NullPointerException on button's OnClickListen

I'm writing an Android game app for an Artificial Intelligence semester project. I'm getting a NullPointerException on Line 38 of the main class, which is the setOnClickListener for the new game button from the launch screen.

Relevant sections Class WW3Activity:

public class WW3Activity extends Activity
{
public boolean DebugMode = false;
private String buildMenuEmplacement = ""; 

//[Initialization] Called when the app is first launched
@Override
public void onCreate(Bundle savedInstanceState)
{           
    super.onCreate(savedInstanceState);
    setContentView(R.layout.startscreen);

    //[Initialization] Makes the button resources available to the class
    final Button newGame = (Button) findViewById(R.id.buttonNewGame);
    final Button loadGame = (Button) findViewById(R.id.buttonLoadGame);
    final Button exitGame = (Button) findViewById(R.id.buttonExit);
    final Button about = (Button) findViewById(R.id.buttonAbout);
    final Button troll = (Button) findViewById(R.id.buttonTroll);
    final Button debug = (Button) findViewById(R.id.buttonDebug);

    //[Action] When user pushes the New Game button
    newGame.setOnClickListener(new View.OnClickListener()
    {
        public void onClick(View v)
        {
            // Perform action on click
            setContentView(R.layout.main);
            buttonInitialize();
        }
    });
    //[Action] When user pushes the Exit Game button
    exitGame.setOnClickListener(new View.OnClickListener()
    {
        public void onClick(View v)
        {
            // Perform action on click
            finish();
        }
    });
    //[Action] When user pushes the Debug Mode button
    exitGame.setOnClickListener(new View.OnClickListener()
    {
        public void onClick(View v)
        {
            // Perform action on click
            DebugMode = true;
        }


      });
    }

    //[Initialization] Gets the emplacements ready to be pushed
   public void buttonInitialize()
   {
        final ImageView pe1 = (ImageView) findViewById(R.id.playerEmplacement1);
        final ImageView pe2 = (ImageView) findViewById(R.id.playerEmplacement2);
        final ImageView pe3 = (ImageView) findViewById(R.id.playerEmplacement3);
        final ImageView pc = (ImageView) findViewById(R.id.playerCity);
        final ImageView ee1 = (ImageView) findViewById(R.id.enemyEmplacement1);
        final ImageView ee2 = (ImageView) findViewById(R.id.enemyEmplacement2);
        final ImageView ee3 = (ImageView) findViewById(R.id.enemyEmplacement3);
        final ImageView ec = (ImageView) findViewById(R.id.enemyCity);

        registerForContextMenu(findViewById(R.id.playerEmplacement1));
        registerForContextMenu(findViewById(R.id.playerEmplacement2));
        registerForContextMenu(findViewById(R.id.playerEmplacement3));
        registerForContextMenu(findViewById(R.id.enemyEmplacement1));
        registerForContextMenu(findViewById(R.id.enemyEmplacement2));
        registerForContextMenu(findViewById(R.id.enemyEmplacement3));

        /*empAction(pe1);
        empAction(pe2);
        empAction(pe3);
        empAction(ee1);
        empAction(ee2);
        empAction(ee3);
        empAction(ec);
        empAction(pc);*/
    }

And the LogCat:

