Question

I recently saw the source code of the Google library Gson and saw something really strange (IMO). In the com.google.gson.internal package there are 2 classes which do begin with a “$”.

For example

public final class $Gson$Types { ... }

Does the dollar sign has a effect to development or is it just to say to outside developers something like “please don’t use cause it is a internal class”?

Answer 1

You can use any currency sign or continuation character in an identifier.

By using $, it suggests it is generated e.g. generated by the build, or for internal use only.

Uses for special characters in Java code

Hidden code

It only causes a problem if you have conflicts with generated classes e.g.

class A {
     class B {
     }
}
class A$B {
}

reports

error: duplicate class: A.B

Compiler generated classes do not start with a $ which may be reason this as done.

On Windows 7, the special characters which can appear in a Class are

public static void main(String... args) throws IOException {
    for (char ch = 0; ch < Character.MAX_VALUE; ch++) {
        if (Character.isJavaIdentifierStart(ch) && !Character.isLetter(ch))
            System.out.println("interface " + ch + " { }");
    }
}

interface $ { }
interface _ { }
interface ¢ { }
interface £ { }
interface ¤ { }
interface ¥ { }
interface € { }

Other special characters are allows by Java, but not supported as file names. This may depend on the OS and possibly the language.