why does this work?

[dkoleary]

From: dkoleary <[email protected]>

Hi;

New java programmer. So new, in fact, that I'm still working my way through
the O'Reilly Head First Java book. One of the end of chapter questions
involves identifying if a sample class will compile and what to do to make it
compile if it won't.

The sample class from chapter 4 is:

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

int go(int arg)
{ return arg * 2; }
}

The book says that it'll compile and run, displaying "42 84" and, sure enough,
it does:

$ javac XCopy.java
$ java XCopy
42 84

How come that isn't recursive? XCopy.main() instantiates a new XCopy.
Shouldn't that new XCopy instance also instantiate a new XCopy?

I was figuring this would run until the XCopy.go function tried returning a
number that wouldn't fit in int anymore... That's obviously not the case, but I
don't know why.

Can someone provide the missing concept?

Thanks.

Doug O'Leary

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[Daniel Pitts]

To: dkoleary
From: Daniel Pitts <[email protected]>

Hi;

New java programmer. So new, in fact, that I'm still working my way through

the O'Reilly Head First Java book. One of the end of chapter questions
involves identifying if a sample class will compile and what to do to make it
compile if it won't.

The sample class from chapter 4 is:

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

int go(int arg)
{ return arg * 2; }
}

The book says that it'll compile and run, displaying "42 84" and, sure enough, it does:

$ javac XCopy.java
$ java XCopy
42 84

How come that isn't recursive? XCopy.main() instantiates a new XCopy.

Shouldn't that new XCopy instance also instantiate a new XCopy? new XCopy()
creates a new instance of the XCopy class, which executes Constructors. main
isn't executed again because of this. main is only automatically executed by
the JVM on start-up, once.

I was figuring this would run until the XCopy.go function tried returning a

number that wouldn't fit in int anymore... That's obviously not the case, but I
don't know why.
int can fit any number in the range [-2^31, 2^31). 42 and 84 are both within
that range. It is possible that if you pass in a number with a large enough
magnitude, you will end up with an overflow. In Java (and many 2s-compliment
integer systems), overflow will simply throw-away the upper bits, and you will
have what is called "wrap-around". This actually makes many of the basic
operations easier, because signed numbers and unsigned numbers behave the same
way.


Hopefully this helps.

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[Eric Sosman]

To: dkoleary
From: Eric Sosman <[email protected]>

[...]
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
(e-mail address removed)

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[dkoleary]

To: Daniel Pitts
From: dkoleary <[email protected]>

new XCopy() creates a new instance of the XCopy class, which executes
Constructors. main isn't executed again because of this. main is only
automatically executed by the JVM on start-up, once.

Got it... I do remember reading that somewhere else. main only gets executed
once.

Thanks for the concept correction. Hopefully, it'll stick now

Doug O'Leary

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[Lew]

To: dkoleary
From: Lew <[email protected]>

Got it... I do remember reading that somewhere else. main only gets executed

once.

That isn't strictly true. The 'main()' method is only called for one class one
time by
the JVM when it starts, but nothing prevents code in the program from
explicitly
calling some class's 'main()' at some later time.

Thanks for the concept correction. Hopefully, it'll stick now

Read the Java tutorials. They explain constructors vs. methods and such
concepts.

--
Lew

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[Roedy Green]

To: dkoleary
From: Roedy Green <[email protected]>

How come that isn't recursive?

new allocates some space and XCopy() runs the initialitzer code in the
constructor.

There is no new in the constructor, so why would it be recursive?

There is no more reason for the constructor to be recursive than any other
method. The constructor does not reinvoke the constructor.
--
Roedy Green Canadian Mind Products http://mindprod.com A new scientific truth
does not triumph by convincing its opponents and making them see the light,
but rather because its opponents eventually die, and a new generation grows up
that is familiar with it.
~ Max Planck 1858-04-23 1947-10-04

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[glen herrmannsfeldt]

To: Roedy Green
From: glen herrmannsfeldt <[email protected]>


(snip)

There is no new in the constructor, so why would it be recursive?

There is no more reason for the constructor to be recursive than any
other method. The constructor does not reinvoke the constructor.

But if there IS a new in the constructor...


-- glen

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