Actual size in bytes occupied by an array of x datatype in java?

Question

How can "size in bytes" of a array of some datatype be determined?

What makes me ask this is a question from this icse board question paper.

The question is as follows:

And the accepted answers by the board are (space occupied by one of those data types*no of elements in the array)

For example , one int array of 10 objects is said to take 4*10=40Bytes

But my question is, wouldn't the array have to store it's length and indexes somehow(increasing it's size on memory), or am I misunderstanding the question?

That is just a answer what test checker's will understand. Your understanding is correct. But dare you write it in exam. — Saurabh Batra
– Saurabh Batra, Commented Feb 17, 2014 at 15:23

NPE · Accepted Answer · 2014-02-17 15:26:40Z

7

Your understanding is indeed correct. The official answer adopts a simplistic view of how arrays are stored.

There is a fixed cost associated with every object. Additionally, arrays need to keep track of their size and possibly other things.

The answer also ignores issues such as padding, alignment, etc.

edited Feb 17, 2014 at 15:26

answered Feb 17, 2014 at 15:19

NPE

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5 Comments

Amith KK Over a year ago

So, there is no reliable way to determine it?

NPE Over a year ago

@AmithKK: You could use a memory profiler (VisualVM, YourKit, etc) to determine this at runtime. The answer would of course be JVM-specific.

NPE Over a year ago

@AmithKK: Again, depends on the JVM, on whether it's 32- or 64-bit, etc. I've not seen any official formulas.

Saurabh Batra Over a year ago

@AmithKK these are not formulas, they are profiling tools, much like eclipse.

Amith KK Over a year ago

I know @Batty. You can't use a profiling tool during an exam ;)

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