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Java Forum / General / April 2006what's the meaning of Class<?> and how to use it
The return type of JTable.getColumnClass(int column) is Class<?>. What is the meaning of it and how to use it properly? It seems the following code is okay, Class c = jTable1.getColumnClass(0); Is there another way to use Class<?>? keepiss schreef: > The return type of JTable.getColumnClass(int column) is Class<?>. What > is the meaning of it and how to use it properly? It seems the following [quoted text clipped - 3 lines] > > Is there another way to use Class<?>? It is used intensively in reflection. I suggest you keep away from that a while until you feel you have enough experience to delve into the topic. http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html H. - -- Hendrik Maryns ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html I know 'Class' is mainly used in reflection. What I don't understand is <?>. What does it mean? > I know 'Class' is mainly used in reflection. What I don't understand is > <?>. What does it mean? http://java.sun.com/docs/books/tutorial/extra/generics/index.html /Thomas  SignatureThe comp.lang.java.gui FAQ: ftp://ftp.cs.uu.nl/pub/NEWS.ANSWERS/computer-lang/java/gui/faq http://www.uni-giessen.de/faq/archiv/computer-lang.java.gui.faq/ > I know 'Class' is mainly used in reflection. What I don't understand is > <?>. What does it mean? An attempt to explain <?> is given in <http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf> (chapter "4 Wildcard"). May be you can make a sense of it. (I don't, but I'm new to generics anyway :-( ) Signature"Thomas:Fritsch$ops:de".replace(':','.').replace('$','@') Thomas Fritsch schreef: >> I know 'Class' is mainly used in reflection. What I don't understand is >> <?>. What does it mean? [quoted text clipped - 4 lines] > May be you can make a sense of it. (I don't, but I'm new to generics > anyway :-( ) The <T> in Class<T> is the class it actually represents. So the Class object belonging to YourClass is Class<YourClass>. This has all kinds of advantages in making sure the right class object is given in method arguments etc. If a method returns Class<?>, this means it can return any Class object. H. - -- Hendrik Maryns ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html > Thomas Fritsch schreef: >>An attempt to explain <?> is given in [quoted text clipped - 9 lines] > > If a method returns Class<?>, this means it can return any Class object. Ah, thanks! So Class<?> is pretty much the same as Class. A somewhat related question: How do I get the concrete semantic meaning of a <T> or <E> parameter found in the javadoc class declaration? For example in: public class Class<T> extends Object public interface Collection<E> extends Iterable<E> Surely we can make clever guesses about what the <T> or <E> parameter in a particular class is intended to mean, by reading the javadoc of the methods of that class. But shouldn't there be a more formal description, too? Might be as a new javadoc tag, for example: public class Class<T> extends Object @genericparam T this class public interface Collection<E> extends Iterable<E> @genericparam E the type of objects contained in this Collection Signature"Thomas:Fritsch$ops:de".replace(':','.').replace('$','@') Thomas Fritsch schreef: >> Thomas Fritsch schreef: >>> An attempt to explain <?> is given in [quoted text clipped - 11 lines] >> > Ah, thanks! So Class<?> is pretty much the same as Class. Well, yes, it is the generified version. Class is the erasure type. Read up on generics if this sounds unclear to you. Also see http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#Wha t%20does%20the%20type%20parameter%20of%20class%20java.lang.Class%20mean? shortened: http://tinyurl.com/hk39y > A somewhat related question: > How do I get the concrete semantic meaning of a <T> or <E> parameter > found in the javadoc class declaration? For example in: > public class Class<T> extends Object > > public interface Collection<E> extends Iterable<E> You (almost) can’t. The information is erased. At runtime, it is an Object. For details and tricks, browse the above site. > Surely we can make clever guesses about what the <T> or <E> parameter in > a particular class is intended to mean, by reading the javadoc of the [quoted text clipped - 5 lines] > public interface Collection<E> extends Iterable<E> > @genericparam E the type of objects contained in this Collection Most of the time, the E will be unknown at the moment the class is written, thus nothing sensible can be said about it. Everything you mention here is indeed in the documentation, and is sort of ‘evident’ as it is the purpose of generics. Class is the noticeable exception. H. - -- Hendrik Maryns ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html > Thomas Fritsch schreef: >> Surely we can make clever guesses about what the <T> or <E> parameter in [quoted text clipped - 11 lines] > mention here is indeed in the documentation, and is sort of ?evident? as > it is the purpose of generics. Class is the noticeable exception. The other noticeable exception from self-evidence is public abstract class Enum <E extends Enum<E>> But AFAIK that class is only for internal use by the compiler (as superclass of all enum types). Hence incomprehensibility is no problem here. ;-) Signature"Thomas:Fritsch$ops.de".replace(':', '.').replace('$', '@') Free MagazinesGet these publications absolutely FREE for up to 12 months. 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