10-24 13:29:12.297: D/dalvikvm(440): GC_FOR_ALLOC freed 40K, 4% free 6356K/6595K, paused 177ms
10-24 13:29:12.303: I/dalvikvm-heap(440): Grow heap (frag case) to 6.653MB for 409616-byte allocation
10-24 13:29:12.503: D/dalvikvm(440): GC_FOR_ALLOC freed <1K, 5% free 6755K/7047K, paused 128ms
10-24 13:29:12.614: D/dalvikvm(440): GC_FOR_ALLOC freed 400K, 8% free 6762K/7303K, paused 57ms
10-24 13:29:12.663: D/AndroidRuntime(440): Shutting down VM
10-24 13:29:12.663: W/dalvikvm(440): threadid=1: thread exiting with uncaught exception (group=0x40014760)
10-24 13:29:12.683: E/AndroidRuntime(440): FATAL EXCEPTION: main
10-24 13:29:12.683: E/AndroidRuntime(440): java.lang.RuntimeException: Unable to start activity ComponentInfo{edu.mbijou.cosc473.ww3/edu.mbijou.cosc473.ww3.WW3Activity}: java.lang.NullPointerException
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1815)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread.access$500(ActivityThread.java:122)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.os.Looper.loop(Looper.java:132)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread.main(ActivityThread.java:4123)
10-24 13:29:12.683: E/AndroidRuntime(440):  at java.lang.reflect.Method.invokeNative(Native Method)
10-24 13:29:12.683: E/AndroidRuntime(440):  at java.lang.reflect.Method.invoke(Method.java:491)
10-24 13:29:12.683: E/AndroidRuntime(440):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
10-24 13:29:12.683: E/AndroidRuntime(440):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
10-24 13:29:12.683: E/AndroidRuntime(440):  at dalvik.system.NativeStart.main(Native Method)
10-24 13:29:12.683: E/AndroidRuntime(440): Caused by: java.lang.NullPointerException
10-24 13:29:12.683: E/AndroidRuntime(440):  at edu.mbijou.cosc473.ww3.WW3Activity.onCreate(WW3Activity.java:38)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.Activity.performCreate(Activity.java:4397)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
10-24 13:29:12.683: E/AndroidRuntime(440):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1779)
10-24 13:29:12.683: E/AndroidRuntime(440):  ... 11 more

startscreen.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <ImageView
        android:id="@+id/logo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="493dp"
        android:layout_marginTop="217dp"
        android:src="@drawable/logo" >
    </ImageView>
    <Button
        android:id="@+id/buttonNewGame"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/logo"
        android:layout_marginLeft="200dp"
        android:layout_marginTop="133dp"
        android:clickable="true"
        android:text="@string/bNewGame" >
    </Button>
    <Button
        android:id="@+id/buttonLoadGame"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/buttonNewGame"
        android:layout_alignLeft="@+id/buttonTroll"
        android:layout_alignRight="@+id/buttonTroll"
        android:clickable="true"
        android:text="@string/bLoadGame" >
    </Button>    <Button
        android:id="@+id/buttonExit"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/buttonLoadGame"
        android:layout_alignBottom="@+id/buttonLoadGame"
        android:layout_alignLeft="@+id/buttonDebug"
        android:layout_alignRight="@+id/buttonDebug"
        android:layout_toRightOf="@+id/buttonLoadGame"
        android:clickable="true"
        android:text="@string/bExit" >
    </Button>
    <Button
        android:id="@+id/buttonAbout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/buttonNewGame"
        android:layout_alignRight="@+id/buttonNewGame"
        android:layout_below="@+id/buttonNewGame"
        android:layout_marginTop="50dp"
        android:text="@string/bAbout" />
    <Button
        android:id="@+id/buttonTroll"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/buttonAbout"
        android:layout_alignBottom="@+id/buttonAbout"
        android:layout_centerHorizontal="true"
        android:text="@string/bTrollface" />
    <Button
        android:id="@+id/buttonDebug"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/buttonTroll"
        android:layout_toRightOf="@+id/buttonTroll"
        android:layout_marginLeft="200dp"
        android:text="@string/bDebug" />
</RelativeLayout>

I originally had a NullPointerException because I declared the set of buttons before I changed the layout view to startscreen, but I fixed that, and then this happened. Technically this IS homework, but I don't think that it warrants the homework tag because this isn't a homework-y type of problem, because the problem has nothing to do with the actual assignment itself.

回答1:

The only reason why findViewById() would return null is because the View you are looking for does not belong to the current View. (The View you set with setContentView()). If R.id.buttonNewGame is in R.layout.startscreen, try to clean and rebuild your project.

回答2:

It would seem that newGame is null. Do you have multiple layouts named startscreen.xml? A common cause of this exception would be creating buttonNewGame in one version but not the other; this would cause the code to build correctly but fail at runtime, exactly as you're describing, when the active layout failed to specify the button.

回答3:

I see this error periodically, the trouble in my case is that android inpropertly maintaining same id's in different layouts. Make sure that your id's are unique not inside only one layout, but in entire application.

回答4:

Set a break point on line 38 and launch the debugger. You're most likely trying to reference a button outside of the xml file R.layout.startscreen which would throw a NullPointerException.