Re: How would you invoke arrayList.get() through reflection in 1.4 ??

From:
markspace <nospam@nowhere.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 05 Nov 2009 07:46:00 -0800
Message-ID:
<hcurvr$1h9$1@news.eternal-september.org>
S?bastien de Mapias wrote:

Hi,

It seems to be pretty hard to invoke the List get(int) method through
reflection. I didn't manage to have my code working with my 1.4
compiler.


It isn't. Your code is pretty bad. I'll make some more comments about
that in a sec, you're making things way harder than need to be. First,
a direct answer to your question:

    Method m = obj.getClass().getMethod( "get", Integer.TYPE );

Likely you have "Integer.class" or similar, you have to use the type for
a primitive, not the object Integer.

Ok, on to comments.

To sum up I do the following:

Method method;
method = [some more code...];
if (method.getReturnType().toString().equals("interface
java.util.List"))


That line above drives me nuts. Why do a string compare? Why not just
compare to the class itself?

 > if (method.getReturnType() == List.class )

Not sure what the confusion is with that.

{

  // how many refs does our List contain ?
  int n = sizeOfCollection(method.invoke(root, (Object[])null));


That line is a terrible idea. More later.

  // let's get the actual list
  Object list = method.invoke(root, (Object[])null);


At this point you could just cast to a list, you know. This is the
biggest "wtf?" in your code for me.

     List<?> list = (List) obj;
     for( Object o : list ) {
        System.out.println( o );
     }

There's your "reflective" way to get all members of the list from an Object.

  // now trying to invoke its 'get()' for every element it
  // contains:
  Class listClass = Class.forName(list.getClass().getName());
  Method m2 = listClass.getDeclaredMethod("get", ???); //<= what to
put here ?


See above.

  for (int i=0; i<n; i++) {
      Object o = m2.invoke(list, i); //<= doesn't compile
    ...
  }


Ditto.

  [...]
}

private int sizeOfCollection(Object obj)
{
  return new StringTokenizer(obj.toString(), ",").countTokens();
}


A close second place for "wtf?". Please. What if your string(s)
contain commas themselves? This can't work in the general case. Bad
bad code, bad idea. Just cast to a list and then call the normal
methods, like ".size()".

Here's my reflective example:

package oldlist;

import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.List;

public class Main {
     public static void main(String[] args) throws NoSuchMethodException
     {
        Object x = Arrays.asList( "red", "fish", "blue", "fish" );
        reflectList( x );
     }

     private static void reflectList( Object obj )
            throws NoSuchMethodException
     {
        if( obj instanceof List ) {
           List<?> list = (List) obj;
           for( Object o : list ) {
              System.out.println( o );
           }
        }
        Method m = obj.getClass().getMethod( "get", Integer.TYPE );
        System.out.println( m );
     }
}

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