Java - parse user defined functions

Question

I am working on a program, where I want users to define a simple functions like randomInt(0,10) or randomString(10) instead of static arguments. What is the best way to parse and process such functions ?

I have not found any examples of such problem, the parser does not have to be ultra-efficient, it will not be called often, but mainly I want to focus on good code readability and scalability.

Example of user input:

"This is user randomString(5) and he is randomInt(18,60) years old!"

Expected output(s):

"This is user phiob and he is 45 years old!"

"This is user sdfrt and he is 30 years old!"

This is too broad. You first have to define your exact requirements. For example: is everything that goes " something()" assumed to be function name? What framework are you using for parsing? Where are the functions coming from, and so on? This is simply a lot of work respectively missing context. — GhostCat, Feb 14 '19 at 15:12
I guess you are using string to store user input. So it is string you will need to define methods that will call user desired method. After user input `randomInt(18,60)` I would use substring in loop until it reaches `(` and check if that matches any method that is avaiable. Then you will need again substring to get values — bakero98, Feb 14 '19 at 15:17
Looks like you want some sort of templating enigne. I suggest Googleing for that ("java templating enigne") before (trying) to write your own. — Bart Kiers, Feb 14 '19 at 15:48

tsolakp · Accepted Answer · 2019-02-14T20:26:34.247

One option is to use Spring SPEL. But it forces you to change the expression a little and use Spring library:

The expression can look like this:

'This is user ' + randomString(5) + ' and he is ' + randomInt(18,60) + ' years old!'

or this:

This is user #{randomString(5)} and he is #{randomInt(18,60)} years old!

or you can implement your own by having a custom TemplateParserContext.

And here is the code:

import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;

public class SomeTest {

        @Test
        public void test() {
            ExpressionParser parser = new SpelExpressionParser();

            Expression exp = parser.parseExpression(
                "This is user #{randomString(5)} and he is #{randomInt(18,60)} years old!", 
                new TemplateParserContext() );

            //alternative
            //Expression exp = parser.parseExpression(
            //        "'This is user ' + randomString(5) + ' and he is ' + randomInt(18,60) + ' years old!'");
            //    String message = (String) exp.getValue( new StandardEvaluationContext(this) );


            String message = (String) exp.getValue( new StandardEvaluationContext(this) );
        }

        public String randomString(int i) {
            return "rs-" + i;
        }

        public String randomInt(int i, int j) {
            return "ri-" + i + ":" + "j";
        }

    }

Whatever object you pass to StandardEvaluationContext should have those methods. I put them in the same class that also runs the expression.

score -1 · Answer 2 · answered Feb 14 '19 at 15:20

You could use something such as the following: Warning, I haven't tested it. Just something to get started with

public String parseInput(String input){
    String[] inputArray = input.split(" ");
    String output = "";
    for(String in : inputArray){ //run through each word of the user input
        if(in.contains("randomString(")){ //if the user is calling randomString
            String params = in.replace("randomString(", ""); //strip away function to get to params
            params = in.replace("(", ""); //strip away function to get to params
            String[] paramsArray = params.split(",");  //these are string integers, and could be converted
            //send off these split apart parameters to your randomString method
            String out = randomString(paramsArray); //method parses string integers, outputs string
            output += out + " ";
        }else if(in.contains("randomInt(")){ //if the user is calling randomInt
            String params = in.replace("randomInt(", ""); //strip away function to get to params
            params = in.replace("(", ""); //strip away function to get to params
            String[] paramsArray = params.split(","); //these are string integers, and could be converted
            //send off these split apart parameters to your randomInt method
            String out = randomInt(paramsArray); //method parses string integers, outputs string
            output += out + " ";
        }else{ //if the user is just entering text
            output += in + " "; //concat the output with what the user wrote plus a space
        }

    }
    return output;
}

Sure, I know how to do it "the messy way" but imagine doing this for 20 + users functions, it will be like 1000+ of lines of unreadable code. I am asking for a cleaner solution. — Dakado, Feb 14 '19 at 15:30
I would look into the java Stream methods if you're using a new enough JDK. You could split the user input into words, stream that array, and check each word against a list of trigger functions. The java Stream methodology would definitely give you cleaner code. — Brandon Beiler, Feb 14 '19 at 15:34

Java - parse user defined functions

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