Question

我从responseObject和rootObject列表中获得了一个列表。 responseObject列表中填充了新项目（包含标题，ID，摘要等的对象），我想将它们添加到我的rootObject列表中，但是我收到错误：

java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference

两个列表都是命名结果。有谁知道如何将这些项目添加到我的rootObject 这是代码：

        public void success(RootObject responseObject, Response response) {
            rootObject.nextId = responseObject.nextId;
            rootObject.results.add(responseObject.results.get(0));
        }

我检查了回复，responseObject结果中有项目

整个代码：

public class NewsController {
        private RestAdapter restAdapter;
        public RootObject rootObject = new RootObject();
        public MainActivity ma;
        static final String API_URL = "http://SECRETAPILINK.SECRET/api";
        public void getNews(){
            OkHttpClient mOkHttpClient = new OkHttpClient();
            mOkHttpClient.setConnectTimeout(15000,TimeUnit.MILLISECONDS);
            mOkHttpClient.setReadTimeout(15000, TimeUnit.MILLISECONDS);
            restAdapter = new RestAdapter.Builder().setEndpoint(API_URL).setClient(new OkClient(mOkHttpClient)).setLogLevel(RestAdapter.LogLevel.FULL) .build();
            GetNewsService service = restAdapter.create(GetNewsService.class);

            Callback<RootObject> cb = new Callback<RootObject>() {
                @Override
                public void success(RootObject responseObject, Response response) {
                    Log.v("AASSAASS", responseObject.results.get(0).Title);
                    rootObject.nextId = responseObject.nextId;
                    rootObject.results.addAll(responseObject.results);
                    Log.v("AASSAASS", responseObject.results.get(0).Title);

                }

                @Override
                public void failure(RetrofitError error) {
                    Log.v("AASSAA", error.toString());
                }
            };
            service.GetNewsItems(cb);
        }
    }

Answer 1

你还没有初始化rootObject.results arrayList，这就是它抛出NPE的原因。

为了解释，让我们看一个小例子

class Ideone
{
    List<Integer> list;

    public static void main (String[] args) throws java.lang.Exception
    {
        try {
            Ideone id = new Ideone();
            id.list = new ArrayList<Integer>();
            id.list.add(1);
            id.list.add(2);

            Ideone copyId = new Ideone();
            copyId.list = new ArrayList<Integer>();
            copyId.list.addAll(id.list);

            Ideone npeId = new Ideone();
            npeId.list.add(id.list);      //->nullpointerexception

        } catch (NullPointerException e) {
            System.out.println(e);
        }
    }
}

在上面的例子中，如果你没有初始化npeId，它将会抛出一个NPE。

在您的代码中，您需要类似

的内容

rootObject.results = new ArrayList<...>();

Java / Android将列表项添加到其他列表中

1 个答案: