It's not limited to 64. There is a just a different implementation chosen if there are more than 64 items. I haven't looked at the source, but I would guess that a RegularEnumSet
is implemented as a bit mask using a single long
.
How the limit number of enumeration in EnumSet come from?
Question
I found that a limit number of enumeration (64) is used in EnumSet. Please see a method in EnumSet source code below (This code is captured from JDK 1.7).
/**
* Creates an empty enum set with the specified element type.
*
* @param elementType the class object of the element type for this enum
* set
* @throws NullPointerException if <tt>elementType</tt> is null
*/
public static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) {
Enum[] universe = getUniverse(elementType);
if (universe == null)
throw new ClassCastException(elementType + " not an enum");
if (universe.length <= 64)
return new RegularEnumSet<>(elementType, universe);
else
return new JumboEnumSet<>(elementType, universe);
}
From the above code we can see: if the length of Enum array is less than 64, RegularEnumSet is used. Otherwise, JumboEnumSet is used instead.
Here my questions are as follows:
- How does this limit number of enumeration (64) comes from or how to calculate it as 64?
- Why RegularEnumSet and JumboEnumSet are both used, What good is there in doing that?
Solution
OTHER TIPS
Its not, its limited to the different types available in the enum :
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