Java Forum / General / December 2007
Is clone a member function for array types?
tam@milkyway.gsfc.nasa.gov - 03 Dec 2007 16:02 GMT According to the JLS clone is a public method for all array types:
6.4.5 Members of an Array Type
... The public method clone, which overrides the method of the same name in class Object and throws no checked exceptions. The return type of the clone method of an array type T[] is T[]. ...
However if I run the following program (JDK 1.5.0_04-b05 on Linux):
import java.lang.reflect.*; public class Test {
public static void main(String[] args) { int[] x = {1,2,3};
System.out.println("Class:"+x.getClass()); Class cls = x.getClass(); Method[] methods= cls.getMethods(); for(Method meth: methods) { System.out.println("Method:"+meth); } } }
I get the output
Class:class [I Method:public native int java.lang.Object.hashCode() Method:public final native java.lang.Class java.lang.Object.getClass() Method:public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException Method:public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException Method:public fina lean java.lang.Object.equals(java.lang.Object) Method:public final native void java.lang.Object.notify() Method:public final native void java.lang.Object.notifyAll() Method:public java.lang.String java.lang.Object.toString()
The clone method is missing.
The JavaDocs for the Class.getMethods is:
Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces.
So I should have gotten all the public methods, and clone is supposed to be public, but it's not showing up.... Does anyone have any idea what I'm missing?
Regards, Tom McGlynn
Daniel Pitts - 03 Dec 2007 17:06 GMT > According to the JLS clone is a public method for all array types: > [quoted text clipped - 53 lines] > Regards, > Tom McGlynn hmm... Try using getDeclaredMembers to see. It could be that array cloning is a special case, look at the VM spec to see if there are any hints there. Array types *are* a special case in the JVM.
 Signature Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
tam@milkyway.gsfc.nasa.gov - 03 Dec 2007 17:31 GMT On Dec 3, 12:06 pm, Daniel Pitts <newsgroup.spamfil...@virtualinfinity.net> wrote:
> t...@milkyway.gsfc.nasa.gov wrote: > > According to the JLS clone is a public method for all array types: [quoted text clipped - 35 lines] > > java.lang.InterruptedException > > Method:public fina lean java.lang.Object.equals(java.lang.Object) Seem to have accidentally done some editing here. This should be 'public boolean"...
> > Method:public final native void java.lang.Object.notify() > > Method:public final native void java.lang.Object.notifyAll() [quoted text clipped - 19 lines] > cloning is a special case, look at the VM spec to see if there are any > hints there. Array types *are* a special case in the JVM. Do you mean getDeclaredMethods? I tried that but it returns an empty array. That's OK in principle since the method might be declared in some superclass of [I but getMethods should find it regardless of where it's declared.
Although arrays are special, it seems very peculiar for there to be objects of any type with methods not visible by reflection that may still be invoked non-reflectively.
Regards, Tom McGlynn
tam@milkyway.gsfc.nasa.gov - 03 Dec 2007 17:31 GMT On Dec 3, 12:06 pm, Daniel Pitts <newsgroup.spamfil...@virtualinfinity.net> wrote:
> t...@milkyway.gsfc.nasa.gov wrote: > > According to the JLS clone is a public method for all array types: [quoted text clipped - 35 lines] > > java.lang.InterruptedException > > Method:public fina lean java.lang.Object.equals(java.lang.Object) Seem to have accidentally done some editing here. This should be 'public boolean"...
> > Method:public final native void java.lang.Object.notify() > > Method:public final native void java.lang.Object.notifyAll() [quoted text clipped - 19 lines] > cloning is a special case, look at the VM spec to see if there are any > hints there. Array types *are* a special case in the JVM. Do you mean getDeclaredMethods? I tried that but it returns an empty array. That's OK in principle since the method might be declared in some superclass of [I but getMethods should find it regardless of where it's declared.
Although arrays are special, it seems very peculiar for there to be objects of any type with methods not visible by reflection that may still be invoked non-reflectively.
Regards, Tom McGlynn
Roedy Green - 03 Dec 2007 17:22 GMT > Does anyone have any idea what I'm missing? My guess is arrays are handled as a special case. For example you can iterate over them with for:each even though they don't implement Iterable.
 Signature Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com
tam@milkyway.gsfc.nasa.gov - 03 Dec 2007 23:26 GMT On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> wrote:
> On Mon, 3 Dec 2007 08:02:51 -0800 (PST), t...@milkyway.gsfc.nasa.gov > wrote, quoted or indirectly quoted someone who said : [quoted text clipped - 7 lines] > Roedy Green Canadian Mind Products > The Java Glossaryhttp://mindprod.com The JLS explicitly mentions that arrays can be iterated over in the enhanced for loop. Unless there is something I'm missing here [which is quite likely but I'd like to know what it is], there seems to be a clear incompatibility between the JLS specification of array members and Class.getMethods results for arrays. There's nothing I've found in the documentation that suggests arrays should be handled specially here.
Regards, Tom McGlynn
Mike Schilling - 04 Dec 2007 00:30 GMT > On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> > wrote: [quoted text clipped - 6 lines] >> iterate over them with for:each even though they don't implement >> Iterable.
