![]() ![]() Both stack allocations and heap allocations can be contiguous. in every programming language runtime I've ever encountered where it makes sense to talk about allocations at all.Ī sequence of allocations is contiguous if there is no gap between the individual allocations. When we talk about memory or disc allocation, the word "contiguous" simply means "without any gaps".Ī single stack or heap memory allocation is always contiguous. It is a great book IMO and covers these topics in easy to understand ways. If you are starting out, I highly recommend head first java. This is true for any method, if you have a method that creates an object but doesn't return the reference to that object, then you can never access the object and the java garbage collector comes along at a later time and cleans up all the objects on the heap that don't have references pointing to them. Lastly, the main method is a stack frame too, so when main finishes, you no longer have a reference to a Person object or access to any temporary variables that may have existed in main. This means that the memory required for this Person object is the required memory to store a reference variable of type String (bit pattern of the memory address of a String object on the heap) and memory to hold the bit pattern of an int. Using the example above, a Person has 2 instance variables, a String name and int age. When a method is called, a new stack frame is created which allocates temporary memory for the duration of the method call. The important part to understand is, no memory is required to store an objects methods. new keyword means java will allocate memory on the heap for the space required to store the Person objects instance variables.Person person = new Person("Steven", 30) This allows you to reference the object to change the objects state at a later time. The pointer contains a memory address that leads to the location of the object stored on the heap. Thus, you can only access an object when the method is in "scope". The key point that helped me was that, a pointer to an object is kept in a stack frame, it has a little bit of temporary memory allocated that exists for the life time of the method call. When I was learning I found it hard to work out the difference between the two. The heap is different because it is where objects live, not "pointers" to objects. Once the loop is finished, the stack frame would be popped off the stack, which means all the temporary memory allocated for the local variables and method parameters that existed are released back into the system. Since Java is a garbage-collected language, their memory will be automatically recovered for reuse sometime after that last reference to them goes away - again, the details are too large a topic to cover here, but basically you can trust that the GC comes through periodically to pick up the clothes we dropped on the floor and toss them into the laundry.Ī stack frame only exists for the life of a method call, which means that memory is allocated to provide storage for all your local variables and method parameters that are used in some way that assist helping the method achieve its goals of whatever task it set out to achieve.Įxamples of memory storage in a stack frame are temporary pointers that are used to keep track of an index position in an array which you are iterating through. The important thing is that since they were obtained from the heap, their lifetime is independent of the stack frame. ![]() ![]() There are several reasons for this which, frankly, unless you're designing a programming language you don't really need to know about and it's too large a topic to cover here. Instead, they are allocated from the heap, on demands, when the new operation is performed. In Java, objects are never allocated from automatic storage. Obviously this isn't useful for anything which is intended to persist past the end of that function call. That's how most local variables are handled. This is what I've understood - I'll try to speak about it with examples.The question isn't contiguous versus heap, but automatic versus heap.Īutomatic storage is set up on entry to a block of code - traditionally on entry to a function or method - and discarded when that function returns, so its memory space on the stack can be reused by the next function call. Reading a book I had a problem understanding how Java allocate memory in heap and in stack. I want to study Java again, because I leave it some years ago. ![]()
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