android - ListView NullpointerException 问题

Question

我尝试创建一个 XML 文件，该文件采用像这样的个人适配器设置的简单 listeView:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">
    <ListView
        android:id="@+id/listpromo"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"/>
</LinearLayout>

和我的 Activity :

 public class ListSample_category3 extends Activity 
    {

        public final static String ITEM_TITLE = "title";
        public final static String ITEM_COMPANY= "company";
        public final static String ITEM_DATE = "availabledate";
        private ListView list;

        public Map<String,?> createItem(String title, String company, String date) 
        {
            Map<String,String> item = new HashMap<String,String>();
            item.put(ITEM_TITLE, title);
            item.put(ITEM_COMPANY, company);
            item.put(ITEM_DATE, date);
            return item;
        }

        @Override
        public void onCreate(Bundle bundle) 
        {
            super.onCreate(bundle);

            List<Map<String,?>> security = new LinkedList<Map<String,?>>();
            security.add(createItem("Remember passwords", "Cisco", "2012-02-10"));
            security.add(createItem("Clear passwords", "Microsoft", "2012-02-10"));
            security.add(createItem("Show security warnings", "Apple", "2012-02-10"));

            // create our list and custom adapter
            SeparatedListAdapter adapter = new SeparatedListAdapter(this);

            adapter.addSection("Categorie 1", new ArrayAdapter<String>(this,
                R.layout.list_item_category3, new String[] { "First item", "Item two" }));

            adapter.addSection("Categorie 2", new SimpleAdapter(this, security, R.layout.list_complex_category3, 
                new String[] { ITEM_TITLE, ITEM_COMPANY, ITEM_DATE }, new int[] { R.id.list_complex_title, R.id.list_complex_company, R.id.list_complex_date }));

            // create ListView
            //ListView list = new ListView(this);
            list = (ListView) findViewById(R.id.listpromo);

            // OnItemClickListener on all items
            OnItemClickListener listener = new OnItemClickListener()
            {
                  public void onItemClick(AdapterView<?> parent, View view, int position, long id)
                  {

                  }
            };
            list.setOnItemClickListener(listener);
            list.setItemsCanFocus(true);

            list.setAdapter(adapter);
            this.setContentView(list);

        }
    }

但是我有一个 NullPointerException :

    08-05 17:00:26.304: E/AndroidRuntime(3226): FATAL EXCEPTION: main
08-05 17:00:26.304: E/AndroidRuntime(3226): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dev.prixo/com.dev.prixo.OngletsActivity}: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dev.prixo/com.dev.prixo.ListSample_category3}: java.lang.NullPointerException
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.os.Looper.loop(Looper.java:130)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.main(ActivityThread.java:3687)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at java.lang.reflect.Method.invokeNative(Native Method)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at java.lang.reflect.Method.invoke(Method.java:507)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at dalvik.system.NativeStart.main(Native Method)
08-05 17:00:26.304: E/AndroidRuntime(3226): Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dev.prixo/com.dev.prixo.ListSample_category3}: java.lang.NullPointerException
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.startActivityNow(ActivityThread.java:1491)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:676)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.widget.TabHost.setCurrentTab(TabHost.java:348)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.widget.TabHost.addTab(TabHost.java:227)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at com.dev.prixo.OngletsActivity.onCreate(OngletsActivity.java:34)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
08-05 17:00:26.304: E/AndroidRuntime(3226):     ... 11 more
08-05 17:00:26.304: E/AndroidRuntime(3226): Caused by: java.lang.NullPointerException
08-05 17:00:26.304: E/AndroidRuntime(3226):     at com.dev.prixo.ListSample_category3.onCreate(ListSample_category3.java:77)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-05 17:00:26.304: E/AndroidRuntime(3226):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
08-05 17:00:26.304: E/AndroidRuntime(3226):     ... 20 more

请告诉我该怎么做..

这是我的 SeperatedListAdapter:

public class SeparatedListAdapter extends BaseAdapter 
{
    public final Map<String,Adapter> sections = new LinkedHashMap<String,Adapter>();
    public final ArrayAdapter<String> headers;
    public final static int TYPE_SECTION_HEADER = 0;

    public SeparatedListAdapter(Context context) 
    {
        headers = new ArrayAdapter<String>(context, R.layout.list_header_category3);
    }

    public void addSection(String section, Adapter adapter) 
    {
        this.headers.add(section);
        this.sections.put(section, adapter);
    }

    public Object getItem(int position) 
    {
        for(Object section : this.sections.keySet()) 
        {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;

            // check if position inside this section 
            if(position == 0) return section;
            if(position < size) return adapter.getItem(position - 1);

            // otherwise jump into next section
            position -= size;
        }
        return null;
    }

    public int getCount() 
    {
        // total together all sections, plus one for each section header
        int total = 0;
        for(Adapter adapter : this.sections.values())
            total += adapter.getCount() + 1;
        return total;
    }

    public int getViewTypeCount() 
    {
        // assume that headers count as one, then total all sections
        int total = 1;
        for(Adapter adapter : this.sections.values())
            total += adapter.getViewTypeCount();
        return total;
    }

    public int getItemViewType(int position) 
    {
        int type = 1;
        for(Object section : this.sections.keySet()) 
        {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;

            // check if position inside this section 
            if(position == 0) return TYPE_SECTION_HEADER;
            if(position < size) return type + adapter.getItemViewType(position - 1);

            // otherwise jump into next section
            position -= size;
            type += adapter.getViewTypeCount();
        }
        return -1;
    }

    public boolean areAllItemsSelectable() 
    {
        return false;
    }

    public boolean isEnabled(int position) 
    {
        return (getItemViewType(position) != TYPE_SECTION_HEADER);
    }

    public View getView(int position, View convertView, ViewGroup parent) 
    {
        int sectionnum = 0;
        for(Object section : this.sections.keySet()) {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;

            // check if position inside this section 
            if(position == 0) return headers.getView(sectionnum, convertView, parent);
            if(position < size) return adapter.getView(position - 1, convertView, parent);

            // otherwise jump into next section
            position -= size;
            sectionnum++;
        }
        return null;
    }

    public long getItemId(int position) 
    {
        return position;
    }
}

Answer 1

如果不设置 contentView，就不能说 findViewById()。您必须设置包含 ListView 的 XML 布局。

setContentView(R.layout.yourlayout)。否则，您的 ListView 列表 将为空。

所以在您的 onCreate() 中:

protected void onCreate(Bundle bundle) {
     super.onCreate(bundle);
     setContentView(R.layout.yourlayout);
}

并删除 onCreate() 底部的这一行:

setContentView(listview);