Programming Tutorials

A Stack Class in Java

By: aathishankaran in Java Tutorials on 2007-03-05  

To show the real power of classes, this article will conclude with a more sophisticated example. 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, 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.

class Stack {
     int stck[] = new int[10];
     int tos;
     stack() {
      tos = -1;
     }

void push (int item) {
     if ( tos == 9)
          System.out.println(“Stack is full.”);
     else
          stck[++tos] = item;
}

int pop() {
     if(tos < 0) {
          system.out.println(“Stack underflaow.”);
          return 0;
     }
     else
          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 and 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();
     for (int i=0; i<10; i++) mystack1.push(i);
     System.out.println(“Stack in mystack1:”);
     for (int i=0; i<10; i++)
          System.out.println(mystack1.pop());
     }
}

This program generates the following output:

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

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.