public class BoundedStack {

	private Object[] s;
	private int sp;
	
	public BoundedStack(int size){
		s = new Object[size];
		sp = 0;
	}
	
	public void push(Object entry){
		if(sp == s.length)
			throw new IndexOutOfBoundsException("Stack full");
		s[sp++] = entry;
	}
	
	public void pop(){
		checkNotEmpty();
		s[--sp] = null;
		// --sp; is a slower alternative to the above line.
		// It does less but leaves more data reachable, slowing garbage collection.
	}

	private void checkNotEmpty() {
		if(sp == 0)
			throw new IndexOutOfBoundsException("Stack empty");
	}
	
	public Object top(){
		checkNotEmpty();
		return s[sp-1];
	}
	
}