package DataStructures;

import java.util.NoSuchElementException;

/**
 * An array implementation of a stack
 * @author Peter Williams
 */

public class StackArray implements Stack {

    private Object[] stack;
    private int top; // for storing next item

    public StackArray() {
        stack = new Object[1];
        top = 0; 
    }
  
    public boolean isEmpty() {
        return top == 0;
    }
  
    public void push(Object item) {
        if (top == stack.length) {
            // expand the stack
            Object[] newStack = new Object[2*stack.length];
            System.arraycopy(stack, 0, newStack, 0, stack.length);
            stack = newStack;
        }
        stack[top++] = item;
    }
  
    public Object pop() {
        if (top == 0) {
            throw new NoSuchElementException();
        } else {
            return stack[--top];
        }
    }
}