注册监听器时出现NullPointerException

Question

Plase have a look at the following code

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

<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>

When I run this, I get a NullPointerException . The error is happening in this place.

cards.setOnClickListener(new CardsClicked());

Below is the error log

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

What is causing this error?

Answer 1

UPDATE:

There are two possibilities that can happen:

1. You have two similar xml files with similar names. In one You don't have defined @id
2. You edited xml file and add an @id but somehow this xml is not rebuilded

Try clean and rebuild project and check if you don't have similar files.

I compile your code and works fine. I reproduced error when I create similar file with the same @id (card_layout) add remove it from xml bonded to Activity

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

Old

You are sure its not line below ? Because I don't see any initialization of animation1 variable.

animation1.setAnimationListener(new AnimationEvent());

Answer 2

试试这个改变：

private class CardsClicked implements View.OnClickListener

Answer 3

Your Animation animation1, animation2; are null you have not initialized the animations for your instance. That is why its giving null pointer exception.

Make sure you give the animation file before starting animation as below:

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

I hope you have already defined a listener new AnimationEvent() and implemented its methods to start the animation. As you have not shared any code of this listener.

Answer 4

I found the issue. The Landscape layout is controlled by the XML file in layout-land folder. In that folder, the ID for the LinearLayout was not provided.