How to convert Strings in functions in Java?

Question

I'm trying to make a function that takes a String and a double as parameters in Java, like this:

public static double calc(String fx, double arg) {
    return fx.convertToFunction(arg);   // Pseudocode

}

For instance, to calculate the cosine of a number (PI, for example), the code would be:

calc("cos", Math.PI);

and the function calc has to convert "cos" into Math.cos().

Is that at least possible?

you should put if/else state that takes input fx, and use it in Math class. using string as a function name is too complicated — Adem, Dec 15 '14 at 11:27
You're either looking for reflection or a better code structure. — Tom, Dec 15 '14 at 11:27
Why do you have a string in the first place? Is this from user input via a console for example? — musefan, Dec 15 '14 at 11:31

score 4 · Answer 1 · edited May 23 '17 at 10:29

4

What you are looking for is called Reflection in Java.

Have a look at this topic: How do I invoke a Java method when given the method name as a string?

Without the required checks for Exceptions your code may look something like this:

//assume that you have an Object that you want to invoke the method on
MyObject obj = new MyObject();
//the variable `method` will hold your function
java.lang.reflect.Method method;
//paremters can be provided to identify a specific method among overloaded methods
method = obj.getClass().getMethod(methodName, param1.class, param2.class, ..);
//invoke the method of your object
method.invoke(obj, arg1, arg2,...);

edited May 23 '17 at 10:29

Community

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answered Dec 15 '14 at 11:27

Dennis

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This should rather be a comment, since it doesn't provide answer to the question – Konstantin Yovkov Dec 15 '14 at 11:27
1

Thank you guys, I tried to present the essence of the referenced post in my eddited answer. – Dennis Dec 15 '14 at 12:50

score 2 · Answer 2 · answered Dec 15 '14 at 11:28

2

An easiest approach to what you want to do is..

You can easily do it using if else or switch

   calc("cos", Math.PI);

in the function check it as

 if(stringValue.equals("cos")){
  call the cos function
 }

answered Dec 15 '14 at 11:28

Danyal Sandeelo

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Are you going write like this for all the possible scenarios? – Abimaran Kugathasan Dec 15 '14 at 11:28
I think given the limited examples from the OP that this is a good suggestion. – musefan Dec 15 '14 at 11:29
@AbimaranKugathasan depends on the requirements..if there are like 2 or 3 functions it is okay then..else a number of approaches can be used to solve it dynamically.. – Danyal Sandeelo Dec 15 '14 at 11:30

score 0 · Answer 3 · answered Dec 15 '14 at 11:29

0

No, you can use a design pattern or implement with switch:

switch (command) {
case "cos" : return Math.cos(dblParam);
}

answered Dec 15 '14 at 11:29

ACV

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m0skit0 · Answer 4 · 2014-12-15T11:39:50.143

Really not in an easy way. You can define a Map to relate a name with a method (or better, a functional interface), or use reflection.

For example (using a functional interface, Java 7 syntax):

// Define functions
final Runnable cosFunction = new Runnable() {    
    @Override
    public void run() {
        // Do cosine
    }
}

// Store functions
Map<String, Runnable> functionMap = new HashMap<>();
functionMap.put("cos", cosFunction);

// Retrive functions
final Runnable function = functionMap.get("cos");
function.run();

Note that using Runnable doesn't allow you to return a value.

In functional programming this is way easier, I suggest you look into Clojure or Scala, which are JVM-based.

score 0 · Answer 5 · answered Dec 15 '14 at 11:36

I suggest you reconsider whether this is a good idea, however you can do this.

public static double calc(String fx, double arg) {
    try {
        Method math = Math.class.getMethod(fx, double.class);
        return (Double) math.invoke(null, arg);
    } catch (Exception e) {
        throw new IllegalArguementException(e);
    }
}

score 0 · Answer 6 · answered Dec 15 '14 at 11:37

You need to use reflection api as below:

String methodName = "cos";
double arg = Math.PI;

try {
    Method method = Math.class.getDeclaredMethod(methodName,
            double.class);//get cos method from math class that accepts double as parameter
    method.setAccessible(true);//i trust this call
    double time = (Double) method.invoke(null, arg);//invoke the method with PI
    System.out.println(time);
} catch (Exception e) {//handle all exceptions or individual according to business need
    e.printStackTrace();
}

score 0 · Accepted Answer · answered Dec 15 '14 at 11:38

Java provides a facility called reflection. This provides methods to access methods, variables etc. by name. Most of these facilities are on the java.lang.Class object.

Class.getMethod(name, args) on java.lang.Math to get a java.reflect.Method reference
Call the method reference with method.invoke(null, args). The null here is the instance, which doesn't make sense for static methods

Code

public static void main(String[] args) throws Exception {
    System.out.println(calc("cos", Math.PI));
    System.out.println(calc("sin", Math.PI /2));
}

public static double calc(String fx, double arg) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
    return (Double) Math.class.getMethod(fx, double.class).invoke(null, arg);
}

How to convert Strings in functions in Java?

7 Answers7