Multiple .class files generated for a class?

Out of curiosity, why are sometimes multiple Java .class files generated for a class after compilation? For example, my application has six classes. For one class, a total of 10 .class files has been generated, starting from MyClass#1 up to MyClass#10.

These are for inner classes and static nested classes. The ones with numbers are anonymous inner classes.

For example:


class Foo {
   class Bar { }
   static class Baz { }
   void run() {
      Helper t = new Helper() {
         int helpMethod() {
            return 2;
         }
      };
    }
}

This will produce class files Foo.class, Foo$Bar.class, Foo$Baz.class and Foo$1.class (for the implementation of the Helper interface)

You get more .class fils from a single source file if

the class contains inner classes or static inner classes. Inner classes can nest. Their names are <outer class name>$<inner class name>.
inner interfaces which are always static.
anonymous inner classes (which in fact are plain inner classes without a name)
package access interfaces and classes before and after your main class. You can have an arbitrary number of package access classes and interfaces in a single Java source file. Usually small helper objects that are only used by the class are just put into the same file.

One java source file can generate multiple class files, if your class contains inner classes. Anonymous inner classes are represented by your numbered class files.

Every class in java belongs to a .java-file, but a .java-file can contain multiple classes. That includes inner and anonymous classes. The .class-files generated for inner classes contain a '$' in their name. Anonymous inner classes get numbers.

To add to the answers above, this is another good example of generated inner classes based on Comparators (each Comparator is compiled in a different MyClass$X.class):

public class MyClass {
    ...

    public void doSomething() {
        ...
        Collections.sort(list, new Comparator<MyObj>() {
            public int compare(MyObj o1, MyObj o2) {
                ...
            }
        });
        ...
    }

    ...
}

If there is one X.java file and if it contains 4 collections.sort() {} then after compilation X.class,X$1.class,X$2.class,X$3.class,X$4.class will get generated.

In case of inner class and static inner class more .class files get generated.

Rakesh Juyal

More than one class will be generated on compilation, Only if your class is having inner class.

refer: Why does Java code with an inner class generates a third SomeClass$1.class file?

来源：https://stackoverflow.com/questions/1031962/multiple-class-files-generated-for-a-class

标签

java

class

javac