public class objstack implements ObjStackI {
    private objlistnode head=null;

    public void push(Object n) {
        head = new objlistnode(n, head);
    }
    public Object pop() {
        Object v = head.val;
        head = head.next;
        return v;
    }
    public boolean isempty() {
        return head==null;
    }
    public String toString() {
        String s="";
        for (objlistnode n=head; n!=null; n = n.next)
            s = n.val+" "+s;
        return s;
    }
    private class objlistnode {
        private Object val=null;
        private objlistnode next=null;
        objlistnode(Object v, objlistnode l) {
            val = v;
            next = l;
        }
    }
}