PersistenceDelegate for typesafe enum pattern

I've converted my code over from using int constants to the typesafe enum pattern in Effective Java. Unfortunately, the pattern only goes into serialization using Serializable and does not go into how to write a proper PersistenceDelegate that parallels the safety precautions of the Serializable enum. I've tried to do a direct mirroring of the readResolve() method given in the book inside of the intiatiate method of my PersistenceDelegate, but the XMLEncoder keeps rejecting the output and I get an InstantiationException, which shouldn't happen since the method call should not instantiate any new instances since the enums are static.
class PositionBiasModePersistenceDelegate extends PersistenceDelegate {
    protected Expression instantiate(Object oldInstance, Encoder out) {
        PositionBiasMode mode = (PositionBiasMode) oldInstance;
        return new Expression(mode, PositionBiasMode.VALUES, "get", new Object[] {new Integer(PositionBiasMode.VALUES.indexOf(mode))});
public abstract class PositionBiasMode {
    private final String name;
    private static int nextOrdinal = 0;
    private final int ordinal = nextOrdinal++;
    /** Creates a new instance of PreferredPositionMode */
    private PositionBiasMode(String name) {
        this.name = name;
    public String toString() {
        return name;
    public abstract boolean complies(PositionBias pb);
    public static final PositionBiasMode IGNORE = new PositionBiasMode("Ignore") {
        public boolean complies(PositionBias pb) {
            return true;
    public static final PositionBiasMode FRONT = new PositionBiasMode("Front") {
        public boolean complies(PositionBias pb) {
            return pb.hasBias() && pb.getPositionBias() < 0 ? false : true;
    public static final PositionBiasMode BACK = new PositionBiasMode("Back") {
        public boolean complies(PositionBias pb) {
            return pb.hasBias() && pb.getPositionBias() >= 0 ? false : true;
    private static final PositionBiasMode[] PRIVATE_VALUES = {IGNORE, FRONT, BACK};
    public static final List VALUES = Collections.unmodifiableList(Arrays.asList(PRIVATE_VALUES));
}

Yeah I tried this too. I think the instantiation exception is something to do with it trying to recreate the MyEnum.VALUES List. I don't understand enough about the process to know how to fix that though.
The approach I eventually went for was to add a couple of things to the enum class as follows:
// add to MyEnum class:
public static final PersistenceDelegate PERSISTENCE = new PersistenceDelegate()
     protected boolean mutatesTo(Object oldInstance, Object newInstance)
          return (MyEnum)oldInstance == (MyEnum)newInstance;
     protected Expression instantiate(Object oldInstance, Encoder out)
          MyEnum enum = (MyEnum)oldInstance;
          return new Expression(enum, new MyEnum.Factory(), "forOrdinal", new Object[]{new Integer(enum.ordinal)});
public static final class Factory
     public MyEnum forOrdinal(int ordinal)
          return PRIVATE_VALUES[ordinal];
// usage:
XMLEncoder enc = new XMLEncoder(...);
enc.setPersistenceDelegate(MyEnum.class, MyEnum.PERSISTENCE);Not entirely sure it's the ideal approach but maybe it'll work for you. Cheers.

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The predecessor to the "typesafe enum pattern"

In 1998, Michael Bridges published the following on Dr.Dobb's Portal
(http://www.ddj.com/windows/184403569):
class BookAgeLevel
private BookAgeLevel(){}
// Following are all the BookAgeLevel objects
// there will ever be.
final static BookAgeLevel TODDLER = new BookAgeLevel();
final static BookAgeLevel CHILD = new BookAgeLevel();
final static BookAgeLevel TEENAGER = new BookAgeLevel();
final static BookAgeLevel ADULT = new BookAgeLevel();
}In 1999, Nigel Warren and Philip Bishop refers to the exact same pattern as the "typesafe
constant idiom" in their book "Java in Practice" (page 48):
public final class Alienrace
public static final AlienRace BORG = new Alienrace();
public static final AlienRace FERENGI = new Alienrace();
private AlienRace() {}
}In 2000, Joshua Bloch refers to the following code as the "typesafe enum pattern" (http://java.sun.com/developer/Books/shiftintojava/page1.html):
// The typesafe enum pattern
public class Suit {
    private final String name;
    private Suit(String name) { this.name = name; }
    public String toString() { return name; }
    public static final Suit CLUBS = new Suit("clubs");
    public static final Suit DIAMONDS = new Suit("diamonds");
    public static final Suit HEARTS = new Suit("hearts");
    public static final Suit SPADES = new Suit("spades");
}

sunfun99 wrote:
No point, just interesting. There is no question attached to this.OK - I still have a load of code in both C++ and Java that uses this Pattern ( it ain't broke so I ain't fixing it) and I first started using this in C++ in about 1987 (God 20 years ago). I didn't invent it for myself so it must be older than that.

