Difference Between Stack and Heap

What Is Stack?

Each Java virtual machine thread has a private Java virtual machine stack, created at the same time as the thread. A Java virtual machine stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Because the Java virtual machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java virtual machine stack does not need to be contiguous.

The Java virtual machine specification permits Java virtual machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java virtual machine stacks are of a fixed size, the size of each Java virtual machine stack may be chosen independently when that stack is created. A Java virtual machine implementation may provide the programmer or the user control over the initial size of Java virtual machine stacks, as well as, in the case of dynamically expanding or contracting Java virtual machine stacks, control over the maximum and minimum sizes.

The following exceptional conditions are associated with Java virtual machine stacks:

  • If the computation in a thread requires a larger Java virtual machine stack than is permitted, the Java virtual machine throws a StackOverflowError.
  • If Java virtual machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java virtual machine stack for a new thread, the Java virtual machine throws an OutOfMemoryError

The Stack section of memory contains methods, local variables and reference variables.

What Is Heap?

The Java virtual machine has a heap that is shared among all Java virtual machine threads. The heap is the runtime data area from which memory for all class instances and arrays is allocated.

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java virtual machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

The heap mainly store objects create using or class level variables.

The following exceptional condition is associated with the heap:

  • If a computation requires more heap than can be made available by the automatic storage management system, the Java virtual machine throws an OutOfMemoryError

The Heap section contains Objects (may also contain reference variables).

Points to Remember:

  • When a method is called , a frame is created on the top of stack.
  • Once a method has completed execution , flow of control returns to the calling method and its corresponding stack frame is flushed.
  • Local variables are created in the stack
  • Instance variables are created in the heap & are part of the object they belong to.
  • Reference variables are created in the stack.

What if Object has a reference as its instance variable?


public static void main(String args[]){

A parent = new A();
//more code

}

class A{

B child = new B();
int e;
//more code

}

class B{

int c;
int d;
//more code

}

In this case , the reference variable “child” will be created in heap ,which in turn will be pointing to its object, something like the diagram shown below.

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About Sanju
I am Software Programmer. I am working in JAVA/J2EE Technologies.

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