Why does Java generate Multiple .class files on compilation?

Question

In Java, on compilation we get a .class file for each class( including nested classes and interfaces) defined in the source file.

What is the reason for this multiple .class file generation?
Is it for simplifying the reusablity of the class?
Why not generate one .class for one .java file?

Answer 1

The JVM needs to be able to find the code for a given class, given its name. If there's potentially no relationship between the source filename and the code filename, and you want the code filename to be based on the source filename, how would you expect it to load the code?

As an example: suppose I were to compile Foo.java which contains class Bar.

Another class then refers to Bar, so the JVM needs the code for it... how would you suggest it finds the file?

Note that in .NET there's a separate of unit of deployment called the assembly - and a reference to a type includes the assembly name as well, but that's slightly different from what you were proposing.

Answer 2

In response to @Jon Skeet's rhetorical question:

Another class then refers to Bar, so the JVM needs the code for it... how would you suggest it finds the file?

Suppose (hypothetically) that the Java classfile format represented nested / inner classes by embedding them in the classfile for the outermost class. The binary name for the Bar is "Lsome/pkg/Foo$Bar;". The class loader could split the name at the "$" character, use the first part to locate the classfile for Foo, and then navigate to the embedded Bar class representation.

I think that the real reason that inner/nested classes have separate classfiles is historical. IIRC, Java 1.0 did not support nested or inner classes, and hence the corresponding classfile formats did not need to deal with them. When Java 1.1 was created (supporting inner/nested classes), Sun wanted the classfile format to be compatible with the classfiles produced by the Java 1.0 compiler. So they chose to implement inner / nested classes as separate classfiles, using the reserved "$" character in the binary classname.

A second possible reason is that the flat format simplifies class loading compared to a hypothetical embedded format.

And finally, there was (and still is) no compelling reason for them NOT to use a flat file format. It maybe creates some minor head-scratching when some programmer wants to load inner classes using Class.forName() but that is pretty rare occurrence ... and the solution is straight-forward.

Answer 3

That's is a design decision regarding a compilation unit, made by the developers. Compiled classes are usually combined in a jar file.

Extract from Java Language Spec

7.3 Compilation Units CompilationUnit is the goal symbol (§2.1) for the syntactic grammar (§2.3) of Java programs.

Types declared in different compilation units can depend on each other, circularly. A Java compiler must arrange to compile all such types at the same time.