Why do threads share the heap space?

Question

Threads each have their own stack, but they share a common heap.

Its clear to everyone that stack is for local/method variables & heap is for instance/class vari

Answer 1

The Heap is just all memory outside of the stack that is dynamically allocated. Since the OS provides a single address space then it becomes clear that the heap is by definition shared by all threads in the process. As for why stacks are not shared, that's because an execution thread has to have its own stack to be able to manage its call tree (it contains information about what to do when you leave a function, for instance!).

Now you could of course write a memory manager that allocated data from different areas in your address space depending on the calling thread, but other threads would still be able to see that data (just like if you somehow leak a pointer to something on your thread's stack to another thread, that other thread could read it, despite this being a horrible idea)