Typesafe enum pattern

Hi all,
In the Effective Java Programming Language Guide by: Joshua Bloch, chapter: 5, item: 21, I came across the following:
+Constants maybe added to a typesafe enum class w/o recompiling its clients b/c the public static object reference fields containing the enumeration constants provide a layer of insulation bet. the client and the enum class.+
What does that mean?
The constants themselves are never compiled into clients as they are in the more common int enum pattern and its String variant.
What does that mean?
Can someone please explain the above in more detail or guide me in another doc./article which helps me get the point?
Any help is greatly appreciated.
Edited by: RonitT on Apr 20, 2009 2:53 PM

In any case, that item should be updated now to advise you to use the Enum types introduced in 1.5 instead of rolling yer own. You are right, I'll use that instead - looks like I have an old edition of book!
Anyway. just for me to understand what you mentioned, since the client code sees the value of RANDOM -- like 17 and not 42 -- so let say my client code has the constants 0 thru 10 and now I need to add new constants from 11 to 20, if I'm confident that my client will never need these new constants, I don't need to give them the new code which contains the values 11 thru 20, is that right? That was you were trying to tell me?
Thanks again.

ORing TypeSafe enums together

I have tranaslated an enum type from C using the "typesafe enum pattern" (see, e.g. Bloch: Effective Java, p.104). I want to OR two of the enum values together. E.g.
Classname.FIRST | Classname.SECOND
but because they are instances of a class and not int values I get an "operator cannot be applied..." error from the compiler. How can I augment/modify the class to make this operation legal and correct?
Many thanks.

You would need to associate an integer ordinal with each enum. Josh demonstrates something like this later in the chapter for an enum that implements Comparable. Since you want to work with bitwise values, you'll need to make each successive ordinal a power of two so each enum maps to one bit. It might look something like this: public class MyEnum implements Comparable {
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        int thisOrd = nextOrdinal ;
        nextOrdinal <<= 1 ;
        return thisOrd ;
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    private final String name ;
    /** bitwise ordinal for this enum */
    private final int ordinal = getNextOrdinal() ;
    private MyEnum(String name)    { this.name = name ; }
    public String toString()       { return name ; }
    public int getOrdinal()        { return ordinal ; }
    public int compareTo(Object o) { return ordinal - ((MyEnum)o).ordinal ; }
    static public final MyEnum FIRST = new MyEnum("First") ;
    static public final MyEnum SECOND = new MyEnum("Second") ;
    static public final MyEnum THIRD = new MyEnum("Third") ;
    static public final MyEnum FOURTH = new MyEnum("Fourth") ;
} Then, two enums are "OR'd" together like so: MyEnum.FIRST.getOrdinal() | MyEnum.SECOND.getOrdinal().

Typesafe enums - 1.5

In the interview Joshua Bloch states the the new 1.5 feature "Typesafe enums - Provides all the well-known benefits of the Typesafe Enum pattern"
Does it also provide the pitfalls? The typesafe enum pattern is nice. However, in a multi-classloader environment it can behave unexpectedly. Similar to the Singleton pattern, it is generally best to avoid it in app-server code.
Does the 1.5 implementation essentially compile to the pre-1.5 manual implementation or does it deal with this issue?

you can overcome the and .equals() failing with a very minor amount of code. Basically you need a set of all possible typesafe values.
Additionally you need to implement readResolve to prevent serialization from cloning.
Finally - it's just never a good idea to use == for equality comparison. It should only be used for identity comparison (and even then most of the time you should have equals perform the identity comparison).
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Mapping typesafe enums

