注冊監聽器時出現NullPointerException

Question

Plase看看下面的代碼

MainActivity.java

public class MainActivity extends Activity {

    private TextView words;
    private Map<String, String> wordsMap;
    private List wordList;
    private Animation animation1, animation2;
    private LinearLayout cards;

    private int displayIndex = 0;
    private static final int ENGLISH = 1;
    private static final int BRAZIL = 2;
    private int languageDisplaying;

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

        words = (TextView)findViewById(R.id.word);
        cards = (LinearLayout)findViewById(R.id.cards_layout);

        languageDisplaying = ENGLISH;

        animation1 = AnimationUtils.loadAnimation(this, R.anim.to_middle);
        animation2 = AnimationUtils.loadAnimation(this, R.anim.from_middle);

        wordsMap = new HashMap<String, String>();
        wordsMap.put("Dog", "Perro");
        wordsMap.put("Cat", "Gato");
        wordsMap.put("Universe", "Universo");
        wordsMap.put("Telephone", "Teléfono");
        wordsMap.put("KeyBoard", "TecladoDel");
        wordsMap.put("Country", "País");

        wordList = new ArrayList<String>();
        wordList.add("Dogsssssssssss");
        wordList.add("Cat");
        wordList.add("Universe");
        wordList.add("Telephone");
        wordList.add("KeyBoard");
        wordList.add("Country");

        words.setText(wordList.get(0).toString());

        //Registering Listeners
        words.setOnClickListener(new TextClicked());
        cards.setOnClickListener(new CardsClicked());
        animation1.setAnimationListener(new AnimationEvent());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    //Animation Listener
    private class AnimationEvent implements AnimationListener
    {
        String str = "";

        @Override
        public void onAnimationEnd(Animation arg0) {
            // TODO Auto-generated method stub
            if(languageDisplaying==ENGLISH)
            {
                str = wordList.get(displayIndex).toString();


                words.setText(wordsMap.get(str));
                words.setTextColor(Color.BLUE);

                languageDisplaying = BRAZIL;

            }
            else
            {
                str = wordList.get(displayIndex).toString();

                words.setText(str);
                words.setTextColor(Color.BLACK);

                languageDisplaying = ENGLISH;
            }
        }

        @Override
        public void onAnimationRepeat(Animation animation) {
            // TODO Auto-generated method stub

        }

        @Override
        public void onAnimationStart(Animation animation) {
            // TODO Auto-generated method stub

        }

    }

    private OnTouchListener textViewSwiped = new OnSwipeTouchListener()
    {
        public boolean onSwipeLeft() 
         {
            return true;
         }
    };

    private class TextClicked implements OnClickListener
    {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            words.startAnimation(animation1);



        }

    }

    private class CardsClicked implements View.OnClickListener
    {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            //showMenu();

        }

    }

    private void showMenu()
    {
        final Dialog dialog = new Dialog(this);
        dialog.setContentView(R.layout.glass_menu);
        dialog.show();
    }

}

activity_main.xml中

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

<TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="202dp"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="100 de 554" />

    <LinearLayout 
        android:id="@+id/cards_layout"
        android:layout_width="300dp"
        android:layout_height="100dp"
        android:background="@drawable/round_corners"
        android:clickable="true"
        android:gravity="center">

        <TextView
        android:id="@+id/word"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="Text"
        android:textSize="50sp" />

    </LinearLayout>

</LinearLayout>

當我運行它時，我得到一個NullPointerException 。 錯誤發生在這個地方。

cards.setOnClickListener(new CardsClicked());

以下是錯誤日志

12-09 06:05:24.040: E/AndroidRuntime(2424): FATAL EXCEPTION: main
12-09 06:05:24.040: E/AndroidRuntime(2424): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.googleglass_demo/com.example.googleglass_demo.MainActivity}: java.lang.NullPointerException
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.os.Looper.loop(Looper.java:137)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread.main(ActivityThread.java:5041)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at java.lang.reflect.Method.invokeNative(Native Method)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at java.lang.reflect.Method.invoke(Method.java:511)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at dalvik.system.NativeStart.main(Native Method)
12-09 06:05:24.040: E/AndroidRuntime(2424): Caused by: java.lang.NullPointerException
12-09 06:05:24.040: E/AndroidRuntime(2424):     at com.example.googleglass_demo.MainActivity.onCreate(MainActivity.java:71)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.Activity.performCreate(Activity.java:5104)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
12-09 06:05:24.040: E/AndroidRuntime(2424):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
12-09 06:05:24.040: E/AndroidRuntime(2424):     ... 11 more

導致此錯誤的原因是什么？

Answer 1

更新：

有兩種可能發生的可能性：

1. 您有兩個類似名稱的類似xml文件。 在一個你沒有定義@id
2. 您編輯了xml文件並添加了@id，但不知道這個xml是否已被重建

嘗試清理並重建項目，並檢查是否沒有類似的文件。

我編譯你的代碼，工作正常。 當我使用相同的@id（card_layout）創建類似的文件時，我再現了錯誤，從xml綁定到Activity中刪除它

05-02 09:06:19.514: ERROR/AndroidRuntime(519): FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.stacktest/com.example.stacktest.MyActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
    at android.app.ActivityThread.access$2300(ActivityThread.java:125)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:4627)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:521)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.example.stacktest.MyActivity.onCreate(MyActivity.java:38)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
    ... 11 more

舊

你確定它不在下面嗎？ 因為我沒有看到animation1變量的任何初始化。

animation1.setAnimationListener(new AnimationEvent());

Answer 2

試試這個改變：

private class CardsClicked implements View.OnClickListener

Answer 3

你的Animation animation1, animation2; 如果為null，則表示您尚未初始化實例的動畫。 這就是它給出空指針異常的原因。

確保在開始動畫之前給出動畫文件，如下所示：

  // load the animation
    animation1= AnimationUtils.loadAnimation(getApplicationContext(),
            R.anim.<yourfileName>);  
  animation1.setAnimationListener(new AnimationEvent());

我希望你已經定義了一個偵聽器new AnimationEvent()並實現了它的方法來啟動動畫。 因為您還沒有共享此偵聽器的任何代碼。

Answer 4

我發現了這個問題。 Landscape布局由layout-land文件夾中的XML文件控制。 在該文件夾中，未提供LinearLayout的ID。