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Java Forum / General / July 2006Does Object extend Object?
The javadoc on the class Object is as follows: Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class. Does this mean that Object implicitly extends Object? It passes all the inheritance tests such as implementing the methods of the parent class, the "is a" relationship etc. So does it in fact extend itself or should it say "Every OTHER class has Object as a superclass? Thanks for your thoughts! > The javadoc on the class Object is as follows: > [quoted text clipped - 3 lines] > > Does this mean that Object implicitly extends Object? Yes, technically, Object extends Object. What does this mean? Really not much, except that it sometimes simplifies reflection code... i.e. Class<?> c = ...; while (c.getSuperClass() != Object.class) { ... } works even if c is Object.class (for the most obvious intended behaviors).  SignatureChris Smith - Lead Software Developer / Technical Trainer MindIQ Corporation This post led me to an obvious conclusion of just trying it... The following code: Object o = new Object(); Class c = o.getClass(); Class c2 = c.getSuperclass(); System.err.println(c.getName()); System.err.println(c2.getName()); yields: java.lang.Object Exception in thread "main" java.lang.NullPointerException at main(Main.java:15) So apparently Object does not in fact extend Object... and the javadoc is inaccurate. > Yes, technically, Object extends Object. What does this mean? Really > not much, except that it sometimes simplifies reflection code... i.e. JLS3 8.1.4 p184 "... the class Object ... has no direct superclass." Ugly API wart... As an experiment I tried to define Object as package java.lang; public class Object extends java.lang.Object { ... }. javac complains about cyclic inheritance. > Class<?> c = ...; > while (c.getSuperClass() != Object.class) { ... } ^getSuperclass Tom Hawtin SignatureUnemployed English Java programmer http://jroller.com/page/tackline/ > > Yes, technically, Object extends Object. What does this mean? Really > > not much, except that it sometimes simplifies reflection code... i.e. > > JLS3 8.1.4 p184 "... the class Object ... has no direct superclass." > > Ugly API wart... Whoops! I was sure I saw that somewhere. Apparently not. SignatureChris Smith - Lead Software Developer / Technical Trainer MindIQ Corporation > ... > So does it in fact extend itself or should it say "Every OTHER class > has Object as a superclass? > ... I would think that having a class extend itself doesn't make a lot of sense, and would involve some serious problems. First of all, the cyclic inheritance restriction comes into play and forbids this. If that were somehow overridden, then there would be an infinite regress in any method that invoked super(). Then there's the problem of duplication of data. If field X is defined in Object, is it also defined in the parent of Object? And in the parent of the parent, etc.? All of the above really reduce to one objection, not three, but they illustrate the multiple facets of the contradiction involved in self-inheritance. Alan >> ... >> So does it in fact extend itself or should it say "Every OTHER class [quoted text clipped - 3 lines] > I would think that having a class extend itself doesn't make > a lot of sense, and would involve some serious problems. Back in CS101, I seem to recall being taught that the parent node of any tree was the node itself. I guess if Object doesn't extend Object, then the Java class hierarchy isn't really a tree... > First of all, the cyclic inheritance restriction comes into play > and forbids this. > > If that were somehow overridden, then there would be an > infinite regress in any method that invoked super(). No, just methods that don't check for the terminal case of Object. Even without Object extending Object, the regression has to check for a base case. The regression or while-loop that invokes super() has to know when to end, anyway. > Then there's the problem of duplication of data. If field X > is defined in Object, is it also defined in the parent of Object? > And in the parent of the parent, etc.? Object doesn't have any public fields, so I'm not sure that's an issue. > All of the above really reduce to one objection, not three, > but they illustrate the multiple facets of the contradiction > involved in self-inheritance. Ruby has a single root Object class, and its superclass() method returns the equivalent of null. I wonder how Python handles this. > Back in CS101, I seem to recall being taught that the parent node of any > tree was the node itself. I guess if Object doesn't extend Object, then > the Java class hierarchy isn't really a tree... I think, for convenience in writting recursive code, it's useful to treat any node within a tree as a tree as well. But when we need the precision (such as now), I think it's useful to distinguish between the tree and the nodes that make up the tree. If a tree has a parent, that parent would be a forest, not a tree, nor a node. In most contexts, a tree is nothing more than a connected acyclic graph. Under this definition, the Java class hierarchy is a tree; if you include interfaces, it is no longer a tree. Taken literally, if any node is connected to itself, that's a cycle, and thus not a tree. - Oliver >> Back in CS101, I seem to recall being taught that the parent node of >> any tree was the node itself. I guess if Object doesn't extend [quoted text clipped - 10 lines] > include interfaces, it is no longer a tree. Taken literally, if any node > is connected to itself, that's a cycle, and thus not a tree. Excellent point. :) * Jeffrey Schwab: > Ruby has a single root Object class, and its superclass() method returns > the equivalent of null. I wonder how Python handles this. Python's class <type 'object'> has no bases either: >>> object <type 'object'> >>> object.__bases__ () (object.__bases__ is an empty list since Python allows multiple inheritance but object doesn't have a base.) But the root of Python's inheritance hierarchy is quite interesting. Since classes are objects, too, <type 'object'> is an instance of <type 'type'>, the mother of all types. At the same time, <type 'type'> is an instance if itself and derives from <type 'object'>. %-) Good resource: <http://www.cafepy.com/article/python_types_and_objects/> J. SignatureThy lyrics in pop songs seem to describe my life uncannily accurately. [Agree] [Disagree] <http://www.slowlydownward.com/NODATA/data_enter2.html> > The javadoc on the class Object is as follows: > [quoted text clipped - 3 lines] > > Does this mean that Object implicitly extends Object? No, just sloppy wording. "A superclass" is odd too; generally, a class (other than Object itself) has a single superclass, which might, of course, not be Object. Better would be "Every class other than Object itself has Object as an ancestor." Note that Object is also the superclass of all interfaces. You an view this fact as the reason that all methods defined by Object can be invoked on interface-typed references. Free MagazinesGet these publications absolutely FREE for up to 12 months. 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