Java Byte/byte array space efficiency comparison

Question

I need to create a space efficient 2D array for a large number of 8 bit values. I began writing my class using a few layers of abstraction and generics to allow for code reuse. Once I got to implementing the concrete class it occurred to me I cannot pass in a primitive type as a generic class argument and I would have to use a wrapper class. Because I am concerned about space efficiency, I need to know: what is the space efficiency difference between a Byte array using the wrapper class compared to a primitive byte array ?

Answer 1

Yes, primitives are light-weight compared to corresponding Wrapper class objects.

You can read about it here : Primitives vs Wrappers

Answer 2

Accroding to http://www.javamex.com/tutorials/memory/object_memory_usage.shtml

byte[] size ~= 12 + length

Byte[] size ~= 12 + 20 * length (20 = 16 + 4 the size of 1 Byte object + 4 bytes reference)

so, Byte[] may take 20 times more memory than byte[]. It is actually maximum, it depends on the way you create a Byte. new Byte is always a new Object, Byte.valueOf is always a cached instance. It also depends on CPU, for x64 each reference takes 8 bytes.

Answer 3

Watch this other question: Wrappers of primitive types in arraylist vs arrays

The big issue with double versus Double is that the latter adds some amount of memory overhead -- 8 bytes per object on a Sun 32-bit JVM, possibly more or less on others. Then you need another 4 bytes (8 on a 64-bit JVM) to refer to the object.

So, assuming that you have 1,000,000 objects, the differences are as follows:

double[1000000]

8 bytes per entry; total = 8,000,000 bytes

Double[1000000]

16 bytes per object instance + 4 bytes per reference; total = 20,000,000 bytes

Whether or not this matters depends very much on your application. Unless you find yourself running out of memory, assume that it doesn't matter.