JAVA - Difference between Class class and *.class files?

Question

There's something I don't understand between *.class files and Class class API. Let's me explain :

I have a file A.java representing a java class :

public class A { ... }

Class<?> clazz = A.class;

When I compile A.java, I get a A.class file (the byte code).

Is there any relation between the A.class file (bytecode) and clazz which represents the instance class (A.class) ? Are they the same thing ?

Thank you

Answer 1

*.class files are files on the disk, A.class is a class object in the memory.

Answer 2

Class<?> clazz = A.class;

is equivalent to

Class<?> clazz = Class.forName("A");

except that for the class literal, A.class, the compiler will check that A is available and accessible at compile-time, so its availability will be mandatory at runtime. Therefore, you don’t need to catch checked exceptions.

If A is not abstract and has a default constructor, you would get the same via

Class<?> clazz = new A().getClass();

Of course, all these constructs depend on the availability of a class definition for A at runtime, which is typically delivered in a class file name A.class, but the fact that one of these source code constructs looks similar to the file name has no relevance.

Note that for nested classes, the source code representation and the file name will differ, e.g. when using Map.Entry.class to refer to the nested type java.util.Map.Entry, the class file’s name will be Map$Entry.class. You could also add an import statement for this class and refer to it via Entry.class, showing that this construct is subject to the standard source code name resolution rules and not connected to the file name of the compiled class.