Rogue class file - Outer$1.class

[cfmdobbie]

Hey peeps,

Here's a bit of a wierd one for you; I'd be very grateful if anyone can give me a reason why the following happens, could tell me whether or not it's a bug, and whether it has been fixed in the latest JDK - I'm running JDK 1.4.1_01:

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public class Outer {       private class Inner       {       }       Inner i = new Inner() ; }

When this class is compiled it produces three class files, Outer.class, Outer$Inner.class and Outer$1.class.  What is Outer$1?  It is generated whenever the class is compiled, but can be deleted without affecting the class' functionality.  When decompiling the three class files together, a single file equivalent to the above is generated - subsequently compiling this file produces the three class files again.

Decompiling just the Outer$1.class file produces the following:

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class Outer$1 { }

Decompiling just the Outer$Inner.class file produces the following:

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private class Outer$Inner {       private Outer$Inner()       {             super();       }       Outer$Inner(Outer$1 outer$1)       {             this();       } }

So what is that second constructor?  Why can life continue fine without the class existing?  And is this expected behaviour?

Cheers,
Charlie.

[erikd]

That *is* weird  

When I try in Eclipse, I get the expected 2 classes (so not the Outer$1 class), but using javac I get the weird 3rd one...

[sma]

1.4.2 beta also generates that $1 class.  Eclipse 2.1 doesn't. The eclipse-generated Outer$Inner class has just one constructor:

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Outer$Inner(Outer o) {   super();   this$0 = o; }

javac probably wants to make sure to create a unique method signature but I don't really know why.

Games published by our own members! Check 'em out!

[snak]

It's a class generated to send information to the CIA.  That's why when you decompile it you dont see anything.  True story

[jbanes]

It's a "go between" class that links used methods and variables between the internal and parent classes. Since your example doesn't actually access anything inside internal class or vica-versa, the "go between" is unnecessary. Javac usually generates it anyway. I'll make a bet that Eclipse is using Jikes instead.

[cfmdobbie]

Ah, good.  I won't worry about it then.

I was developing an ultra-clever DisplayMode filtering class and had a load of anonymous inner classes at one point.  I then removed them all in favour of one named inner class.  But one unnamed class file remained...

I have noticed that it doesn't appear if the inner class is anything but private though.  I must decompile the classes and see what goes on in that case.

Cheers, everyone!