public JSONObject putOnce(String key, Object value) throws JSONException {
        if (key != null && value != null) {
            if (this.opt(key) != null) {
                throw new JSONException("Duplicate key \"" + key + "\"");
            }
            this.put(key, value);
        }
        return this;
    }
@author Robert HG (254963746@qq.com) on 12/28/15.
lts-core/src/main/java/com/github/ltsopensource/json/JSONObject.java