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Chapter: Java The Complete Reference : The Java Language : Introducing Classes

A Stack Class - Java

While the Box class is useful to illustrate the essential elements of a class, it is of little practical value.

A Stack Class


While the Box class is useful to illustrate the essential elements of a class, it is of little practical value. To show the real power of classes, this chapter will conclude with a more sophisticated example. As you recall from the discussion of object-oriented programming (OOP) presented in Chapter 2, one of OOP’s most important benefits is the encapsulation of data and the code that manipulates that data. As you have seen, the class is the mechanism by which encapsulation is achieved in Java. By creating a class, you are creating a new data type that defines both the nature of the data being manipulated and the routines used to manipulate it. Further, the methods define a consistent and controlled interface to the class’ data. Thus, you can use the class through its methods without having to worry about the details of its implementation or how the data is actually managed within the class. In a sense, a class is like a “data engine.” No knowledge of what goes on inside the engine is required to use the engine through its controls. In fact, since the details are hidden, its inner workings can be changed as needed. As long as your code uses the class through its methods, internal details can change without causing side effects outside the class.

To see a practical application of the preceding discussion, let’s develop one of the archetypal examples of encapsulation: the stack. A stack stores data using first-in, last-out ordering. That is, a stack is like a stack of plates on a table—the first plate put down on the table is the last plate to be used. Stacks are controlled through two operations traditionally called push and pop. To put an item on top of the stack, you will use push. To take an item off the stack, you will use pop. As you will see, it is easy to encapsulate the entire stack mechanism.


Here is a class called Stack that implements a stack for up to ten integers:


// This class defines an integer stack that can hold 10 values 

class Stack {


int stck[] = new int[10]; int tos;


     Initialize top-of-stack Stack() {


tos = -1;




     Push an item onto the stack void push(int item) {




System.out.println("Stack is full."); else


stck[++tos] = item;




     Pop an item from the stack


int pop() { if(tos < 0) {


System.out.println("Stack underflow."); return 0;






return stck[tos--];






As you can see, the Stack class defines two data items and three methods. The stack of integers is held by the array stck. This array is indexed by the variable tos, which always contains the index of the top of the stack. The Stack( ) constructor initializes tos to –1, which indicates an empty stack. The method push( ) puts an item on the stack. To retrieve an item, call pop( ). Since access to the stack is through push( ) and pop( ), the fact that the stack is held in an array is actually not relevant to using the stack. For example, the stack could be held in a more complicated data structure, such as a linked list, yet the interface defined by push( ) and pop( ) would remain the same.


The class TestStack, shown here, demonstrates the Stack class. It creates two integer stacks, pushes some values onto each, and then pops them off.



     class TestStack  {        

     public static    void main(String args[]) {

     Stack  mystack1 =     new  Stack();

     Stack  mystack2 =     new  Stack();


     push some numbers onto the stack for(int i=0; i<10; i++) mystack1.push(i); for(int i=10; i<20; i++) mystack2.push(i);


     pop those numbers off the stack System.out.println("Stack in mystack1:"); for(int i=0; i<10; i++)




System.out.println("Stack in mystack2:"); for(int i=0; i<10; i++)








This program generates the following output:


Stack in mystack1: 9 8 7 6 5 4 3 2 1 0


Stack in mystack2: 19 18 17 16 15 14 13 12 11 10


As you can see, the contents of each stack are separate.


One last point about the Stack class. As it is currently implemented, it is possible for the array that holds the stack, stck, to be altered by code outside of the Stack class. This leaves Stack open to misuse or mischief. In the next chapter, you will see how to remedy this situation.

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