Reflection – Calling functions

With c++ you can call functions as a reference instead of the actual name, as long as the reference is linked to the function itself (I did a post about it function pointers and also within csharp delegates)

To start with, we need to sort out what function we want to use to call, I am going to pick a function that returns a value and also passes parameters (kinder covers all aspects really), so this is going to be the function

public String addsTogether(String st1, String st2)

So to start off , lets build up the parameters to pass to the function, we have two string parameters, so we need to have a array of 2 strings.

String[] passingParameters = { new String("genux"), new String("was here")};

Now we need to setup a link to the function that we want to call, we setup a class instance with using the forName (which is the function name). Also because the function takes 2 String parameters we need to setup blank classes that are both Strings in types.

Class callingClass = Class.forName("CallMethodClass");
 
Class[] callingParameters = new Class[2];
callingParameters[0] = String.class;
callingParameters[1] = String.class;

this is the main part, this is the link, we create a Method (from the java.lang.reflect package) that links to the class function/method “addsTogether” (the function at the start of this post) and also the parameters type and length (2 strings).

Method callingMethod = callingClass.getMethod("addsTogether", callingParameters);

that is about it, all we need to do is to call the function now and that is it, so we use the Method link above variables invoke method, this takes the class that the function is wrapped within and also the parameters that we want to send to the function (method and function are kinder interchangeable) and that is about it. Since the returning value is a String (and we know this) we will need to case the returning type into that by using (String) otherwise it would be a type of Object (the base object type in java)

String result = (String)callingMethod.invoke(new CallMethodClass(), passingParameters);

Here is the source code in full (Save as CallMethodClass.java if you want to compile it up)

import java.lang.reflect.Method;
 
public class CallMethodClass {
 
	// adds together the two strings with inserting a space between them
	public String addsTogether(String st1, String st2)
	{
		return st1.concat(new String(" ").concat(st2));
	}
 
	public static void main(String[] args) {
		try
		{
			String[] passingParameters = { new String("genux"), new String("was here")};
			// setup the link to the class to call
			Class callingClass = Class.forName("CallMethodClass");
			// passing parameters to the callingClass, the method has two String parameters
			Class[] callingParameters = new Class[2];
			callingParameters[0] = String.class;
			callingParameters[1] = String.class;
			// and now setup the method to call
			Method callingMethod = callingClass.getMethod("addsTogether", callingParameters);
 
			// call the method and get the return (convert to a String)
			// pass in the parameters here, when calling (invoking) the method
			String returnValue = (String)callingMethod.invoke(new CallMethodClass(), passingParameters);
 
			// and now output the result
			System.out.println("OUTPUT : "+returnValue);
		} catch (Exception e)
		{
			System.out.println(e.getMessage());
		}
	}
}

here is the output of the program

OUTPUT : genux was here

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