what is the signficance of the Java class files of the sort myClass$1.class?

Question

I have a number of .java classes compiled with Eclipse and over in the /bin directory I see that not only do I have various .class files corresponding to my Java classes but also a few with a dollar sign in the file name.

Example: I have a class called RangeFinder and in the /bin I see a RangeFinder.class and also a RangeFinder$1.class.

What is the significance of the latter?

(I am on Ubuntu and I am using Eclipse EE Indigo.)

It's an anonymous inner class. See http://stackoverflow.com/questions/380406/java-inner-class-class-file-names — NPE, Aug 02 '11 at 14:05

Péter Török · Accepted Answer · 2011-08-02T14:09:33.183

3

These are anonymous inner classes in bytecode form. The compiler gives them numerical names starting with 1 (it is not allowed in Java to have a class name starting with a number, but it is possible in the bytecode, so the compiler does it to avoid name clashes, I guess). Normal (named) inner classes are named like OuterType$InnerType.class.

edited Aug 02 '11 at 14:09

answered Aug 02 '11 at 14:03

Péter Török

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the inner class name does not start w/ a number though. And having class name org.yyy.XXX$1 is valid, no clashes w/ bytecode. inner classes are absolutely normal classes for the verifier and the JIT/JVM. The only difference is the metadata (inner/outer info) that's not used by the runtime itself. – bestsss Aug 02 '11 at 14:13
@bestsss, for the sake of precision, what I meant is that code like `class 1 { }` is not valid Java, regardless of whether it is at outer level or inside another class. – Péter Török Aug 02 '11 at 14:23
*for the sake of precision, what I meant is that code like class 1 { }* i know you did – bestsss Aug 02 '11 at 14:44

score 2 · Answer 2 · answered Aug 02 '11 at 14:12

Each .java source file can contain definitions for multiple Java classes. However, every .class file can only contain the bytecode for a single class.

So, if you have a file Foo.java that looks like this:

public class Foo {
    public class Inner {
        …
    }

    public void method() {
        widget.addListener(new Listener() {
            public void listen() {…}
        }
    }
}

class Bar {
    …
}

It will compile into the following files:

Foo.class
Foo$Inner.class
Foo$1.class
Bar.class

Anonymous classes are assigned numbers as "names", I believe in the order in which they're found in the enclosing class.

*I believe in the order in which they're found in the enclosing class.* yes, it is so — bestsss, Aug 02 '11 at 15:38

what is the signficance of the Java class files of the sort myClass$1.class?

2 Answers2