I have used the following code to create a sorted set of results. Your context variable would be the context you wish to list (eg. "o=kumar").

LdapContext ctx = new InitialLdapContext(env, null);

Control[] ctxControls = new Control[]{

     new SortControl(new String[]{"cn"}, Control.CRITICAL);

};

ctx.setRequestControls(ctxControls);

NamingEnumeration results = ctx.list(context);