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copied!<ol> <li>"Where are the heap and stack allocated?" The <a href="https://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap">accepted answer to this question covers this.</a> Each thread gets its own stack and they all share one heap. The operating system controls the exact memory locations of the stacks and heap items and it varies.</li> <li>"Why is stack memory easier to access" Each thread has its own stack, so there are fewer concurrency issues. The stack and heap are both eligible for caching in the <a href="http://en.wikipedia.org/wiki/Memory_hierarchy" rel="nofollow noreferrer">L1, L2, and L3 portions of the memory hierarchy</a>, so I disagree with Daniel's answer here. Really I would not say that one kind of memory is particularly easier to access than the other.</li> <li>"Why can't we allocated objects in stack memory?" This is a design decision taken by the JVM. In other languages like C/C++ you can allocate objects on the stack. Once you return from the function that allocated that <a href="http://en.wikipedia.org/wiki/Call_stack" rel="nofollow noreferrer">stack frame</a> such objects are lost. A common source of errors in C/C++ programs is sharing a pointer to such a stack allocated object. I bet that's why the JVM designers made this choice, though I am not sure.</li> <li>The PermGen is another piece of the heap. Constant strings are stored here for the lifetime of the JVM. It <a href="https://stackoverflow.com/questions/3796427/in-java-is-permanent-generation-space-garbage-collected">is garbage collected just like the rest of the heap.</a></li> </ol>

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