Stack & Heap

Java Stack and Heap: Java Memory Allocation Tutorial

What is Stack Memory?

Stack in java is a section of memory which contains methods, local variables, and reference variables. Stack memory is always referenced in Last-In-First-Out order. Local variables are created in the stack.

What is Heap Memory?

Heap is a section of memory which contains Objects and may also contain reference variables. Instance variables are created in the heap

Memory Allocation in Java

The JVM divided the memory into following sections.

Difference between Stack and Heap

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Let’s take an example to understand this better.

Consider that your main method calling method m1

In the stack java, a frame will be created from method m1.

Java Stack and Heap

The variable X in m1 will also be created in the frame for m1 in the stack. (See image below).

Java Stack and Heap

Method m1 is calling method m2. In the stack java, a new frame is created for m2 on top of the frame m1.

Java Stack and Heap

Java Stack and Heap

Variable b and c will also be created in a frame m2 in a stack.

Java Stack and Heap

Same method m2 is calling method m3. Again a frame m3 is created on the top of the stack (see image below).

Java Stack and Heap

Java Stack and Heap

Now let say our method m3 is creating an object for class “Account,” which has two instances variable int p and int q.

The statement new Account() will create an object of account in heap.

Java Stack and Heap

The reference variable “ref” will be created in a stack java.

Java Stack and Heap

The assignment “=” operator will make a reference variable to point to the object in the Heap.

Java Stack and Heap

Once the method has completed its execution. The flow of control will go back to the calling method. Which in this case is method m2.

Java Stack and Heap

The stack from method m3 will be flushed out.

Java Stack and Heap

Since the reference variable will no longer be pointing to the object in the heap, it would be eligible for garbage collection.

Java Stack and Heap

Once method m2 has completed its execution. It will be popped out of the stack, and all its variable will be flushed and no longer be available for use.

Likewise for method m1.

Eventually, the flow of control will return to the start point of the program. Which usually, is the “main” method.

What if Object has a reference as its instance variable?

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.

Java Stack and Heap

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