java - SimpleCursorAdapter 抛出 NullPointerException

Question

我在这里发现了类似的问题，但似乎没有一个解决方案有效。

这是我的自定义适配器的构造函数。

public ContactAdapter(Context context, int layout, Cursor c, String[] from, int[] to)
    {
        super(context, layout, c, from, to, 
    }

这是应该填充 ListView 的 Activity 。

public class ContactList extends Activity {

    private static final String KEY_ID = "_id";
    private static final String KEY_NAME = "name";
    private static final String KEY_PH_NO = "phone_number";

    public static final String[] from = new String[] { KEY_ID, KEY_NAME,
            KEY_PH_NO };

    public static final int[] to = new int[] { R.id.textID, R.id.nameText,
            R.id.numberText };

    ListView listView;

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

        DatabaseHandler db = new DatabaseHandler(this);

        Cursor cur = db.getAllContacts();

        ContactAdapter adapter = new ContactAdapter(this, R.layout.custom_list, cur, from, to);

        listView.setAdapter(adapter);


    }

它抛出一个空指针异常。这是我的堆栈树。

    08-12 11:39:17.647: E/AndroidRuntime(6902): FATAL EXCEPTION: main
    08-12 11:39:17.647: E/AndroidRuntime(6902): java.lang.RuntimeException: Unable to start activity ComponentInfo
{com.example.databaseproject/com.example.databaseproject.ContactList}: java.lang.NullPointerException
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.os.Handler.dispatchMessage(Handler.java:99)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.os.Looper.loop(Looper.java:137)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread.main(ActivityThread.java:5103)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at java.lang.reflect.Method.invokeNative(Native Method)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at java.lang.reflect.Method.invoke(Method.java:525)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at dalvik.system.NativeStart.main(Native Method)
    08-12 11:39:17.647: E/AndroidRuntime(6902): Caused by: java.lang.NullPointerException
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at com.example.databaseproject.ContactList.onCreate(ContactList.java:42)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.Activity.performCreate(Activity.java:5133)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
    08-12 11:39:17.647: E/AndroidRuntime(6902):     ... 11 more

请询问是否需要任何其他代码示例来跟踪问题。

PS:我检查了创建的数据库。它有值(它不是空的)。如果我评论这一行，它不会抛出错误。

listView.setAdapter(adapter);

Answer 1

您有一个成员变量 ListView listView; 从未初始化，因此包含 null 值。我认为 NullPointerException 是可以 self 解释的。

为了避免这种情况，您必须创建 ListView 的实例并将其分配给您的成员 listView ，就像您对字符串和数组 from 所做的那样和到。