I have a class, one of it's attributes is a typesafe enum:
public class Title {
     TitleType type;
     int titleId;
     String titleName;
     public TitleType getTitleType() {
          return this.type;
public class TitleType {
     private final String name;
     private final int typeId;
     private TitleType(String name, int typeId) {
          this.name = name;
          this.typeId = typeId;
     public String toString() {
          return this.name;
     public int toInt() {
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     public static final TitleType SUB_TITLE = new TitleType("SUB", 2);
}My table is:
TITLES (
title_id numeric,
title_type varchar,
title_name varchar)
How do I map the TitleType class/attributes' String value to the column title_type in titles?
For a title with TitleType TitleType.MAIN_TITLE my table should look like:
title_id     title_type     title_name
1          MAIN          "The Guiness book of records"Regards,
Anthony

Thanks Doug,
I created the AfterLoad class and unmapped the field.
For some reason though all the titles loaded from the table are coming through as just "MAIN" TitleType.
Heres the AfterLoad class:
public class AfterLoad {
     public static void afterLoadTitle(Descriptor descriptor) {
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          otm.setAttributeName("titleType");
          otm.setFieldName("TITLE_TYPE");
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          otm.addConversionValue(TitleType.SUB_TITLE.toString(), TitleType.SUB_TITLE);
          otm.addConversionValue(TitleType.TRANSLATED_TITLE.toString(), TitleType.TRANSLATED_TITLE);
          otm.setDescriptor(descriptor);
          descriptor.addMapping(otm);
}Any idea why?
Regards,
Anthony

Extending typesafe enum

Hello,
I know that this topic is already cleared that not possible to extend typesafe enum.
But I don't have any other solution than doing this.
Here is my problem.
I have 2 package.
- package generated
- package design
The purpose of the programs is to wrapping package generated in package design, that the user of this program should not use even the smallest part of the package generated.
package generated contains class A, B, and enum Type
Here are the declarations:
File: generated/Type.java
package generated;
package generated;
enum Type
read, write, readwrite;
}File: generated/B.java
package generated;
public class B
int num;
void setNum(int num){this.num = num;}
int getNum(){ return num;}
}File: generated/A.java
package generated;
public Class A
int num;
Type type;
B b;
void setNum(int num){this.num = num;}
int getNum(){ return num;}
void setType(Type type){this.type = type;}
Type getType(){ return type;}
void setB(B b){this.b = b;}
B getB(){ return b;}
}Now inside package design:
File: design/B.java
package design;
public class B
extends generated.B
}File: design/A.java
package design;
public Class A
extends generated.A
}Here is the problem:
File: App.java
import design.*;
class App{
public static void main(String args[])
A a = new A();
B b = new B();
a.setB(b);
//This part is not expected, while it's using the type of package generated
//a.setType( generated.Type.write);
}The problem here is by setting the type of class A. While the method setType of design.A is inherited from generated.A, I cannot setting the value of this type without using the Type from the package generated, even when I add a enum of Type in package design, it still doesn't work, while there couldn't be casted from design.Type to generated.Type.
This problem is not arised with setB(), while design.B is the subclass of generated.B, and thus can be casted automatically.
So I have no Idea how can I solve this thing.
As a note, the package generated cannot be changed.
If there are anybody have ever faced the same problem, maybe can share their experience.
Thanks a lot in advance.
Best regards,
Heru

Hi,
thanks for the reply.
I'm actually making an API of a data source using JAXB to generate Java Code from an XML schema.
This generated package is going to have features added along the time. So the wrapper I make is to assure that the API still work when new functions/features added. If I don't create any Wrapper, my written code (extension of the generated code) would be replaced by the new generated code each time the new features added.
The reason I restrict the user to access the generated package, is because I don't want to confuse the user to access around the generated package and the wrapper package. The purpose of this wrapper is to completely wrapping whole the function of the generated code.
Hopefully my explanation is understandable.. :-)
Do you have any other advice how to accomplish this?
Thanks.
Best regards,
Heru

Typesafe enums with private access - strange or useful?

Given the class listed at the bottom, you'll notice that it's possible to specify a typesafe enum, and restrict that enum to clients in such a way that - and here's the strange part - the constant class is not exported in the api, despite the fact that it's required as an argument to the constructor.
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MyClass myClass = new MyClass(constant);Anyone else noticed this? (Still coming to terms with the meaning of it).
/k1
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The problem I'm having comes from the fact that, in
my program, I can add the integer constants together.
Is there a similar way to do that with typesafe
enums, and if so, how should the enum class be set
up? Thanks a lot. Take care.Something like
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PersistenceDelegate for typesafe enum pattern

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