ArrayStack.java

//stack.ArrayStack
//Michael Crumm 2010-10-04

package stack;

//An ArrayStack uses an internal Object array to store elements.
public class ArrayStack implements Stack
{
	private int size = 0;
	private Object[] element;
	
	//Initialize an ArrayStack of size 100
	public ArrayStack()
	{
		this(100);
	}
	
	//Initialize an ArrayStack of size theSize
	public ArrayStack(int theSize)
	{
		this.element = new Object[theSize];
	}
	
	//post: returns number of elements in this stack
	public int size()
	{
		return this.size;
	}
	
	//post: returns true if this stack is empty
	public boolean isEmpty()
	{
		if (this.size() > 0) {
			return false;
		} else {
			return true;
		}
	}
	
	//post: theElement is added to the top of this stack
	public void push(Object theElement)
	{
		if(this.element.length == this.size) {
			doubleArrayCapacity();
		}
		
		this.element[this.size] = theElement;
		this.size++;
	}
	
	//pre: stack not empty
	//post: removes top element and returns it
	public Object pop()
	{
		if(this.isEmpty()) {
			throw new StackEmptyException("Could not pop.");
		}

		this.size--;

		Object removedElement = this.element[this.size];
		this.element[this.size] = null;
		
		return removedElement;
	}
	
	//pre: stack not empty
	//post: returns the top element
	public Object peek()
	{
		if(this.isEmpty()) {
			throw new StackEmptyException("Could not peek.");
		}
		
		return this.element[(this.size-1)];
	}
	
	//returns a String representation of the ArrayStack
	public String toString()
	{
		String retval = ""; 
		String sep = "";

		int count = this.size-1;		
		while(count >= 0) {
			retval += sep + this.element[count].toString();
			sep = ",";
			count--;
		}

		return retval;
	}
	
	//doubles size of this.element while retaining its data
	private void doubleArrayCapacity()
	{
		Object[] bigArray = new Object[2*this.size];
		int count = 0;
		while(count < this.size) {
			bigArray[count] = this.element[count];
			count++;
		}
		this.element = bigArray;
	}
}