Java ArrayList类成员为空

我的模型课如下

public class mf_menu_data {
private static String TAG = "mf_menu_data class";
private String business;
private String description;
private ArrayList<String> categories;

public String cats;

public String getBusiness(){
    return business;
}

public String getDescription(){
    return description;
}

public ArrayList<String> getCategories(){
    return categories;
}

public mf_menu_data(String JSONin) {
    try {
        ArrayList<String> categories = new ArrayList<String>();
        JSONArray my_menus = new JSONArray(JSONin);
        for (int i = 0; i < my_menus.length(); i++) {
            JSONObject my_menu = my_menus.getJSONObject(i);
            business = my_menu.getString("m_business");
            description = my_menu.getString("m_description");
            Log.v(TAG, "m_business, m_description");
            JSONArray my_categories = my_menu.getJSONArray("menu_cat_set");
            for (int j = 0; j < my_categories.length(); j++) {
                JSONObject my_category = my_categories.getJSONObject(j);
                categories.add(my_category.getString("mc_menu"));
                categories.add(my_category.getString("mc_category_elGR"));
                categories.add(my_category.getString("mc_category_enUS"));
                categories.add(my_category.getString("mc_category_deDE"));
                categories.add(my_category.getString("mc_category_frFR"));
                categories.add(my_category.getString("mc_category_itIT"));
                categories.add(my_category.getString("mc_category_ruRU"));
                categories.add(my_category.getString("mc_category_zhCN"));
                Log.v(TAG, "mc_category");
            }
            cats = categories.toString();
        }
    }
    catch (JSONException e) {
        Log.v(TAG, e.getLocalizedMessage());
    }

}

menu_data.getCategories.toString返回null！如果我在模型类中使用字符串变量 公共字符串s=this.getCategories.toString 我可以看到我的数据……情况是，我的Json反序列化程序在我的模型类中有数据，但由于某些原因，它在主活动中消失了

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有什么想法吗？

这一行应该更改ArrayList categories=新建ArrayList；到this.categories=new ArrayList；您有两个ArrayList类别的声明；谢谢，就这样。。。。。。

protected void onPostExecute(String result) {

            mJSONData = new Helpers().CreateJSON(result);
            mf_menu_data menu_data = new mf_menu_data(mJSONData);                
            String s = menu_data.getBusiness()+" "+
                       menu_data.getDescription()+" "+
                       menu_data.getCategories().toString();
            Toast.makeText(getBaseContext(), s , Toast.LENGTH_LONG).show();
            TextView textView = (TextView)findViewById(R.id.tv1);
            textView.setText(mJSONData);
            Log.d("onPostExecute","mJSONData");

        }
    }