Question

The Code:

package com.keyoti.rapidSpell;

import java.util.Comparator;

// Referenced classes of package com.keyoti.rapidSpell:
//            RapidSpellChecker

class RapidSpellChecker$CompareL
    implements Comparator
{

    public int compare(Object a, Object b)
    {
        return (int)(100D * (suggestionScore2b(topWord, (String)b) - suggestionScore2b(topWord, (String)a)));
    }

    public void with(String w)
    {
        topWord = w;
    }

    private String topWord;

    RapidSpellChecker$CompareL()
    {
    }
}

This is the one the many classes in the application.

What does the $ sign in class RapidSpellChecker$CompareL implements Comparator signify?Is it simply the class name or has some significance?

Answer 1

I suspect this is decompiled code. (See at the bottom for more information.) The $ shows that it’s a nested class within RapidSpellChecker. So the code would originally have looked something like this:

public class RapidSpellChecker
{
    // Other code withing RapidSpellChecker

    static class CompareL implements Comparator
    {
        // Code for compare, with etc
    }
}

I’ve shown this as a static nested class, because the code you’ve shown doesn’t have any implicit reference to an instance of RapidSpellChecker. If it did, the original code would have been like this:

public class RapidSpellChecker
{
    // Other code withing RapidSpellChecker

    class CompareL implements Comparator
    {
        // Code for compare, with etc
    }
}

In this case it’s an inner class.

See the Java tutorial on nested classes for more information.

---

EDIT: I originally thought this was invalid code; that you couldn’t use $ in an identifier in Java to start with. It turns out I’m wrong. From the Java Language Specification, section 3.8:

The $ character should be used only in mechanically generated source code or, rarely, to access preexisting names on legacy systems.

So it’s valid, just discouraged.