Question

I have a need to store multiple datatypes(like int or string mostly) inside a two dimensional array. Using Object[][] does solve the problem. But is it a good way to do so ??

How does the Object[][] array then reserve the heap space ? I mean, in accordance with which datatype? Does it leads to any wastage of resources ?

I was trying to do something like this:-

Object[][] dataToBeWritten={ {"Pami",34,45},
{"Ron","x",""},
{"spider","x",""}
};

Edit: You may suggest any better alternatives also if there exists any..

Answer 1

See How to calculate the memory usage of a Java array and Memory usage of Java objects: general guide.

For example, let’s consider a 10×10 int array. Firstly, the “outer” array has its 12-byte object header followed by space for the 10 elements. Those elements are object references to the 10 arrays making up the rows. That comes to 12+4*10=52 bytes, which must then be rounded up to the next multiple of 8, giving 56. Then, each of the 10 rows has its own 12-byte object header, 4*10=40 bytes for the actual row of ints, and again, 4 bytes of padding to bring the total for that row to a multiple of 8. So in total, that gives 11*56=616 bytes. That’s a bit bigger than if you’d just counted on 10*10*4=400 bytes for the hundred “raw” ints themselves.

I think this is for Hotspot only though. References to any object are, just link ints, 4 byte each, regardless of the actual object, or the object being null. Spare requirement for the objects themselves is a whole different story though, as the space isn’t reserved or anything the like at array creation.