为什么我的应用程序会因 NullPointerException 而崩溃，为什么我的适配器（可能）为空？

Question

当我的应用程序尝试获取适配器上的位置时，它崩溃了，因为显然我的适配器没有正确填充。

数据通过 Retrofit 输入良好，并且我的自定义适配器（ NewsAdapter ，基于 ArrayAdapter ）之前已填充，但我必须对其进行一些调整，现在我根本无法使其正常工作。

这个想法是让每个新闻源可用，获取每个特定新闻源的文章，然后用这些文章填充我的 GridView。 这是我的代码：

NewsAdapter ：

public class NewsAdapter extends ArrayAdapter<Articles_Map> {
    Context mContext;
    ArrayList<Articles_Map> articles;

    public NewsAdapter(Context c, int resource) {
        super(c, resource);
        this.mContext = c;
    }

    public NewsAdapter(Context c, int resource, ArrayList<Articles_Map> articles) {
        super(c, resource, articles);
        this.mContext = c;
        this.articles = articles;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // get the property we are displaying
        Articles_Map article = articles.get(position);

        // get the inflater and inflate the XML layout for each item
        LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
        View view = inflater.inflate(R.layout.article_layout, null);

        ImageView thumbnail = (ImageView) view.findViewById(R.id.thumbnail);
        TextView title = (TextView) view.findViewById(R.id.title);
        TextView description = (TextView) view.findViewById(R.id.description);

        Picasso.with(mContext).load(article.urlToImage).into(thumbnail);
        title.setText(article.title);
        description.setText(article.description);

        return view;
    }
}

NewsAPI_Map :

public class NewsAPI_Map {
    String status;
    String source;
    ArrayList<Articles_Map> articles;

    public NewsAPI_Map(String status, String source, ArrayList<Articles_Map> articles) {
        this.status = status;
        this.source = source;
        this.articles = articles;
    }
}

Articles_Map _地图：

public class Articles_Map {
    String title;
    String description;
    String url;
    String urlToImage;

    public Articles_Map(String title, String description, String url, String urlToImage) {
        this.title = title;
        this.description = description;
        this.url = url;
        this.urlToImage = urlToImage;
    }
}

ListNewsActivity.java :

@Override
protected void onCreate(Bundle savedInstanceState) {
    /* ... */

    // parameters for Sources endpoint
    String category = "sport";
    String language = "en";
    String country = "us";

    // Sources endpoint
    Sources_Interface client_sources = NewsAPI_Adapter.createService(Sources_Interface.class);
    Call<Sources_Map> call_sources = client_sources.getData(category, language, country);

    call_sources.enqueue(new Callback<Sources_Map>() {
        @Override
        public void onResponse(Call<Sources_Map> call_sources, Response<Sources_Map> response) {
            if (response.body() != null) {
                final NewsAdapter nAdapter = new NewsAdapter(ListNewsActivity.this,
                        R.layout.article_layout);

                for (Sources_Content source : response.body().sources) {
                    if (source.sortBysAvailable.contains("latest")) {
                        // Articles endpoint
                        NewsAPI_Interface client = NewsAPI_Adapter.createService(NewsAPI_Interface.class);
                        Call<NewsAPI_Map> call = client.getData(source.id, "apiKeyHere");

                        call.enqueue(new Callback<NewsAPI_Map>() {
                            @Override
                            public void onResponse(Call<NewsAPI_Map> call, Response<NewsAPI_Map> response) {
                                if (response.body() != null) {
                                    ExpandableHeightGridView gv_content = (ExpandableHeightGridView) findViewById(R.id.gv_content);
                                    nAdapter.addAll(response.body().articles);
                                    System.out.println("Count: " + nAdapter.getCount() + "\n" +
                                            "Body: " + response.body().articles + "\n");
                                    gv_content.setAdapter(nAdapter);
                                    gv_content.setExpanded(true);
                                }
                            }

                            @Override
                            public void onFailure(Call<NewsAPI_Map> call, Throwable t) {
                                System.out.println("An error ocurred!\n" +
                                        "URL: " + call.request().url() + "\n" +
                                        "Cause: " + t.getCause().toString());
                            }
                        });
                    }
                }
            }
        }

