• Understand stack and heap.
  • Understand the variables declared inside of methods are allocated on the stack.
  • Understand that objects are allocated on the heap.
  • Stack and Heap

    We know the following with regard to computers, information, programs, and memory.

    • A computer is a machine that stores and manipulates information under the control of a changeable program.
    • A computer stores information in memory.
    • Memory is a sequence of bytes.
    • A byte is 8-bits of information.
    • A bit is a binary digit - either 1 or 0.
    • Computer information must be represented as binary in memory.
    • Primary types have specific byte allocations - for examples, byte is one byte, char is two, int is 4 and double is eight.
    • Reference types have 4 bytes allocated which contain a reference (or pointer) to the object which is in heap memory.

    We have studied figures of primitive types, reference types, and memory where the memory shows objects in heap memory. The following is one from Java Strings.

    Primitive and Reference Memory

    This section describes two types of memory - stack and heap - that are fundamental in computer programming.

    Stacks in Everyday Life

    Most people have a stack somewhere in the life. For example, you probably have a stack of books in your room or office. The following figure is a stack of my books. An old Linux for Dummies is on the bottom - I have not read it in a while. On top of my Linux book is one I read to my granddaughter - Hop of Pop. On the top of my stack is My Diary. My stack of books has a top - the last book I put on the stack is on top. I have to remove the top book before I can get to the next book and so on. My stack of books has the attribute - last book in is the first book out - which is shortened to last in first out - which is shortened to LIFO. A stack is a LIFO.

    Stack of Books

    Stacks for Method Variables

    Stacks are useful in computer programming. We will study stacks in Abstract Data Types. At this point, we want to understand how method variables are allocated on a stack. Consider three methods, where main calls methodB, which calls methodC. The following code snippet demonstrates this calling sequence.

    public static int methodC(int x, int y) {
       int z = x + y;
       return z;
    public static int methodB(int a, int b) {
       int c = a * methodC(b, 10);
       return c;
    public static void main(String[] args) {
       Scanner in = new Scanner(System.in);
       System.out.print("Enter a number: ");
       int i = in.nextInt();
       System.out.print("Enter a number: ");
       int j = in.nextInt();
       System.out.print(methodB(i, j));

    The method calls are like stacking books. main is on the bottom of the stack. When main calls methodB, methodB is placed on top of main. Likewize methodC is placed on top of methodB. When methodC returns, it is removed (or popped) from the stack. When methodB returns, it is popped from the stack. Notice the LIFO mechanism at work. The variables of the called method are what is actually placed on the stack. The JVM calls main which starts the stacking of method variables. We create a sequence of figures to watch the stack grow and shrink. The method variables include the parameters. The variables for these three methods are the following.

    • main
      • args, in, i, and j
    • methodB
      • a, b, and c
    • methodC
      • x, y, and z

    In the following sequence of three figures, the stack will grow upward. When the JVM calls main, the stack contains mains variables as follows.

    main stack

    When main calls methodA, the stack contains mains variables and methodAs variables. methodAs variables on on the top of the stack.

    methodA stack

    When methodA calls methodB, the stack contains mains variables, methodAs variables, and methodBs variables on on the top of the stack.methodBs variables as follows.

    methodA stack

    As each method returns, the variables on the stack for that method are popped off the stack. This popping is the reverse order of the previous three figures.

    Stack, Heap, Memory Allocation, and Garbage Collection

    The computers you purchase today have a lot of memory - typically have 4 to 8 gigabytes of memory - a gigabyte is a billion bytes. Memory is shared among the various programs running on your computer, which probably include a browswer (Chrome, Firefox, Safari, etc), a word processor (MS-Word), a game, and your Java IDE. When running a Java program, the JVM performs memory management. Memory management includes

    • Allocating memory for code
    • Allocating memory for stack. A program has a large but finite amount of memory allocated to its stack. If a program calls too many methods, it may overflow its stack space and get an overflow stack exception. When we study Recursion, we may encounter a recursive method that does not properly terminate, which easily generates a stack overflow.
    • Allocating memory for heap. A program gets a large block of memory allocated for its heap. Initially all bytes in the heap are marked as free, which means they can be used to allocate objects. Creating Strings and calling a constructors allocate memory for an object in the heap. When an object is referenced by a variable, the memory occupied by the object is marked as used. Eventually, an object is not referenced by a variable. The JVM has a garbage collector that periodically examines the heap for objects that are no longer referenced. The garbage collector returns those bytes to being free so thay can be reused for another object. The following code snippet demonstrates an object (for Person("Gusty",22)) that is created, but immediately unreferenced.
    Person p = new Person("Gusty", 22);
    p = new Person("JerriAnne", 21);

    Memory management is a popular subject in computers science. You will study memory management in CPSC 405 - Operating Systems. The following are a few tidbits to whet your appetite.

    • You can imagine the memory in the heap can become fragmented with lots of used and free block mingled together. Sometimes the fragmented free blocks must be coalesced together.
    • You can also imagine multiple memory managers. The operating system (LINUX, Windows, MACOS) has to manage memory for the various programs running. JVAM has to manage memory that is allocated to it from the OS.
    • The C programming language forces the programmer to explicitly allocate memory by calling functions such as malloc() and free(). These functions use operating system services that manage memory.
    • Java allocates memory for objects with the new operator. Java does not have a free() method. Programmers consume memory and when it is no longer referenced, the garbage collector does the free.

    When a Java program executes,

    • the JVM allocates memory for the code and places the code in it.
    • allocates memory for the program stack.
    • allocates memory for the heap.
    • places the String[] arg on the stack.
    • Call the main method.
    • As main declares variables and calls methods,
      • the stack grows and shrinks
      • the heap get objects created with the new operator

    The following figure demonstrates the stack and heap for the method memDemo.

    Stack and Heap

    String is Immutable

    In Java Strings we learned that String is immutable. Now that we understand stack and heap, the following figure demonstrates String and its immutability.

    Immutable Strings

    Tags: class object