Java封装接口统一返回数据模板

现在大多数都使用前后端分离开发模式，前端通过Ajax请求访问后台服务器，后台返回JSON数据供前端操作，这里编写一个统一返回数据模板类，方便日后操作

public class R extends HashMap<String, Object>{

    public R(int code) {
        this.put("code", code);
        this.put("time", System.currentTimeMillis());
    }

    public static R success() {
        return new R(100);
    }

    public static R success(String msg) {
        R r = success();
        r.put("msg", msg);
        return r;
    }

    public static R error() {
        return new R(200);
    }

    public static R error(String msg) {
        R r = error();
        r.put("msg", msg);
        return r;
    }

    public static R error(int code, String msg) {
        R r = new R(code);
        r.put("msg", msg);
        return r;
    }

    @SuppressWarnings("unchecked")
    public R setAttribute(String key, Object value) {
        String data = "data";
        if (!this.containsKey(data)) {
            this.put(data, new HashMap<String, Object>());
        }
        ((HashMap<String, Object>) this.get(data)).put(key, value);
        return this;
    }

}