Why does Java uses heap for memory allocation?

Question

I just read this statement in a java book saying Objects in java reside on a heap. Is a heap used because it is the best way to store data and retrieve data fast ?

Answer 1

It's an artifact of the underlying computing model. Memory to the operating system looks like a big, mostly contiguous space in which data can be read and written by address. The operating system allows processes to grab a block of memory (a large contiguous space, usually at least one page of a couple K) and to do with that as they like, by using memory addresses and read/write operations.

The java heap builds on that, i.e. to the programmer it just looks like a big bag of memory (it of course is not, i.e. garbage collection routinely moves stuff around) to which he gets "addresses" (references really, they are not actually addresses) for data (objects) written to this memory space. This allows you maximum flexibility to build more specialised data structures on top of that.

Remember that it acts like a "heap" to the programmer, because that allows you the necessary flexibility, but it doesn't have to be implemented as such. It's a piece of memory managed by the garbage collector, and there are a bunch of data structures it uses to do its job, which you could or could not consider part of the heap, i.e. it's memory used and allocated by the JVM, but usually only the memory accessible to the programmer is considered to be "the heap" in this context.