0 
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.joericharduz.akita/com.joericharduz.akita.MainActivity}: java.lang.NullPointerException

代码是这样的：

Cursor cursor = null;
    if (inputText == null || inputText.length() == 0) {
        cursor = myDataBase.query(true, GL_TABLE, new String[] { GL_ID,
                GL_FK, GL_LANG, GL_VALUE }, GL_FK + " like '%" + inputText
                + "%'", null, null, null, null, null);

    } else {
        cursor = myDataBase.query(true, GL_TABLE, new String[] { GL_ID,
                GL_FK, GL_LANG, GL_VALUE }, GL_VALUE + " like '%"
                + inputText + "%'", null, null, null, null, null);
    }
    if (cursor != null) {
        cursor.moveToFirst();
    }

    String[] columns = new String[] { GL_FK, GL_LANG, GL_VALUE };

    int[] to = new int[] { R.id.tvWord, R.id.tvMeaning, R.id.tvKanji };

    dataAdapter = new SimpleCursorAdapter(mContext, R.layout.listword,
            cursor, columns, to, 0);

    MainActivity mainActivity = new MainActivity();
    ListView listView = mainActivity.wordlist;
    // Assign adapter to ListView
    listView.setAdapter(dataAdapter);

    listView.setOnItemClickListener(new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> listView, View view,
                int position, long id) {
            // Get the cursor, positioned to the corresponding row in the
            // result set
            Cursor cursor = (Cursor) listView.getItemAtPosition(position);

            // Get the state's capital from this row in the database.
            String countryCode = cursor.getString(cursor
                    .getColumnIndexOrThrow("code"));
            Toast.makeText(mContext, countryCode, Toast.LENGTH_SHORT)
                    .show();
        }
    });

我跟着这个教程。它从 SQLite 数据库中获取数据并放入 listview。当我尝试对此进行测试时，它给出了错误：java.lang.RuntimeException：无法实例化活动。不知道什么可能导致这个问题。请给一些建议如何解决这个问题。

完整的堆栈跟踪：

    08-05 21:36:01.375: W/dalvikvm(2367): threadid=1: thread exiting with uncaught exception (group=0x40018578)
08-05 21:36:01.382: E/AndroidRuntime(2367): FATAL EXCEPTION: main
08-05 21:36:01.382: E/AndroidRuntime(2367): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.joericharduz.akita/com.joericharduz.akita.MainActivity}: java.lang.NullPointerException
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1573)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.os.Looper.loop(Looper.java:130)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread.main(ActivityThread.java:3687)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at java.lang.reflect.Method.invokeNative(Native Method)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at java.lang.reflect.Method.invoke(Method.java:507)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at dalvik.system.NativeStart.main(Native Method)
08-05 21:36:01.382: E/AndroidRuntime(2367): Caused by: java.lang.NullPointerException
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.Activity.findViewById(Activity.java:1647)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at com.joericharduz.akita.MainActivity.<init>(MainActivity.java:20)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at java.lang.Class.newInstanceImpl(Native Method)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at java.lang.Class.newInstance(Class.java:1409)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
08-05 21:36:01.382: E/AndroidRuntime(2367):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1565)
08-05 21:36:01.382: E/AndroidRuntime(2367):     ... 11 more
08-05 21:37:13.851: W/dalvikvm(2579): threadid=1: thread exiting with uncaught exception (group=0x40018578)
08-05 21:37:13.937: E/AndroidRuntime(2579): FATAL EXCEPTION: main
08-05 21:37:13.937: E/AndroidRuntime(2579): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.joericharduz.akita/com.joericharduz.akita.MainActivity}: java.lang.NullPointerException
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1573)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.os.Looper.loop(Looper.java:130)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread.main(ActivityThread.java:3687)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at java.lang.reflect.Method.invokeNative(Native Method)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at java.lang.reflect.Method.invoke(Method.java:507)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at dalvik.system.NativeStart.main(Native Method)
08-05 21:37:13.937: E/AndroidRuntime(2579): Caused by: java.lang.NullPointerException
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.Activity.findViewById(Activity.java:1647)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at com.joericharduz.akita.MainActivity.<init>(MainActivity.java:20)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at java.lang.Class.newInstanceImpl(Native Method)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at java.lang.Class.newInstance(Class.java:1409)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
08-05 21:37:13.937: E/AndroidRuntime(2579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1565)
08-05 21:37:13.937: E/AndroidRuntime(2579):     ... 11 more
4

2 回答 2

2

检查MainActivity.java第 20 行。由于在调用 findViewById 时发生 NPE (NullPointerException)，这可能是因为您在 NULL Activity 或具有 NULL 窗口的 Activity 上调用 findViewById（AOSP 中 findViewById 的实现）只是调用 getWindow().findViewById())。

仔细检查您是否仅在 Activity 处于正确状态时才调用 findViewById() 。也就是说，您应该等到收到onCreate回调后再尝试与窗口或视图有关的任何事情。

此外，用类似的东西实例化一个 Activitynew MainActivity()是非常奇怪的，除非你试图完成一些非常不寻常和高级的事情，否则它可能无法工作。

于 2013-08-05T17:30:16.813 回答
1

空指针异常是我们见过的最漂亮的异常。找出哪条线构成这个 NPE，然后回溯！它确定你在 NULL OBJECT 上调用了一些东西！

于 2013-08-05T17:39:29.900 回答