String to Executable Code

[THEMAN101]

I have a string variable, that holds a mathematical equation such as 4+5-6/(9+3) now i would like to be able to take this string variable and have the java compiler execute a line of code.

Such as

if i wanted to execute this command to store it in a var, but dont want to have the user pick the equation then i would do:

sum = 4+5-6/(9+3);

and sum woul equal 8.5.

now what if it was like this:

String equation;
equation = textFeild.getText();

sum = // Now what would i put here

Hope this explains it.

If you need more explination just post.
Thanks all.

 

[icujc]

Your going to have to change data types from string to Double or Float here is an example call to change to Double:

stringToDouble

double val = Convert.stringToDouble (digitString, defaultValue);

stringToFloat

float val = Convert.stringToFloat (digitString, (float) defaultValue);

Hope that gets you started

 

[THEMAN101]

Ok i looked in the java docs, and found none of that. Convert is what type of object.

Is there seriously no way to have the java compiler execute a line of code thats a string. This seems like it would be a great addition to a language.

 

[sedj]

>>>> Is there seriously no way to have the java compiler execute a line of code thats a string. This seems like it would be a great addition to a language.


First, the compiler does not execute code, the interpreter does.
Second, a String is a Java object, so how would you execute that ? Its not machine code is it !
There are ways of intialiasing and invoking objects on the fly in java using reflection, but this is not necessary for this.

So, for example, to show you a simple example (the algorithm to work out how to break an "equation" from a String to its operators and numeric data will be much trickier, but thats something you will have to work out.

	public int plusCalc(int a, int b) {
		return a + b;
	}

	public int minusCalc(int a, int b) {
		return a - b;
	}

 	public static void main(String args[]) throws Exception {
		String eq = "4+5";
		int i = Integer.parseInt(eq.charAt(0));
		char oper = eq.charAt(1);
		int j = Integer.parseInt(eq.charAt(2));
		
		int calc = -1;		
		
		if (oper == '+') {
			calc = plusCalc(i, j);
		} else if (oper == '-') {
			calc = minusCalc(i, j);
		}
		
	}

 

[stevexff]

Sounds like you need a scripting language, not Java. Rather than asking for a new language feature that would be a hacker's wet dream, try perl instead...

 

[JavaDude32]

Well unless you find a scripting language that will let you execute code just like that, you might just want to do a google search for code that deals with Expression Trees.

 

[hologram]

If you only need a calculator, you can find one in JavaWorld (author Allen Holub).

http://www.javaworld.com/javaworld/jw-07-1999/jw-07-toolbox.html?

(Part 6)

sedj wrote "First, the compiler does not execute code, the interpreter does." --> So a solution would be to compile the "code" and then execute it. Can you (THEMAN101) post your code when you've worked this out ? I know it's possible ...

 

[stefanwagner]

Another way to endless luck (without perl, python, ruby, ...) might be found at the project 'bsh'.
bsh stands for 'beanshell' which is a java shell, written in java and using java-syntax.
I only had a brief look at it, but I guess with some investigations you will find the right classes to invoke, since one of it's features is, to evaluate arithmetic expressions.