> The JLS explicitly mentions that arrays can be iterated over in the > enhanced for loop. Unless there is something I'm missing here [which [quoted text clipped - 3 lines] > in the documentation that suggests arrays should be handled specially > here. Of course arrays can be used in enhanced for loops; that doesn't imply that they implement Iterable. The compiler knows that it's an array, and can emit code accordingly. E.g.
for (int entry : i) { System.out.println(entry); }
generates
int arr$[] = i; int len$ = arr$.length; for(int i$ = 0; i$ < len$; i$++) { int entry = arr$[i$]; System.out.println(entry); }
Daniel Pitts - 04 Dec 2007 00:41 GMT > On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> > wrote: [quoted text clipped - 19 lines] > Regards, > Tom McGlynn array.length is actually a special JVM instruction, not a field access.
 Signature Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
Patricia Shanahan - 04 Dec 2007 01:00 GMT >> On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> >> wrote: [quoted text clipped - 18 lines] >> > array.length is actually a special JVM instruction, not a field access. At the Java source language level, according to the JLS, length is a public final field member of an array type, so it *should* show up in reflection. However, according to the API documentation for java.lang.Class, "Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions."
Patricia
Stefan Ram - 04 Dec 2007 01:11 GMT >However, according to the API documentation for >java.lang.Class, "Every array also belongs to a class that is >reflected as a Class object that is shared by all arrays with >the same element type and number of dimensions." It is always on my mind, whether an array is an instance of a class. What you quote sounds as if this is so.
On the other hand, the JLS3 says:
»An object (§4.3.1) is a dynamically created instance of a class type or a dynamically created array.«
JLS3, 4
http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html
»An object is a class instance or an array.«
JLS3, 4.3.1
http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.3.1
If an array is an instance of a class, then why does the JLS adds »... or an array« in the quotations? It would be sufficient to state »An object is a class instance.«, because this would include arrays.
tam@milkyway.gsfc.nasa.gov - 04 Dec 2007 14:34 GMT On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> wrote:
> On Mon, 3 Dec 2007 08:02:51 -0800 (PST), t...@milkyway.gsfc.nasa.gov > wrote, quoted or indirectly quoted someone who said : [quoted text clipped - 7 lines] > Roedy Green Canadian Mind Products > The Java Glossaryhttp://mindprod.com You're right! I've looked more carefully at the JavaDocs and I see that you're right and the description for getMethods explicitly mentions that only the methods inherited from Object are returned: "Array classes return all the (public) member methods inherited from the Object class."
Not sure how I missed that in my first read through, but I should have been more careful when posting. It's still a bit of a gotcha, but my apologies for misleading anyone that it is not consistent with the documentation.
I see that the discussion of getFields similarly talks in detail about not handling the length attribute that Patricia talks about elsewhere in this thread.
This came up in a routine that was trying to do a generic clone. I was checking that an object implemented Cloneable and had a public clone method reflectively, It looks like I need to check for arrays explicitly also.
Regards, Tom McGlynn
Daniel Pitts - 04 Dec 2007 17:27 GMT > On Dec 3, 12:22 pm, Roedy Green <see_webs...@mindprod.com.invalid> > wrote: [quoted text clipped - 32 lines] > Regards, > Tom McGlynn Not to mention Arrays are a special case anyway. I believe that if you clone a multi-dimensional array, you in fact only clone the left-most dimension, and therefor have shared elements.
Not to mention that performance wise, .clone on arrays(in currently implementations) tends to be slower than creating a new array and using System.arrayCopy. Also, in Java 6 java.util.Arrays has some very nice methods for copying and resizing arrays. No deep copy though :-(
Why, may I ask, are you trying to create a generic clone? What problem will be solved by it? Its been my experience that using reflection should be a last resort, or used only in framework libraries/APIs.
 Signature Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
tam@milkyway.gsfc.nasa.gov - 04 Dec 2007 21:00 GMT On Dec 4, 12:27 pm, Daniel Pitts <newsgroup.spamfil...@virtualinfinity.net> wrote:
> t...@milkyway.gsfc.nasa.gov wrote: ...
> Not to mention Arrays are a special case anyway. > I believe that if you clone a multi-dimensional array, you in fact only > clone the left-most dimension, and therefor have shared elements. Actually I think that a deep clone of arrays would have been the special case, were that to have been Java's behavior. Java does not have multidimensional arrays. It only has arrays of arrays. The default behavior of shallow cloning seems perfectly in keeping with the shallow cloning of objects generally.
> Not to mention that performance wise, .clone on arrays(in currently > implementations) tends to be slower than creating a new array and using > System.arrayCopy. Also, in Java 6 java.util.Arrays has some very nice > methods for copying and resizing arrays. No deep copy though :-( The library that this came up in dates back all of the way to Java 1.0 in its origins, though clearly it's been updated a bit since I don't think getMethods was available till at least 1.1 (Maybe 1.2?). A deep copy of arrays is another of the functions it provides.
> Why, may I ask, are you trying to create a generic clone? What problem > will be solved by it? Its been my experience that using reflection > should be a last resort, or used only in framework libraries/APIs. The method is used in a library which reads a data format (FITS) which can contain a variety of objects including arrays of any primitive type, and tables with columns of heterogeneous data. It's often necessary to make duplicates of a structure and having a single method to do so is very convenient. At one time using clone seemed simplest since it allows me to use a common method for both my custom classes, primitive arrays and the a few standard Java classes that may be included in the structures conveniently. In fact this method is not used much -- the deep array copy I mentioned above is used in most of the cases where I anticipated this method might be used. That uses reflection too but not the classes of java.lang.reflect. It's not liable to the bug that lay hidden in the generic clone for so long.
When one is dynamically constructing objects using reflection can be the natural approach and that's one of the things that the library is supporting -- that might be what you have in mind as a 'framework/ API'.
Regards, Tom McGlynn
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