java why different memory usage for primitive versus object array in 2D

Question

Relevant code

int row = 100000;
int col = 18;

Object[][] objectArray = new Object[row][1];
int[][] intArray = new int[row][1];

System.out.println("Size of objectArray  = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(objectArray) + " bytes");
System.out.println("Size of intArray     = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(intArray) + " bytes");

Object[][] objectMatrix = new Object[row][col];
int[][] intMatrix = new int[row][col];

System.out.println("Size of objectMatrix = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(objectMatrix) + " bytes");
System.out.println("Size of intMatrix    = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(intMatrix) + " bytes");

Relevant output

Size of objectArray  = 4000024 bytes
Size of intArray     = 4000024 bytes
Size of objectMatrix = 17600024 bytes
Size of intMatrix    = 10400024 bytes

If instead of 1D (number of cols=1), I have 2D (number of cols > 1), the object matrix takes more space.

Can someone explain the reason?

Edit: Added another case with just one row

    int row = 1;
    int col = 2;

    Object[][] objectArray = new Object[row][1];
    int[][] intArray = new int[row][1];

    System.out.println("Size of objectArray  = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(objectArray) + " bytes");
    System.out.println("Size of intArray     = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(intArray) + " bytes");

    Object[][] objectMatrix = new Object[row][col];
    int[][] intMatrix = new int[row][col];

    System.out.println("Size of objectMatrix = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(objectMatrix) + " bytes");
    System.out.println("Size of intMatrix    = " + net.sourceforge.sizeof.SizeOf.deepSizeOf(intMatrix) + " bytes");

Output

Size of objectArray  = 64 bytes
Size of intArray     = 64 bytes
Size of objectMatrix = 72 bytes
Size of intMatrix    = 64 bytes

I guess maybe an object array takes more space. (Dunno why this would be, but it's possible, depending on the internal implementation.) — Hot Licks
– Hot Licks, Commented Apr 13, 2012 at 18:02
(And, of course, on a full 64-bit Java implementation, pointers are twice as big as ints.) — Hot Licks
– Hot Licks, Commented Apr 13, 2012 at 18:03
I mentioned that in the second comment. Why did you not reply sooner? — Hot Licks
– Hot Licks, Commented Apr 13, 2012 at 21:22
@trutheality -- Not for an array of 1, since they round to the same block size. — Hot Licks
– Hot Licks, Commented Apr 14, 2012 at 5:34
Object size is rounded to some power of 2, generally either 16 or 32. So an array of 1 4-byte element and an array of 1 8-byte element are likely to round to the same boundary. — Hot Licks
– Hot Licks, Commented Apr 17, 2012 at 15:43

haskovec · Accepted Answer · 2012-04-14 04:03:55Z

1

The size of the reference inside of the object array depends on many factors (32 bit vs 64 bit) or if you are in 64 bit are you running compressedOOPs? Since I typically work in a 64 bit environment I would always expect the Object[] to occupy more memory. On the other hand an int in Java is defined as a 32bit value, so with an int[] you are going to have 32 bits used for each value plus some overhead for the array object itself.

answered Apr 14, 2012 at 4:03

haskovec

3704 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

java why different memory usage for primitive versus object array in 2D

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related