$
is used internally by the compiler to decorate certain names. Wikipedia gives the following example:
public class foo {
class bar {
public int x;
}
public void zark () {
Object f = new Object () {
public String toString() {
return "hello";
}
};
}
}
Compiling this program will produce three .class files:
foo.class
, containing the main (outer) classfoo
foo$bar.class
,
containing the named inner classfoo.bar
foo$1.class
, containing the
anonymous inner class (local to methodfoo.zark
)
All of these class names are valid (as $
symbols are permitted in the JVM specification).
In a similar vein, javac
uses $
in some automatically-generated variable names: for example, this$0
et al are used for the implicit this
references from the inner classes to their outer classes.
Finally, the JLS recommends the following:
The
$
character should be used only in mechanically generated source
code or, rarely, to access preexisting names on legacy systems.