import java.util.EmptyStackException;

/** Implementation of the StackInt interface using
    an array.
    @author Koffman and Wolfgang
*/

public class ArrayStack
{
  // Data Fields
  /** Storage for stack. */
  Object[] theData;
  /** Index to top of stack. */
  int topOfStack = -1; // Initially empty stack
  
  public ArrayStack()
  { theData = new Object[10];  }

  public Object push(Object item) {
    if (topOfStack == theData.length - 1) {
      reallocate();
    }
    topOfStack++;
    theData[topOfStack] = item;
    return item;
  }

  /** Remove and return the top item on the stack.
      pre: The stack is not empty.
      post: The top item on the stack has been
            removed and the stack is one item smaller.
      @return The top item on the stack
      @throws EmptyStackException if the stack is empty
   */
  public Object pop() {
    if (empty()) {
      throw new EmptyStackException();
    }
    return theData[topOfStack--];
  }

  /** Return the top item on the stack
        Pre: The stack is not empty
        Post: The stack remains unchanged
        @return The top item on the stack
        @throws EmptyStackException If the stack
        is empty
    */
    public Object peek() {
        if (empty()) {
            throw new EmptyStackException();
        }
        return theData[topOfStack];
    }

    /** Return true if the stack is empty
        @return True if the stack is empty
    */
    public boolean empty() {
        return topOfStack == -1;
    }

    /** Method to reallocate the array containing the stack data
        Postcondition: The size of the data array has been doubled
        and all of the data has been copied to the new array
    */
    private void reallocate() {
        Object[] temp = new Object[2 * theData.length];
        System.arraycopy(theData, 0, temp, 0, theData.length);
        theData = temp;
    }

}