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Re: problem with compiling abstract class with CW (MAC OS X)

At 11:43 AM -0700 11/25/02, Greg Guerin wrote:
Dmitry Markman <email@hidden> wrote:

public class AlignCommand extends AbstractCommand {
public static abstract class Alignment {
public final static Alignment LEFTS = new Alignment("Lefts") {
public void moveBy(Figure f, Rectangle anchor) {
Rectangle rr = f.displayBox();
f.moveBy(anchor.x-rr.x, 0);
}
};

private Alignment(String newDescription) {
...
}
public abstract void moveBy(Figure f, Rectangle anchor);
}
}

I think the way to avoid the problem is to not declare Alignment as abstract, and to define Alignment.moveBy() as an empty method by default, with an override in each anonymous subclass.

I agree, that should work, but I would consider it a workaround to a compiler bug -- again, assuming the language spec is unambiguous about this.

Since all the possible definitions of Alignment's subclasses are entirely under your control in exactly one source file, I don't see how you're gaining anything by declaring it abstract.

I think it depends how fussy one is about semantics and defensive programming. Declaring moveBy() abstract forces you (and anyone who maintains the class after you) to implement it in each inner subclass. It also prevents any other code within the class from creating a direct instance of Alignment by mistake.

I still think it would be worthwhile to run 'javap' on the output of 'javac', to discover what it's doing about the private constructor. I suspect it's being promoted to the package-visibility

Here's some code you can fool around with, that I compiled with javac. The javap output is semi-Greek to me, but it looks like the compiler creates a special constructor with an extra invisible argument containing the concrete instance being initialized. And if I'm reading correctly, it is indeed package-visible.


----- Foo.java -----
public abstract class Foo
{
public Foo foo =
new Foo("test")
{
public void foo(Object x)
{
}
};

public Foo foo2 =
new Foo("test")
{
public void foo(Object x)
{
System.out.println(x);
}
};

private Foo(String str)
{
}

public abstract void foo(Object x);
}
----- end Foo.java -----


----- output of "javap -c Foo" -----
Compiled from Foo.java
public abstract class Foo extends java.lang.Object {
public Foo foo;
public Foo foo2;
public abstract void foo(java.lang.Object);
Foo(java.lang.String,Foo$1);
}

Method Foo(java.lang.String)
0 aload_0
1 invokespecial #2 <Method java.lang.Object()>
4 aload_0
5 new #3 <Class Foo$1>
8 dup
9 aload_0
10 ldc #4 <String "test">
12 invokespecial #5 <Method Foo$1(Foo,java.lang.String)>
15 putfield #6 <Field Foo foo>
18 aload_0
19 new #7 <Class Foo$2>
22 dup
23 aload_0
24 ldc #4 <String "test">
26 invokespecial #8 <Method Foo$2(Foo,java.lang.String)>
29 putfield #9 <Field Foo foo2>
32 return

Method Foo(java.lang.String,Foo$1)
0 aload_0
1 aload_1
2 invokespecial #1 <Method Foo(java.lang.String)>
5 return
----- end output of "javap Foo" -----


--Andy
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References: 
 >Re: problem with compiling abstract class with CW (MAC OS X) (From: Greg Guerin <email@hidden>)



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