java - 无法恢复 Android 应用程序中的 Activity 错误

Question

所以我有一个 FragmentActivity，它可以正常工作。

public class ChallongeEvent extends FragmentActivity {
    private TextView tab_text;
    private String EVENT_ID, URL;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.challonge_act_event);
        init();
    }

    private void init() {
        tab_text = (TextView) findViewById(R.id.tab_text);

        Intent intent = getIntent();
        EVENT_ID = intent.getStringExtra("event_id");
        URL = "https://api.challonge.com/v1/tournaments/" + EVENT_ID + ".json";

        String titles[] = new String[] { getString(R.string.details), getString(R.string.players) };
        int numTabs = titles.length;

        EventAdapter adapter = new EventAdapter(getSupportFragmentManager(), titles, numTabs);
        ViewPager pager = (ViewPager) findViewById(R.id.pager);
        pager.setAdapter(adapter);
        pager.setCurrentItem(numTabs - 1);

        SlidingTabLayout sliding_tabs = (SlidingTabLayout) findViewById(R.id.sliding_tabs);
        sliding_tabs.setDistributeEvenly(true);
        sliding_tabs.setViewPager(pager);
    }

    @Override
    public void onResume() {
        super.onResume();
        populate();
    }

    private void populate() {
        AsyncGet fetch = new AsyncGet(new AsyncResponse() {
            @Override
            public void processFinish(String output) {
                for (Fragment fragment : getSupportFragmentManager().getFragments())
                {
                    if (fragment instanceof ChallongePlayers) {
                        ((ChallongePlayers) fragment).parsePlayers(output);
                    } else if (fragment instanceof ChallongeMatches) {
                        ((ChallongeMatches) fragment).parseMatches(output);
                    }
                }
            }
        });

        HttpUrl.Builder urlBuilder = HttpUrl.parse(URL).newBuilder();
        urlBuilder.addQueryParameter("api_key", Challonge.API_KEY);
        urlBuilder.addQueryParameter("include_participants", "1");
        urlBuilder.addQueryParameter("include_matches", "1");
        fetch.execute(urlBuilder.build().toString());
    }
}

当 FragmentActivity 启动时，它从 API 获取 JSON，然后将此 API 的数据发送到分页器中的多个 Fragment。它使用以下代码执行此操作:

        AsyncGet fetch = new AsyncGet(new AsyncResponse() {
            @Override
            public void processFinish(String output) {
                for (Fragment fragment : getSupportFragmentManager().getFragments())
                {
                    if (fragment instanceof ChallongePlayers) {
                        ((ChallongePlayers) fragment).parsePlayers(output);
                    } else if (fragment instanceof ChallongeMatches) {
                        ((ChallongeMatches) fragment).parseMatches(output);
                    }
                }
            }
        });

        HttpUrl.Builder urlBuilder = HttpUrl.parse(URL).newBuilder();
        urlBuilder.addQueryParameter("api_key", Challonge.API_KEY);
        urlBuilder.addQueryParameter("include_participants", "1");
        urlBuilder.addQueryParameter("include_matches", "1");
        fetch.execute(urlBuilder.build().toString());

但是，每个 fragment 中都有一个 SwipeRefreshLayout，我想在 AsyncTask 请求之前激活 .setRefreshing(true)。因此，我将每个 fragment 的 SwipeRefreshLayout 设为公开，并尝试在 AsyncGet fetch = ... 行上方添加以下内容:

        for (Fragment fragment : getSupportFragmentManager().getFragments())
        {
            if (fragment instanceof ChallongePlayers) {
                ((ChallongePlayers) fragment).swipe_container.setRefreshing(true);
            } else if (fragment instanceof ChallongeMatches) {
                ((ChallongeMatches) fragment).swipe_container.setRefreshing(true);
            }
        }

不幸的是，这段代码的第一行产生了一个错误:

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

为什么这行不通？

Answer 1

getFragments() 调用返回的集合尚未初始化，为空。

您可以添加一个 if 检查以确保 getFragments() 在遍历它之前不会返回一个 null 集合:

if(getSupportFragmentManager().getFragments() != null) {
    for (Fragment fragment : getSupportFragmentManager().getFragments())
    {
        if (fragment instanceof ChallongePlayers) {
            ((ChallongePlayers) fragment).swipe_container.setRefreshing(true);
        } else if (fragment instanceof ChallongeMatches) {
            ((ChallongeMatches) fragment).swipe_container.setRefreshing(true);
        }
    }
}