ExtendedStack.java

/**
 *	This class extends the Stack 
 *	so that there's a pushdown(i)
 *	method which copies the elements
 *	at index 0 and inserts it and index i.
 * 	All items beneath index i are pushed down.
 *
 */

import java.util.Stack;

public class ExtendedStack extends Stack {

	@SuppressWarnings("unchecked")
	public static void main(String[] args)
	{
			ExtendedStack e = new ExtendedStack();
			e.push(1);
			e.push(2);			
			e.push(42);
			e.push(1);
	}

	public void pushdown(int i)
	{
		this.insert(i, this.peek());
	}

}