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Java reflection - Invoking Methods with a Variable Number of Arguments

Posted on 2008-10-30
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Last Modified: 2013-11-23
Hi,

My scenario is such that i have a class that receives the name of a method and the qualified name of the class that contains it. Using this info i get a reference to the method. My problem arises when i want to call that method and pass it parameters. I need to create this class such that i am able to call different methods with varying numbers of params. These params will all be numbers.

e.g. public double meth1(double 1,double 2){}
       public double meth2(double 1,double 2,double 3){}

I know its possible to to do this if all my methods had an array as a param but realistically i cant do this.

Anybody have a solution?

Thanks in advance
public NelderMead(){}
	public double run(String qualifiedClassName,String methodName, double[] startparams,int paranum,double TOL){
		
		
		
		//start reflection
		try{
			Class<?> c = Class.forName(qualifiedClassName);
			
			Method meth=null;;
			
			Method[] allMethods = c.getDeclaredMethods();
			for (Method m : allMethods) {
				String mname = m.getName();
				if (mname.equals(methodName))
					meth=m;	
			}
			
			meth.invoke(startparams);
		}
		catch(ClassNotFoundException e){}
		//catch(NoSuchMethodException n){}
		catch(InvocationTargetException in){Throwable cause = in.getCause();
	    err.format("invocation of %s failed: %s%n","", cause.getMessage());}
		catch(IllegalAccessException il){Throwable cause = il.getCause();
	    err.format("invocation of %s failed: %s%n","", cause.getMessage());}

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Question by:billyleo
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6 Comments
 
LVL 60

Expert Comment

by:Kevin Cross
ID: 22840782
Here is the example from Java website on reflection:
http://java.sun.com/docs/books/tutorial/reflect/member/methodInvocation.html

If you have any more questions on it, please post back.
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LVL 60

Expert Comment

by:Kevin Cross
ID: 22840839
You can also look for the method by the number of parameters and their types if you actual know coming in what method you are looking for and its signature or at least the parameters you have to pass to it.

try {
                Class c = Class.forName("MainClass");
                Class[] parameterTypes = new Class[2];
                parameterTypes[0] = double.class;
                parameterTypes[1] = double.class;
                c.getMethod("meth1", parameterTypes);
            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (SecurityException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

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Author Comment

by:billyleo
ID: 22841029
thanks for the input but my problem arises after i get the method that i want to invoke.

I have been reading the link you posted but they only seem to show you how to invoke methods like a 'main' method that thats in an array of params!

Cheers
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LVL 60

Expert Comment

by:Kevin Cross
ID: 22841545
I was trying to simplify and my code example was in a class called MainClass.

Here is a better example:
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;
 
 
public class MyClass {
 
    public MyClass() {
        
    }
    
    public static void main(String args[]) {
        MyClass instance = new MyClass();
        Class c = instance.getClass();
        
        if (args == null || args.length != 2) {
            args = new String[2];
            args[0] = "Test";
            args[1] = "Param";
        }
        List<Class> paramTypes = new ArrayList<Class>();
        for (Object o : args) {
            paramTypes.add(o.getClass());
        }
 
        Method meth=null;;
        
        Method[] allMethods = c.getDeclaredMethods();
        for (Method m : allMethods) {
            String mname = m.getName();
            if (mname.equals("meth1"))
                meth=m; 
        }
        
        try {
            Object o = meth.invoke(instance, args);
            System.out.println(o.toString());
        } catch (IllegalArgumentException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
 
    }
 
    public String meth1(String param1, String param2) {
        System.out.println("Param1: " + param1);
        System.out.println("Param2: " + param2);
        return "Hello World";
    }
}

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Accepted Solution

by:
Kevin Cross earned 500 total points
ID: 22841586
If everything else matches what you have, the net is that I think you need an instance of class holding the method to invoke against.
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Author Closing Comment

by:billyleo
ID: 31511645
Thanks - hopefully this will work - i'll let you know when i get a chance to implement it.

Cheers
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