Java Interface definition for integer stack

// Define an integer stack interface.  
interface IntStack {
  void push(int item); // store an item

  int pop(); // retrieve an item
}

// An implementation of IntStack that uses fixed storage.
class FixedStack implements IntStack {
  private int stck[];
  private int tos;

  // allocate and initialize stack
  FixedStack(int size) {
    stck = new int[size];
    tos = -1;//from   w  w w  . j  av  a 2  s.co m
  }

  // Push an item onto the stack
  public void push(int item) {
    if (tos == stck.length - 1) // use length member
      System.out.println("Stack is full.");
    else
      stck[++tos] = item;
  }

  // Pop an item from the stack
  public int pop() {
    if (tos < 0) {
      System.out.println("Stack underflow.");
      return 0;
    } else
      return stck[tos--];
  }
}

// Implement a "growable" stack.
class DynStack implements IntStack {
  private int stck[];
  private int tos;

  // allocate and initialize stack
  DynStack(int size) {
    stck = new int[size];
    tos = -1;
  }

  // Push an item onto the stack
  public void push(int item) {
    // if stack is full, allocate a larger stack
    if (tos == stck.length - 1) {
      int temp[] = new int[stck.length * 2]; // double size
      for (int i = 0; i < stck.length; i++) {
        temp[i] = stck[i];
      }
      stck = temp;
      stck[++tos] = item;
    } else
      stck[++tos] = item;
  }

  // Pop an item from the stack
  public int pop() {
    if (tos < 0) {
      System.out.println("Stack underflow.");
      return 0;
    } else {
      return stck[tos--];
    }
  }
}

public class Main {
  public static void main(String args[]) {
    FixedStack mystack1 = new FixedStack(5);
    FixedStack mystack2 = new FixedStack(8);

    // push some numbers onto the stack
    for (int i = 0; i < 5; i++) {
      mystack1.push(i);
    }
    for (int i = 0; i < 8; i++) {
      mystack2.push(i);
    }

    // pop those numbers off the stack
    System.out.println("Stack in mystack1:");
    for (int i = 0; i < 5; i++) {
      System.out.println(mystack1.pop());
    }
    System.out.println("Stack in mystack2:");
    for (int i = 0; i < 8; i++) {
      System.out.println(mystack2.pop());
    }
    DynStack mydstack1 = new DynStack(5);
    DynStack mydstack2 = new DynStack(8);

    // these loops cause each stack to grow
    for (int i = 0; i < 12; i++) {
      mydstack1.push(i);
    }
    for (int i = 0; i < 20; i++) {
      mydstack2.push(i);
    }

    System.out.println("Stack in mystack1:");
    for (int i = 0; i < 12; i++)
      System.out.println(mydstack1.pop());

    System.out.println("Stack in mystack2:");
    for (int i = 0; i < 20; i++)
      System.out.println(mydstack2.pop());

    IntStack mystack; // create an interface reference variable
    DynStack ds = new DynStack(5);
    FixedStack fs = new FixedStack(8);

    mystack = ds; // load dynamic stack
    // push some numbers onto the stack
    for (int i = 0; i < 12; i++)
      mystack.push(i);

    mystack = fs; // load fixed stack
    for (int i = 0; i < 8; i++)
      mystack.push(i);

    mystack = ds;
    System.out.println("Values in dynamic stack:");
    for (int i = 0; i < 12; i++)
      System.out.println(mystack.pop());

    mystack = fs;
    System.out.println("Values in fixed stack:");
    for (int i = 0; i < 8; i++)
      System.out.println(mystack.pop());
  }
}