5 ms·

Don't Classes in Java get boiled off in the compiler? There are no "class objects" at runtime that I've ever seen.

• lucifer 17y ago
  // unseen? if (foo.getClass() instanceof someClassObject) ...
  • blasdel 17y ago
    In this case isn't someClassObject just a literal that's resolved at compile-time and erased into a simple identifier symbol? You can't pass someClassObject as an argument to anything but an operator like new or instanceof, and you can't assign to or from it, it doesn't itself belong to a class. It isn't an object.
    • lucifer 17y ago
      > In this case isn't someClassObject just a literal that's resolved at compile-time and erased into a simple identifier symbol? No. Its a variable assigned in runtime. public void yesYouCan (Object foo, Class<?> someClassObject) { // compile time? if(foo.getClass() instanceof someClassObject) { ... } } Regardless, class and symbolic resolution occur at both compilation (to create the binary clsss files) and in runtime in the JVM (to load and execute the binary generated by the compiler): http://java.sun.com/docs/books/jls/second_edition/html/execution.doc.html#44487 http://java.sun.com/docs/books/jls/second_edition/html/execu... > You can't pass someClassObject as an argument to anything but an operator like new or instanceof, and you can't assign to or from it, it doesn't itself belong to a class. It isn't an object. No, its quite possible/common to write methods that take class objects as parameters.
  • vlisivka 17y ago
    if(foo instanceof SomeClassObject) ... or if(SomeClassObject.isInstance(foo)) ...
• pvg 17y ago
  There are. Think about classloaders and the whole dynamic code loading thing. Every single object has a class object associated with it.
  • blasdel 17y ago
    But can you pass classes around as objects after they're loaded? Do they have a value beyond their identifier?
    • dminor 17y ago
      Yes, although Java's reflection is somewhat of a pain in the ass and limited in many ways. But you can take a Class object and call newInstance(), and it will invoke the class's no-arg constructor.
      • duskwuff 17y ago
        Moreover, reflection is less of a language feature and more of a very low-level library feature. In ObjC, one can write Class c = [SomeRandomClass class]; id thingy = [[c alloc] init]; but Java won't let you do the equivalent: Class c = SomeRandomClass; Object x = new c;
        • vlisivka 17y ago
          Class c = SomeRandomClass.class; Object x=c.newInstance();
          • duskwuff 17y ago
            Yes, but that's making use of runtime magic (the Class class) which isn't used outside metaprogramming.
    • pvg 17y ago
      Yep, you can pass them around, they do have values, in fact, they can have different values for the same type sometimes. This has been the case since well before Java even had reflection - having them around is fairly central to the Java dynamic loading and security model.
• bad_user 17y ago
  What do you mean? Classes are in some sense like objects, meaning they can hold their own state, but there is no notion of meta-classes like in Smalltalk. The class in the JVM is like a module that holds functions and attributes. And the JVM byte-code is very class-centric. The objects themselves are probably not what you think they are. The JVM currently has no notion of "call method next() on object O" ... that's done with something like ... invoke_virtual Enumerable.next(obj). The bytecode "invoke_virtual" is responsible for doing the single-dispatching (subtype-polymorphism) at runtime, otherwise it looks just like an invoke_static ... so the compiler is crucial for generating the correct byte-code, because the class has to be known at runtime ;)