Copy Tree and Substitute in Java

public static Object copy_tree(Object tree) {
  if ( consp(tree) )
    return cons(copy_tree(first((Cons) tree)),
         (Cons) copy_tree(rest( (Cons) tree)));
  return tree; }

public static Object
  subst(Object gnew, String old, Object tree) {
    if ( consp(tree) )
      return cons(subst(gnew, old,
                        first((Cons) tree)),
           (Cons) subst(gnew, old,
                        rest((Cons) tree)));
  return (old.equals(tree)) ? gnew : tree; }