Re: Extending/replacing a class without a public constructor

From: John C. Bollinger (jobollin_at_indiana.edu)
Date: 06/10/04 

Date: Thu, 10 Jun 2004 15:12:53 -0500

Hugh Beyer wrote:

> I'm having a really frustrating problem and I hope you guys can give me
> some help.
>
> I'm trying to write a test suite for a graphics-heavy app that uses
> MIDP/J2ME. These apps use the Graphics class to do their screen updates.
>
> What I want to do is subclass the Graphics class with my own class that
> captures all calls and writes to a log that my tests can later examine.
> Unfortunately, all the constructors of the Graphics class are private. The

You are mistaken. Both Graphics and Graphics2D (the actual class you
are most likely to run into) have protected no-arg constructors, at
least in Java 1.4.

> system passes in a Graphics class when it's time to update the screen, and
> you can also create a graphics class through a static factory method
> (Image.getGraphics()). But either way, I can't get in and capture the
> method calls to the resulting Graphics object.
>
> Is there any way at all to do this, or am I SOL? Can anyone think of
> another way to accomplish what I want?

You can subclass either class with an implementation that delegates to a
System-created Graphics instance, with logging wrapped around the method
invocations. I suppose that's what you had in mind to begin with. If
aspect-oriented programming were deeply enough supported by Java then
this would be a perfect case for it, but as it is, you probably couldn't
catch the system's manipulations of a Graphics -- only your own code's.

John Bollinger
jobollin@indiana.edu

Relevant Pages

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