Question

When I have a class like this:

class Test {
    private int _id  = 0 ; // 4 bytes
    private int _age = 0 ; // 4 bytes
}

I’m sure that each instance of it will consume more than 8 bytes in memory because of the 2 integers.

But what about methods? If I have a class with a million methods, and 2 instances of it, are the methods going to take twice as much place in memory ?

How does it work?

Thank you.

Answer 1

No. Methods occur only once in memory¹. They don’t vary on a per instance basis, so they don’t need storage on a per-instance basis.

An object in Java basically consists of some fixed-size “housekeeping” (a pointer to the type information including the vtable), potentially GC-related bits (think mark and sweep), information about the monitor for the instance etc – and then the fields.

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¹ This is a bit of a simplification. There may be various representations, such as bytecode, native code etc – but that’s regardless of separate instances.