OnCreate方法中的Android NullPointerException

Question

I have been writing this Android application for the past few days and everything has been working smoothly, until today. In my app I was trying to implement a tabbed view for my application, but as soon as I try to run it my application installs and force closes immediately upon opening. I have been sitting at this for an entire day and cannot for the life of me figure out what the issue is. Hopefully someone can help me out.

E/AndroidRuntime(5469): FATAL EXCEPTION: main
E/AndroidRuntime(5469): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo
{com.stullich.tim.lolbufftimer/com.stullich.tim.lolbufftimer.LoLBuffTimerMain}: java.lang.NullPointerException
E/AndroidRuntime(5469):at android.app.ActivityThread.performLaunchActivity
(ActivityThread.java:1573) 
E/AndroidRuntime(5469): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
E/AndroidRuntime(5469): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
E/AndroidRuntime(5469): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
E/AndroidRuntime(5469): at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(5469): at android.os.Looper.loop(Looper.java:130)
E/AndroidRuntime(5469): at android.app.ActivityThread.main(ActivityThread.java:3691)
E/AndroidRuntime(5469): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(5469): at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(5469): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run
(ZygoteInit.java:907)
E/AndroidRuntime(5469): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:665)
E/AndroidRuntime(5469): at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(5469): Caused by: java.lang.NullPointerException
E/AndroidRuntime(5469): at android.content.ContextWrapper.getResources(ContextWrapper.java:80)
E/AndroidRuntime(5469): at com.stullich.tim.lolbufftimer.LoLBuffTimerMain.<init>(LoLBuffTimerMain.java:14)
E/AndroidRuntime(5469): at java.lang.Class.newInstanceImpl(Native Method)
E/AndroidRuntime(5469): at java.lang.Class.newInstance(Class.java:1409)
E/AndroidRuntime(5469): at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
E/AndroidRuntime(5469): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1565)
E/AndroidRuntime(5469):     ... 11 more

Here is the code for the main activity:

public class LoLBuffTimerMain extends TabActivity{

Resources res = getResources(); 
TabHost tabHost = getTabHost(); 
TabHost.TabSpec spec;  
Intent intent;

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

    intent = new Intent().setClass(this, YourBuffs.class);

    spec = tabHost.newTabSpec("yourbuffs").setIndicator("Your Buffs", 
                        res.getDrawable(R.drawable.tab_view_you))
                  .setContent(intent);
    tabHost.addTab(spec);

    intent = new Intent().setClass(this, EnemyBuffs.class);
    spec = tabHost.newTabSpec("enemybuffs").setIndicator("Enemy Buffs", 
                        res.getDrawable(R.drawable.tab_view_enemy))
                  .setContent(intent);
    tabHost.addTab(spec);
    }
}

and the XML:

<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@android:id/tabhost"
android:layout_width="fill_parent">
    <TabWidget android:id="@android:id/tabs"
        android:layout_height="wrap_content"
        android:layout_width="fill_parent" />
    <FrameLayout android:id="@android:id/tabcontent"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:padding="5dp">        
    </FrameLayout>
</TabHost>

Answer 1

The stack trace points to this line as the source of the exception:

Resources res = getResources(); 

The inherited getResources() method is throwing an exception. That's because you can't call getResources() until later, after the activity is constructed and initialized. Move those member variable initializations into onCreate() and you should be good to go.

Answer 2

The following lines need to be defined inside onCreate

Resources res = getResources(); 
TabHost tabHost = getTabHost(); 

Answer 3

I'm not sure of this, but I think you can do a getTabHost() only after you do a setContentView() . Otherwise getTabHost() returns null (this behavior is similar to findViewById() ).