Why Do Java Compiled Classes Have Dollar Signs?

Java Compiled Classes with Dollar Signs

Java programmers often encounter a peculiar naming convention in their compiled class files. Some classes exhibit an odd pattern of their class name followed by a dollar sign and a number. For instance, you might see files like:

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

Delving into the Cause

Contrary to popular belief, the size of the class does not trigger this behavior. The answer lies in inner classes. When a class contains inner classes, the compiler generates separate class files for those inner classes. The file name follows the pattern:

OuterClass.class
OuterClass$InnerClass.class

Anonymous Inner Classes

Anonymous inner classes, which are defined without a custom name, are assigned numbers. This explains the "Find$1.class" and similar file names.

Example

Consider the following code snippet:

public class OuterClass {
    class InnerClass {
    }

    Serializable anonymous = new Serializable() {
    };
}

The compiler will generate the following class files:

• OuterClass.class
• OuterClass$InnerClass.class
• OuterClass$1.class

Anonymous Classes Considered?

The use of anonymous inner classes has been subject to debate. Some consider it a code smell, while others argue its appropriateness in certain scenarios. Ultimately, the decision to use them is based on specific design requirements.

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