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Android: activity crashes when called.

 
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Hello,

I am making a Rock-Paper-Scissors game.  When I call the activity with no code, it calls ok.  But when I add this bit of code, it crashes straight away:


Can anyone see any problems, or have any advice on how to sort this.  Thank you
 
Bod MacNeil
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I should add that that it not all of the code.  I commented out most of it as a test and it still didn't work.  It complies without any errors also.  
 
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What error is there in the logs?
There should be a stacktrace and exception.
 
Bod MacNeil
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12-14 10:02:54.690 21533-21533/macson.fifteendaysofjava E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{macson.fifteendaysofjava/macson.fifteendaysofjava.Rock_Paper_Scissor}: java.lang.NullPointerException
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1880)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
                                                                             at android.app.ActivityThread.access$600(ActivityThread.java:123)
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                             at android.os.Looper.loop(Looper.java:137)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:4424)
                                                                             at java.lang.reflect.Method.invokeNative(Native Method)
                                                                             at java.lang.reflect.Method.invoke(Method.java:511)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
                                                                             at dalvik.system.NativeStart.main(Native Method)
                                                                          Caused by: java.lang.NullPointerException
                                                                             at android.app.Activity.findViewById(Activity.java:1794)
                                                                             at macson.fifteendaysofjava.Rock_Paper_Scissor.<init>(Rock_Paper_Scissor.java:19)
                                                                             at java.lang.Class.newInstanceImpl(Native Method)
                                                                             at java.lang.Class.newInstance(Class.java:1319)
                                                                             at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
                                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1871)
                                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
                                                                             at android.app.ActivityThread.access$600(ActivityThread.java:123) 
                                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
                                                                             at android.os.Handler.dispatchMessage(Handler.java:99) 
                                                                             at android.os.Looper.loop(Looper.java:137) 
                                                                             at android.app.ActivityThread.main(ActivityThread.java:4424) 
                                                                             at java.lang.reflect.Method.invokeNative(Native Method) 
                                                                             at java.lang.reflect.Method.invoke(Method.java:511) 
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
                                                                             at dalvik.system.NativeStart.main(Native Method) 

 
Dave Tolls
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Caused by: java.lang.NullPointerException
at android.app.Activity.findViewById(Activity.java:1794)
at macson.fifteendaysofjava.Rock_Paper_Scissor.<init>(Rock_Paper_Scissor.java:19)

There is an issue with the view you are attempting to get on line 19.
Does it exist on the page associated with this activity?
 
Bod MacNeil
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hmmm that means this line is causing the problem:
I've just looked and there is a rock2 ImageView in my XML:


above is code from the activity_rock__paper__scissor.xml which has been added to my activity

 
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Line 19 seems to be about scissors2, not rock2.
 
Bod MacNeil
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I missed out the package name when I posted the code here so its a little off.  line 19 is for rock2 on my Android Studio.  I was thinking maybe there is a problem which is triggering this which I haven't spotted yet?
 
Bod MacNeil
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I've tried commenting out line 19 but when I re-run the program the error moved to line 20 and so on.  Its just going thought my instant variables.  Has it anything to do with where I'm putting them?
 
Bod MacNeil
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The code works ok when I comment out these :


Its weird that it only happens at run time.  Any ideas?
 
Bod MacNeil
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Sorry for the constant posts.  But I fixed the problem by putting them in onCreate().  If anyone could explain why this works that would be good

 
Marshal
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Don't know, unless onCreate is some sort of analogue of a constructor. Initialising all fields in the constructor is one way to avoid nulls, so maybe the same happens in onCreate. Otherwise, don't know.
No doubt somebody else will know.
 
Tim Moores
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Oh, I completely missed that.

You had two variables for each GUI element. The ones defined at class level (rock2 etc.) get instantiated when the object is constructed - when there is no layout, and hence they are null.

The objects instantiated in onCreate (rockButton etc) are accessible only in that method, and hence of little use.

The latest code version is doing it correctly.
 
Dave Tolls
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Ha!
Of course.
That's what happens when you don't do this stuff for a while...you miss the forest for the trees...
 
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