CODEWITHSUNDEEP
javamemorymultidimensional-array
Santosh Tiwari
Santosh Tiwari

Reputation: 1216

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

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

Upvotes: 3

Views: 585

Answers (1)

haskovec
haskovec

Reputation: 370

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.

Upvotes: 1

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