1 : reference : http://docs.oracle.com/javase/6/docs/technotes/guides/scripting/programmer_guide/index.html?utm_source=twitterfeed&utm_medium=twitter – M.Sameer Jan 26 '14 at 11:30
you need to define the x via engine.eval. (see edit) – Gergely Králik Jan 26 '14 at 11:31

score 0 · Answer 2 · answered Jan 26 '14 at 11:20

Java is not a dynamically compiled language in this sense.

What you can do is use beanshell for compilation of code at runtime, or use the JavaScript framework to implement the expression.

In Java, the simplest option is to do this

int min = Integer.parseInt(minAsString);
int max = Integer.parseInt(maxAsString);
if (x > min && x < max) {
   // do stuff
}

Using a string as a variable clause in an if statement

2 Answers2