Class.getMethod with variable args - part 2

[Stephen Upton]

Previously, I received an answer on this (see thread topic without the part 2), where I was struggling to find the correct syntax to invoke a method that had a variable argument list using reflection, e.g.,

 class A {
  public void myMethod(int x, String... args) {}
}
 

and I wanted to use reflection to call this method (since I won't know the method name until runtime).

The previous answer was this:

 Method m = AasClass.getMethod("myMethod", Integer.TYPE, String[].class); 

However, I'm still struggling with how to invoke this. I've tried:

Integer i = someinteger;
String[] ss = somestringarray;
 m.invoke(AasInstance, i, ss);  

I get an warning saying I need to cast the ss to Object[], but then when I do that, I get a ClassCastException when the m.invoke is executed.

Hope that explains enough.
thanks!
steve upton

[Jim Yingst]

The previous topic is here. Normally it's preferable to post a followup like this in the same thread as the original. It's easier for one person (you) to do the work of finding the original thread, rather than each reader having to find it themselves.

Can you show what is AasClass? How did you obtain it? If you want the class object associated with A, the easy way is using a class literal: A.class.

Method m = A.class.getMethod("myMethod", Integer.TYPE, String[].class); 
Integer i = someinteger;
String[] ss = somestringarray;
m.invoke(AasInstance, i, ss); 

This code seems to invoke the method just fine. If it's not working for you, can you show the exact error message?

[Stephen Upton]

Hi Jim,

Thanks! I'll work on keeping the topics in the same thread. ;-)

Actually, the problem is more complicated than I posted. I don't know what class A is. I was actually trying to call different methods with differing number of parameters (all Strings, though), so I think I found a way around that complication. However, here's the code I'm actually working with that throws a IllegalAccessArgument (error below) (actionParameters is a String[]):

 
Class<?> effectorClass = effector.getClass();
effectorClassName = effectorClass.getName();
if ( actionParameters[0] == null ) {
Method actionMethod = effectorClass.getMethod(action);
actionMethod.invoke(effector);
}
else {
Method actionMethod = effectorClass.getMethod(action, String[].class);
actionMethod.invoke(effector, (Object[])actionParameters);
}   

and here's the error it throws on the invoke line:

 java.lang.IllegalArgumentException: java.lang.ClassCastException@4977e2
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at jtem.thetester.agents.actions.EffectorAction.execute(EffectorAction.java:69)   

Thank you all for your help.
steve

[Jim Yingst]

What is actionParameters? Is it a String[], or an Object[]? I suspect it's an Object[], and the method requires a String[] - thus you're getting a ClassCastException.

[Stephen Upton]

Hi Jim,

actionParameters is a String[]. If I don't cast to Object[], I get a warning from Eclipse, and if ignore the warning, I still get the same exception.

Oh, well on to change the code to something else that works or to try an experiment on something simple...

[Ilja Preuss]

What JDK are you using?

[Stephen Upton]

1.5.0_13 on Mac OS X

[Roger Chung-Wee]

actionParameters is a String[]. If I don't cast to Object[], I get a warning from Eclipse, and if ignore the warning, I still get the same exception.

You cannot cast an array to another array as there is no hierarchy amongst arrays. You can only cast an array to Object.

[Stephen Upton]

Hi Roger,

You cannot cast an array to another array as there is no hierarchy amongst arrays. You can only cast an array to Object.

I think you hit the nail on the head! After some reflection (pun intended), I realized I was trying to shortcut calling a number of methods using the varargs, but now realize there should still be only one method match.

Thank y'all for your help!
steve
p.s. I LOVE JavaRanch!

[Ilja Preuss]

Originally posted by Roger Chung-Wee:

You cannot cast an array to another array as there is no hierarchy amongst arrays. You can only cast an array to Object.

That's not true - Java arrays are covariant, meaning that the following is perfectly legal:

Object[] array = new String[1];

Now, if you try to put, say, a number into that array, you will get an ArrayStoreException, but that's a different kind of beast from a ClassCastException, obviously...