Java object creation and memory size

Question

Hi All,

I am trying understand about the size that a Java object will be allocated with when created using a new operator.

Consider that i am creating a class `public class NewClass {

NewClass() { }

}`

when i create an instance of NewClass using NewClass nc = new NewClass();. what is the size of the NewClass that gets created in the heap?

~ Jegan

Answer 1

I think you need to use a profiler to measure this. You may use JProfiler or YourKit profilers for this.

Answer 2

Look at this SO Thread.

Answer 3

Profiling is the best way, but you can get a good estimate like so:

8 bytes per object (bare overhead), plus fields.

• Primitive Fields: as listed in Java. Note: booleans need 1 full byte.
• Object fields: 1 pointer (4 bytes on 32-bit VM, 8 on 64-bit), plus size of object itself (if not a reference to a preexisting object)
• Arrays: 4 bytes + object/primitives for elements
• Strings: far, far too much. IIRC, 24 bytes + 2 bytes/character. Might be more.

The final result is increased to the nearest multiple of 8 bytes.

See also my example here for how to calculate memory use on a more complex object. Note: these rules may vary with VMs, and may change as newer versions of the VM come out. My estimate only applies to the Sun JVM, although I suspect IBM's results will be similar.

Answer 4

YourKit profiler shows size of created objects. Here is screenshots from their docs

and this