关于JSP中Cookie的setDomain()和getDomain()方法的疑问

Cookie cookie=new Cookie("username","hugh");
Cookie cookie1=new Cookie("sex","Man");
cookie.setDomain("localhost");
cookie1.setDomain("localhost");
response.addCookie(cookie);
response.addCookie(cookie1);
Cookie[] cookies=request.getCookies();
for(int i=0;i<cookies.length;i++){
    out.println("作用域："+cookies[i].getDomain()+"<br />");
}

显示结果：“作用域：null”
请问为什么得不到传入的值：“localhost”呢？