ordinal() returns inconsistent values?
in [Java Programming]
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From: Todd on 8 Apr 2010 12:13 Hello, I have recently been told that the ordinal() method in a Java enum will not necessarily return the same value in different invocations of the JVM. Has anyone else found this? BTW, the semantics of the enum will stay the same, an enumerated value that was once greater than another will continue to be so. It is just that the absolute value of the declared enumeration will not necessarily be the same from run to run. This indicates to me that one would not be able to retrieve an enumerated element by a stored ordinal value. Further, the ordinal could be a value greater than the number of enumerated elements, making values()[ordinal()] suspect as well. Any insights are appreciated, Todd
From: Mayeul on 8 Apr 2010 13:10 Hello, Todd wrote: > I have recently been told that the ordinal() method in a Java enum > will not necessarily return the same value in different invocations of > the JVM. Has anyone else found this? Never observed this, and it would be contradictory with the JavaDoc of Enum.ordinal() . I often expect ordinal() to behave as specified, and have never noticed this expectation to break anything. > [...] This indicates to me that > one would not be able to retrieve an enumerated element by a stored > ordinal value. Well that /is/ pretty unsafe to do anyway. What if you need to add/remove/swap enum values? I usually store their name instead, or try to think up some safe enough serialization. > [...] -- Mayeul
From: Joshua Cranmer on 8 Apr 2010 13:43 On 04/08/2010 12:13 PM, Todd wrote: > I have recently been told that the ordinal() method in a Java enum > will not necessarily return the same value in different invocations of > the JVM. Has anyone else found this? To do so would contradict the API: public final int ordinal() Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap. Returns: the ordinal of this enumeration constant -- Beware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth
From: Todd on 8 Apr 2010 14:27 On Apr 8, 11:43 am, Joshua Cranmer <Pidgeo...(a)verizon.invalid> wrote: > On 04/08/2010 12:13 PM, Todd wrote: > > > I have recently been told that the ordinal() method in a Java enum > > will not necessarily return the same value in different invocations of > > the JVM. Has anyone else found this? > > To do so would contradict the API: > public final int ordinal() > > Returns the ordinal of this enumeration constant (its position in > its enum declaration, where the initial constant is assigned an ordinal > of zero). Most programmers will have no use for this method. It is > designed for use by sophisticated enum-based data structures, such as > EnumSet and EnumMap. > > Returns: > the ordinal of this enumeration constant > > -- > Beware of bugs in the above code; I have only proved it correct, not > tried it. -- Donald E. Knuth I fully agree. I was told that the JavaDocs were wrong. I tried locating a source on the web to corroborate the assertion, but couldn't find one.
From: John B. Matthews on 8 Apr 2010 15:46
In article <69c37aba-9511-49b4-924f-1ad0236b9ba7(a)z4g2000yqa.googlegroups.com>, Todd <todd.heidenthal(a)gmail.com> wrote: > On Apr 8, 11:43 am, Joshua Cranmer <Pidgeo...(a)verizon.invalid> wrote: > > On 04/08/2010 12:13 PM, Todd wrote: > > > > > I have recently been told that the ordinal() method in a Java > > > enum will not necessarily return the same value in different > > > invocations of the JVM. Has anyone else found this? > > > > To do so would contradict the API: > > public final int ordinal() > > > > Returns the ordinal of this enumeration constant (its position in > > its enum declaration, where the initial constant is assigned an ordinal > > of zero). Most programmers will have no use for this method. It is > > designed for use by sophisticated enum-based data structures, such as > > EnumSet and EnumMap. > > > > Returns: > > the ordinal of this enumeration constant > [...] > I fully agree. I was told that the JavaDocs were wrong. I tried > locating a source on the web to corroborate the assertion, but > couldn't find one. I'd have thought to avoid ordinal(), as suggested by Joshua Bloch, "Item 31: Use instance fields instead of ordinals." Regarding serialization, section 1.12 of "Java Object Serialization Specification," entitled "Serialization of Enum Constants," mentions using "the value returned by the enum constant's name method." <http://java.sun.com/javase/6/docs/platform/serialization/spec/serialTOC.html> -- John B. Matthews trashgod at gmail dot com <http://sites.google.com/site/drjohnbmatthews> |