Why does Java code with an inner class generates a third SomeClass$1.class file? [duplicate]

Question

If I have an inner class, like this:

public class Test
{
    public class Inner
    {
        // code ...
    }

    public static void main(String[] args)
    {
        // code ...
    }
}

When I compile it, I expect it should generate two files:

Test.class
Test$Inner.class

So why do I sometimes see classfiles like SomeClass$1.class, even though SomeClass does not contain an inner class called "1"?

Destefano

Can you clarify why you did not see Test.class and Test$Inner.class? I assume there another SomeClass in the same package that generated the class files you did see? – Alley

Answer 1

The SomeClass$1.class represent anonymous inner class

hava a look at the anonymous inner class section here

Schaaf

Note that the names of inner classes are not specified so they need not be of this form and it would be a bug to rely on that. But that being said, I've not yet seen a compiler that generates different names. – Norine

Answer 2

You'll also get something like SomeClass$1.class if your class contains a private inner class (not anonymous) but you instantiate it at some point in the parent class.

For example:

public class Person {

    private class Brain{
        void ponderLife() {
            System.out.println("The meaning of life is...");
        }
    }

    Person() {
        Brain b = new Brain();
        b.ponderLife();
    }
}

This would yield:

Person.class
Person$Brain.class
Person$1.class

Personally I think that's a bit easier to read than a typical anonymous class especially when implementing a simple interface or an abstract class that only serves to be passed into another local object.

Active

Why does javac do this? This case is more explicitly asked at: #2883681 – Starcrossed

Answer 3

to build up on hhafez : SomeClass$1.class represents anonymous inner classes. An example of such a class would be

public class Foo{
  public void printMe(){
    System.out.println("redefine me!");
  }
}


public class Bar {
    public void printMe() {
    Foo f = new Foo() {
        public void printMe() {
        System.out.println("defined");
        }
    };
    f.printMe();
    }
}

From a normal Main, if you called new Bar().printMe it would print "defined" and in the compilation directory you will find Bar1.class

this section in the above code :

    Foo f = new Foo() {
        public void printMe() {
        System.out.println("defined");
        }
    };

is called an anonymous inner class.

Iva