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

In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.

Answer 2

That's because the idea of threads is "share everything". Of course, there are some things you cannot share, like processor context and stack, but everything else is shared.