Question

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();
        }
    });

我遵循了tutorial。它从SQLite数据库获取数据并放入listview。当我试图测试它时，它给出了错误：java.lang.RuntimeException：无法实例化活动。不知道是什么原因导致这个问题。请提出一些如何解决这个问题的建议。

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

Answer 1

在第20行检查MainActivity.java。由于在调用findViewById时发生了NPE（NullPointerException），这可能是因为您在NULL活动上调用了findViewById，或者在具有NULL的Activity上调用了window（AOSP中findViewById的实现只是调用getWindow（）。findViewById（））。

仔细检查当您的Activity处于正确状态时，您只调用findViewById（）。也就是说，在尝试执行与窗口或视图有关的任何操作之前，您应该等到onCreate回调。

另外，用new MainActivity()之类的东西来实例化一个活动是非常奇怪的，除非你试图完成一些非常不寻常和高级的东西，否则很可能无法工作。

Answer 2

空指针异常是我们能看到的最美丽的例外。找出哪条线制作这个NPE，然后回溯！它肯定你在NULL OBJECT上调用了一些东西！

无法实例化活动：java.lang.NullPointerException

2 个答案: