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Re: why does this work?

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  To: dkoleary
From: Eric Sosman <esosman@ieee-dot-org.invalid>

On 8/8/2012 1:30 PM, dkoleary wrote:
> [...]
> How come that isn't recursive?  XCopy.main() instantiates a new XCopy.
Shouldn't that new XCopy instance also instantiate a new XCopy?

     See Daniel Pitts' explanation.  Another thing you could try to
help you see what's going on is to sprinkle some more println() calls through 
the code to help trace through the execution.  In this case you're interested 
in where the constructor fits with relation to everything else, but there's no 
explicit constructor written in the code.  As I'm sure you've learned, this 
means the compiler will write a simple constructor for you -- but there's no 
way to get the compiler to stick println() calls in what it writes, so your 
recourse is to write your own explicit constructor. With this in mind, the code 
might look like:

        class XCopy {
            public static void main(String[] args) {
                System.out.println("entering main()");
                int orig = 42;
                System.out.println("main() creates an XCopy");
                XCopy x = new XCopy();
                System.out.println("main() created an XCopy");
                int y = x.go(orig);
                System.out.println(orig + " " + y);
                System.out.println("main() is finished");
            }

            int go(int arg) {
                System.out.println("executing go(), arg = " + arg);
                return arg * 2;
            }

            // Explicit constructor, just for the println
            XCopy() {
                System.out.println("constructing an XCopy");
            }
        }

Run this version, study the output, and see if the sequence of events becomes 
clearer.

     This technique is sometimes called "printf debugging" (the name
comes from a different programming language).  Despite its simplicity, it can 
be astonishingly effective, and the overall approach can be used in most 
environments and most languages.  Indeed, Java's various logging frameworks 
(you may learn about them later) are basically just fancied-up versions of 
printf debugging: A piece of the program blurts "Look! I'm *here*, and these 
are a few interesting values."

--
Eric Sosman
esosman@ieee-dot-org.invalid

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why does this work? "dkoleary" <dkoleary@1:261/38.remove-qhs-this> - 2012-08-08 19:04 +0000
  Re: why does this work? "Daniel Pitts" <daniel.pitts@1:261/38.remove-qhs-this> - 2012-08-08 19:04 +0000
    Re: why does this work? "dkoleary" <dkoleary@1:261/38.remove-k2r-this> - 2012-08-09 18:44 +0000
      Re: why does this work? "Lew" <lew@1:261/38.remove-k2r-this> - 2012-08-09 18:44 +0000
  Re: why does this work? "Eric Sosman" <eric.sosman@1:261/38.remove-r72-this> - 2012-08-08 20:06 +0000
  Re: why does this work? "Roedy Green" <roedy.green@1:261/38.remove-k2r-this> - 2012-08-09 18:44 +0000
    Re: why does this work? "glen herrmannsfeldt" <glen.herrmannsfeldt@1:261/38.remove-t9h-this> - 2012-08-10 18:39 +0000

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