实现 Marquee Horizo​​ntalScrollView：致命异常：主要无法启动活动 java.lang.NullPointerException

Question

我正在尝试实现 Marquee Horizo​​ntalScrollView，但是在第 55 行收到一条错误消息，指出 FATAL EXCEPTION: main Unable to start activity java.lang.NullPointerException：

s.fullScroll(HorizontalScrollView.FOCUS_LEFT);

我不确定为什么会发生这种情况。我已经在 XML 中创建/声明了 Horizo​​ntalScrollView：

 <HorizontalScrollView
        android:id="@+id/sc"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentBottom="true"
        android:gravity="bottom"
        android:ellipsize="marquee"
        android:focusable="true"
        android:focusableInTouchMode="true"
        android:marqueeRepeatLimit="marquee_forever"
        android:scrollHorizontally="true"
        android:scrollbars="none" >

并尝试使用以下命令开始滚动过程：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        HorizontalScrollView s =(HorizontalScrollView) findViewById(R.id.sc);
        s.fullScroll(HorizontalScrollView.FOCUS_LEFT);

我用这个例子做到了这一点：

HorizontalScrollView: auto-scroll to end when new Views are added?

但是我不确定为什么会收到这个致命异常/NPE。

建议/意见表示赞赏！

Logcat：

04-07 15:11:23.931: D/AndroidRuntime(11307): Shutting down VM
04-07 15:11:23.931: W/dalvikvm(11307): threadid=1: thread exiting with uncaught exception (group=0x41968ba8)
04-07 15:11:23.931: E/AndroidRuntime(11307): FATAL EXCEPTION: main
04-07 15:11:23.931: E/AndroidRuntime(11307): Process: com.test.app, PID: 11307
04-07 15:11:23.931: E/AndroidRuntime(11307): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.app/com.test.app.Cast}: java.lang.NullPointerException
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.os.Handler.dispatchMessage(Handler.java:102)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.os.Looper.loop(Looper.java:136)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread.main(ActivityThread.java:5017)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at java.lang.reflect.Method.invokeNative(Native Method)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at java.lang.reflect.Method.invoke(Method.java:515)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at dalvik.system.NativeStart.main(Native Method)
04-07 15:11:23.931: E/AndroidRuntime(11307): Caused by: java.lang.NullPointerException
04-07 15:11:23.931: E/AndroidRuntime(11307):    at com.test.app.Cast.onCreate(Cast.java:55)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.Activity.performCreate(Activity.java:5231)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-07 15:11:23.931: E/AndroidRuntime(11307):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-07 15:11:23.931: E/AndroidRuntime(11307):    ... 11 more

Answer 1

您试图在设置内容视图之前找到视图。在找到视图之前添加此行。

setContentView(R.layout.your_xml_name);