Number of bytes allocated to an array after it's declaration in java

Question

In Java, how the check number of bytes allocated to an array after it's declaration? For example:

int[] a = new int[10]; 

Array 'a' is allocated 4*10 bytes since the size of int is 4 bytes and there are 10 integer elements. Is there a non-manual way of doing this?

Answer 1

In Java there is no operator like sizeof in C to get the size of the array element. So there is at least no straightforward way.

By the way, no memory is allocated during declaration, only during initialization.

Declaration:

int[] a;

Initialization:

a = new int[10]; 

Answer 2

Actually the size of int[10] is 56 bytes or more (depending on your JVM).

• 10 * 4 = 40 bytes for the ten ints
• 4 bytes to store the array length
• 8 bytes for the Java object
• 4 bytes to make the size of the object a multiple of 8 (alignment).

But this all depends on the JVM and memory architecture of your operating system. java.lang.instrumentation.Instrumentation provides a way to calculate sizes of an object, as explained in this answer.