[layer8]

I don’t see a difference between pass by copy and pass by value.

The mutability difference is that part of a struct can be modified in place, which value classes can’t: the value of a complete value-class variable (or array slot) can only be modified (reassigned) as a whole. This is presumably because object references to value-class objects can be created, and those objects should be immutable so their identity doesn’t matter.

[_old_dude_]

I think pass by copy is a consequence of being modifiable.

The other solution is to stack allocate and pass a pointer but as i said, unlike in C#, i do not think it's possible to do that in Java.

In Go, you can stack allocate but when you send a pointer (that escapes), the compiler will heap allocate the object.

[layer8]

My point is that pass by copy and pass by value do the same thing, they copy the value representation. In other words, pass by copy means exactly pass by value.

[gf000]

Actually, Java only has pass-by-value, even for reference types. (The same way as C does).

People really misuse/misunderstand this term: Java objects are passed by their pointers ("references") being copied.

The alternative is pass by reference, which is done by e.g. c++, rust, who actually have references (Java doesn't). A good litmus test is whether you can write a swap method that actually changes your local variables.

[steveklabnik]

Rust is also “pass reference by value,” not pass by reference.

[_old_dude_]

For me, the difference is that if methods are inlined, the compiler is still required to do a copy for structs but not for value classes.

I do not know how this is called.

[gf000]

I think that's mostly a semantic difference - Java avoided the problem of strange lifetimes, captures, tearing by fixing the semantics as immutable value objects, while C# has to deal with these issues.

But under the hood it can (and will) do a modification in place.