What's the limit to the number of members you can have in a java enum?

Question

Assuming you have a hypothetical enum in java like this (purely for demonstration purposes, this isn't code i'm seriously expecting to use):

enum Example{
    FIRST,
    SECOND,
    THIRD,
    ...
    LAST;
}

What's the maximum number of members you could have inside that enum before the compiler stops you?

Secondly, is there any performance difference at runtime when your code is referencing an enum with say, 10 members as opposed to 100 or 1,000 (other than just the obvious memory overhead required to store the large class)?

Answer 1

The language specification itself doesn't have a limit. Yet, there are many limitations that classfile has that bound the number of enums, with the upper bound being aruond 65,536 (2^16) enums:

Number of Fields The JVMS 4.1 specifies that ClassFile may have up to 65,536 (2^16) fields. Enums get stored in the classfile as static field, so the maximum number of enum values and enum member fields is 65,536.

Constant Pool The JVMS also specifies that the Constant Pool may have up to 65,536. Constant Pools store all String literals, type literals, supertype, super interfaces types, method signatures, method names, AND enum value names. So there must be fewer than 2^16 enum values, since the names strings need to share that Constant Pool limit.

Static Method Initialization The maximum limit for a method is 65,535 bytes (in bytecode). So the static initializer for the Enum has to be smaller than 64Kb. While the compiler may split it into different methods (Look at Bug ID: 4262078) to distribute the initializations into small blocks, the compiler doesn't do that currently.

Long story short, there is no easy answer, and the answer depends not only on the number of enum values there are, but also the number of methods, interfaces, and fields the enums have!

Answer 2

The best way to find out the answer to this type of question is to try it. Start with a little Python script to generate the Java files:

n = input()
print "class A{public static void main(String[] a){}enum B{"
print ','.join("C%d" % x for x in range(n))
print '}}'

Now try with 1,10,100,1000... works fine, then BAM:

A.java:2: code too large C0,C1,C2,C3,C4,C5,C6,C7,C8,C9,C10,C11,C12,C13,C14,C15,C16,C17,C18,C19,C20,C21,C22,...

Seems like I hit some sort of internal limit. Not sure if it's a documented limit, if it's dependent on the specific version of my compiler, or if its some system dependant limit. But for me the limit was around 3000 and appears to be related to the source code size. Maybe you could write your own compiler to bypass this limit.