        @Override
        public void onFailure(Call<Sources_Map> call_sources, Throwable t) {
            System.out.println("An error ocurred!");
        }
    });
}

日志：

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: pt.ismai.a26800.readr, PID: 8398
                  java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.ArrayList.get(int)' on a null object reference
                      at pt.ismai.a26800.readr.NewsAdapter.getView(NewsAdapter.java:38)
                      at android.widget.AbsListView.obtainView(AbsListView.java:2346)
                      at android.widget.GridView.onMeasure(GridView.java:1065)
                      at pt.ismai.a26800.readr.ExpandableHeightGridView.onMeasure(ExpandableHeightGridView.java:37)
                      at android.view.View.measure(View.java:18804)
                      at android.support.v4.widget.NestedScrollView.measureChildWithMargins(NestedScrollView.java:1411)
                      at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                      at android.support.v4.widget.NestedScrollView.onMeasure(NestedScrollView.java:479)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:700)
                      at android.support.design.widget.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:90)
                      at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1364)
                      at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:765)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                      at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:135)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
                      at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
                      at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
                      at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
                      at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5954)
                      at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
                      at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
                      at android.view.View.measure(View.java:18804)
                      at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2112)
                      at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1228)
                      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1464)
                      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1119)
                      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6060)
                      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
                      at android.view.Choreographer.doCallbacks(Choreographer.java:670)
                      at android.view.Choreographer.doFrame(Choreographer.java:606)
                      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
                      at android.os.Handler.handleCallback(Handler.java:746)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5443)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

Answer 1

Articles_Map article = articles.get(position);

记得在使用前检查null：

if (articles != null)
{
    Articles_Map article = articles.get(position);
    //other code
}

---

在您的 NewsAPI_Map 类中，您忘记为您的对象放置 getter 和 setter。

NewsAPI_Map:

public class NewsAPI_Map {
    String status;
    String source;
    ArrayList<Articles_Map> articles;

    public NewsAPI_Map(String status, String source, ArrayList<Articles_Map> articles) {
        this.status = status;
        this.source = source;
        this.articles = articles;
    }

    public void setStatus(String status){
        this.status = status;
    }

    public String getStatus(){
        return this.status;
    }
    ....
}

由于您的类 getter 和 setter，您的其余 API 可能无法将数据转换为您的对象。

Answer 2

您基本上使用的是第一个构造函数，这意味着您需要传递ArrayList<Articles_Map> 。 使用您的第一个构造函数会使您的 Array 未初始化，这会导致您的Nullpointer Exception 。

final NewsAdapter nAdapter = new NewsAdapter(ListNewsActivity.this,
                        R.layout.article_layout);

而你的要求是第二个。

public NewsAdapter(Context c, int resource, ArrayList<Articles_Map> articles) {
        super(c, resource, articles);
        this.mContext = c;
        this.articles = articles;
    }

您需要通过提供ArrayList<Articles_Map>实例来使用其他重载的构造函数，例如

final NewsAdapter nAdapter = new NewsAdapter(ListNewsActivity.this,
                            R.layout.article_layout, articles);

更新代码

public class NewsAdapter extends ArrayAdapter<Articles_Map> {
    Context mContext;
    ArrayList<Articles_Map> articles;

    public NewsAdapter(Context c, int resource) {
        super(c, resource);
        this.mContext = c;
        this.articles = new ArrayList<>();
    }

    public void add(Articles_Map m){
       articles.add(m);
       notifyDataSetChanged();
    }

    ... more code ...
}

我添加了代码add(Articles_Map m)以便您可以在回调中使用它，在这种情况下您不需要重新初始化您的适配器。

Answer 3

问题是您正在使用第一个构造函数创建适配器，它没有初始化数据集（您的数组列表），因此它为空。 然后您尝试将数据添加到您的数据集“文章”，这将无法工作，因为它没有初始化。

要解决此问题，请进入您在此处定义的 addAll() 方法 nAdapter.addAll(response.body().articles);

并在方法的开头添加 if(articles == null){articles = new ArrayList articles(); }

之后不要忘记调用 .notifyDataHasChanged();

这是您应该知道的基本编程规则，因为 Collections 不是 android 的东西