jvm - Maximum number of parameters in Java method declaration

Question

What is the maximum number of parameters that a method in Java can have and why?

I am using Java 1.8 on a 64-bit Windows system.

All the answers on StackOverflow about this say that the technical limit is 255 parameters without specifying why.

To be precise, 255 for static and 254 for non-static (this will be the 255th in this case) methods.

I thought this could be described in some sort of specification and that there is simply a statically defined maximum number of parameters allowed.

But this was only valid for int and all 4-bytes types. I did some tests with long parameters, and I was only able to declare 127 parameters in that case.

With String parameters, the allowed number I deduced from testing is 255 (it may be because the reference size is 4 bytes in Java?).

But since I am using a 64-bit system, references size should be 8 bytes wide and so with String parameters the maximum allowed number should be 127, similar to long types.

How does this limit is exactly applied?

Does the limit have anything to do with the stack size of the method?

Note: I am not really going to use these many parameters in any method, but this question is only to clarify the exact behavior.

Answer 1

That limit is defined in the JVM Specification:

The number of method parameters is limited to 255 by the definition of a method descriptor (§4.3.3), where the limit includes one unit for this in the case of instance or interface method invocations.

Section §4.3.3 gives some additional information:

A method descriptor is valid only if it represents method parameters with a total length of 255 or less, where that length includes the contribution for this in the case of instance or interface method invocations.

The total length is calculated by summing the contributions of the individual parameters, where a parameter of type long or double contributes two units to the length and a parameter of any other type contributes one unit.

Your observations were spot on, double word primitives (long/double) need twice the size of usual 4 bytes variables and 4 bytes object instance references.

Regarding the last part of your question related to 64bit systems, the specification defines how many units a parameter contribute, that part of the specification must still be complied with even on a 64bit platform, the 64bit JVM will accomodate 255 instance parameters (like your 255 Strings) regardless of the internal object's pointer size.