CODEWITHSUNDEEP
javamemory-management
VJune
VJune

Reputation: 1215

Java object arrays memory requirements

Suppose there is an Integer array in my class:

public class Foo {
   private Integer[] arr = new Integer[20];
   .....

}

On a 64 bit architecture the space requirement for this is ~ (20*8+24) + 24*20 {space required for references + some array overhead + space required for objects}.

Why java stores references to all of the 20 Integer objects? Wouldn't knowing that first memory location and the number of items in the array suffice? (assuming and I also as I read somewhere that objects in an array are placed contiguously anyways). I want to know the reason for this sort of implementation. Sorry if this is a noobish question.

Upvotes: 3

Views: 200

Answers (1)

user395760
user395760

Reputation:

Like every other class, Integer is a reference type. This means it can only be accessed indirectly, via a reference. You cannot store an instance of a reference type in a field, a local variable, a slot in a collection, etc. -- you always have to store a reference and allocate the object itself separately. There are a variety of reasons for this:

  • You need to be able to represent null.
  • You need to be able to replace it with another instance of a subtype (assuming subtypes are possible, i.e. the class is not final). For example, an Object[] may actually store instances of any number of different classes with wildly varying sizes.
  • You need to preserve sharing, e.g. after a[0] = a[1] = someObject; all three must refer to the same object. This is much more important (vital even) if the object is mutable, but even with immutable objects the difference can be observed via reference equality checks (==).
  • You need reference assignment to be atomic (cf. Java memory model), so copying the whole instance is even more expensive than it seems.

With these and many other constraints, always storing references is the only feasible implementation strategy (in general). In very specific circumstances, a JIT compiler may avoid allocating an object entirely and store its directly (e.g. on the stack), but this is an obscure implementation detail, and not widely applicable. I only mention this for completeness and because it's a wonderful illustration of the as-if rule.

Upvotes: 2

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