java compiled classes contain dollar signs

Question

I've been using Eclipse as my IDE. I also use it to export my application into a JAR file. When I look at my classes in the JAR file, a few of my classes contain the name of that class, a dollar sign, then a number. For example:

Find$1.class 
Find$2.class
Find$3.class
Find.class

I've noticed it does this on bigger classes. Is this because the classes get so big, it compiles it into multiple classes? I've googled and looked on multiple forums, and search the Java documentation but have not found anything even related to it. Could someone explain?

Sample code to test this issue https://github.com/Aksi0m/SampleFor123686419 — Prags, Apr 02 '19 at 09:17
As per response from google team https://issuetracker.google.com/issues/123736741, this is fixed internally and will be available in the next release of Navigation. — Prags, Apr 02 '19 at 09:19

score 129 · Accepted Answer · edited Nov 28 '17 at 06:59

129

Inner classes, if any present in your class, will be compiled and the class file will be ClassName$InnerClassName. In case of Anonymous inner classes, it will appear as numbers. Size of the Class (Java Code) doesn't lead to generation of multiple classes.

E.g. given this piece of code:

public class TestInnerOuterClass {
    class TestInnerChild{

    }

    Serializable annoymousTest = new Serializable() {
    };
}

Classes which will be generated will be:

TestInnerOuterClass.class
TestInnerOuterClass$TestInnerChild.class
TestInnerOuterCasss$1.class

Update:

Using anonymous class is not considered a bad practice ,it just depends on the usage.

Check this discussion on SO

edited Nov 28 '17 at 06:59

Eugene S

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answered Jul 09 '12 at 03:45

mprabhat

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Okay, so is it bad to have an anonymous class? I don't quite understand how it works. If I create a new instance variable of a class in a class, that is a anonymous class? – Frizinator Jul 09 '12 at 03:48
No it is not bad to have anonymous class, it depends on the usage. Updated my post – mprabhat Jul 09 '12 at 03:49
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Enums containing a class body will automatically generate an anonymous inner class that extends the enum class, and thus generate another class file with a $ in it. – dlaidlaw Jul 07 '15 at 15:22
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Also if inner class is private one additional class is generated named OuterClass$1.class – SandeepGodara May 05 '16 at 08:44
@mprabhat : is there any mechanism in android studio to see the hierarchy or list of these classes? – user1090751 Mar 05 '18 at 10:43
@SandeepGodara Why is this? – Elisio Quintino Aug 28 '18 at 07:00

score 17 · Answer 2 · edited May 23 '17 at 12:10

17

This is because you have anonymous classes within this larger class. They get compiled using this naming convention.

See The Anonymous Class Conundrum

edited May 23 '17 at 12:10

Community

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answered Jul 09 '12 at 03:45

jjathman

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score 15 · Answer 3 · answered Aug 28 '15 at 15:29

In addition to the above cases presented by @mprabhat, the other cases could be:

if you class contain a enum variable a separate class would be generated for that too. The name of the .class generated would be ClassName$Name_of_enum.
If your class X is inheriting i.e. extending another class Y, then there would be a .class generated with the name ClassName$1.class or ClassName$1$1.class
If your class X is implementing an interface Y, then there would be a .class generated with the name ClassName$1.class or ClassName$1$1.class.

These cases are derivations of my inspection on .class files in jar.

I have a class that meets none of these criteria, and class.getName() is still wrong-- for Group.java, it returns Group_$$_jvst248_20. I have no idea what's going on. Definitely no enums, extends or implements. — Alkanshel, Apr 20 '16 at 20:12

score 0 · Answer 4 · answered Jul 11 '12 at 01:16

To answer your comment about are anonymous classes bad. They are most definately not. Consider this to assign an action listener to a JButton:

JButton button = new JButton(...);
button.addActionListener(new ActionListener() { ... });

or this to do a case insensitive sort by the "name" property

Collections.sort( array, new Comparator<Foo>() {
    public int compare(Foo f1, Foo f2) {
        return f1.getName().toLowerCase().compareTo(f2.getName().toLowerCase());
    }
});

You'll also see a lot of Runnable and Callable done as anonymous classes.

java compiled classes contain dollar signs

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