来自Activity的回调片段中的空指针异常

Question

Before we go on, I know what you're thinking. I initialized my object . I also initialized in the appropriate method (I tried onCreateView and onResume).

My problem is an object (actually, more than one object is throwing a Null Pointer, but they are initialized in the same way) that is throwing a Null Pointer Exception and I can't, for the life of me, figure out why.

Background: I have an Activity that contains a TabHost and a ViewPager. The Activity is also a container for a Fragment that I have (we'll call it Frag A). In my Activity when Frag A is visible the user can click a button that brings up two consecutive DialogFragments. When the second DialogFragment finishes, it calls a method from Frag A that adds some views to Frag A's layout. When the method is called, none of my objects remain initialized despite me trying to do so both in onCreateView and onResume.

Below is applicable code: The exception is being thrown in the last method when the line wpp.addBlankExercise(); is executed. I'm 99% sure that it has something to do with the way the TabHost/ViewPager is using lifecycle callbacks on Frag A. I don't know exactly what part of the TabHost/ViewPager is causing this though.

Activity

    static class DummyTabFactory implements TabHost.TabContentFactory {
    private final Context mContext;

    public DummyTabFactory(Context context) {
        mContext = context;
    }

    @Override
    public View createTabContent(String tag) {
        View v = new View(mContext);
        v.setMinimumWidth(0);
        v.setMinimumHeight(0);
        return v;
    }
}

public TabsAdapter(FragmentActivity activity, TabHost tabHost, ViewPager pager) {
    super(activity.getSupportFragmentManager());
    mContext = activity;
    mTabHost = tabHost;
    mViewPager = pager;
    mTabHost.setOnTabChangedListener(this);
    mViewPager.setAdapter(this);
    mViewPager.setOnPageChangeListener(this);
}

public void addTab(TabHost.TabSpec tabSpec, Class<?> clss, Bundle args) {
    tabSpec.setContent(new DummyTabFactory(mContext));
    String tag = tabSpec.getTag();

    TabInfo info = new TabInfo(tag, clss, args);
    mTabs.add(info);
    mTabHost.addTab(tabSpec);
    notifyDataSetChanged();
}

@Override
public int getCount() {
    return mTabs.size();
}

@Override
public Fragment getItem(int position) {
    TabInfo info = mTabs.get(position);
    return Fragment.instantiate(mContext, info.clss.getName(), info.args);
}

@Override
public void onTabChanged(String tabId) {
    int position = mTabHost.getCurrentTab();
    mViewPager.setCurrentItem(position);
}

@Override
public void onPageSelected(int position) {
    TabWidget widget = mTabHost.getTabWidget();
    int oldFocusability = widget.getDescendantFocusability();
    widget.setDescendantFocusability(ViewGroup.FOCUS_BLOCK_DESCENDANTS);
    mTabHost.setCurrentTab(position);
    widget.setDescendantFocusability(oldFocusability);
}

@Override
public void getChoiceIndex(DialogFragment dialog) {
    amount = QuantityDialogFragment.amount;
    /* We keep track of which exercises are cardio exercises and which exercises are strength exercises here. If it is a cardio
    * exercise then the value is true (first choice) if it's not then the value is false (second example). */
    if (result == 0) {
        // Cardio exercise
        wpp.cardioBoolean = true;
    /* Null Pointer thrown here because the global variables that "addBlankExercise()" uses are no longer initialized. */
        wpp.addBlankExercise();
    } else {
        // Strength exercise
    // Null Pointer also thrown here for the same reason.
        wpp.cardioBoolean = false;
        wpp.addBlankExercise();
    }
}

Here's a small part of addBlankExercise() . It is much longer than this, but it uses the basic pattern seen here:

public void addBlankExercise () {
    printLineNumber();
    System.out.println("addBlankExercise() " + getLineNumber());
    for (int i = 0; i < WorkoutPlannerActivity.amount; i++) {
        printLineNumber();
        if (isCardioList.size() == etExerciseTracker) {
        isCardioList.add(cardioBoolean);
        }
        printLineNumber();
    LinearLayout layoutInfo = new LinearLayout(context);
    layoutInfo.setLayoutParams(new LinearLayout.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT));
    layoutInfo.setOrientation(LinearLayout.HORIZONTAL);
    layoutInfo.setBackgroundColor(colorBlack);
    layoutInfo.setPadding(0, 0, 10, 5);

    editTextTitle = new EditText(context);
    editTextTitle.setHint("Sample Exercise");
    editTextTitle.setTextAppearance(context, R.style.style_exercise_title);
    editTextTitle.setHintTextColor(colorRed);
    editTextTitle.setBackgroundColor(colorAndroidTransparent);
    editTextTitle.setLayoutParams(new LinearLayout.LayoutParams(0, LayoutParams.WRAP_CONTENT, 12));
    editTextTitle.setTag(editTextTitle.getKeyListener());
    editTextTitle.setId(etExerciseTracker);
    etExerciseTracker ++;

Here's the stack trace:

02-08 07:26:46.082: E/AndroidRuntime(6790): FATAL EXCEPTION: main
02-08 07:26:46.082: E/AndroidRuntime(6790): java.lang.NullPointerException
02-08 07:26:46.082: E/AndroidRuntime(6790): at android.view.ViewConfiguration.get(ViewConfiguration.java:318)
02-08 07:26:46.082: E/AndroidRuntime(6790): at android.view.View.<init>(View.java:3234)
02-08 07:26:46.082: E/AndroidRuntime(6790): at android.view.ViewGroup.<init> (ViewGroup.java:420)
02-08 07:26:46.082: E/AndroidRuntime(6790): at android.widget.LinearLayout.<init> (LinearLayout.java:168)
02-08 07:26:46.082: E/AndroidRuntime(6790): at com.nutrifit.Fragments.WorkoutPlannerPopup.addBlankExercise(WorkoutPlannerPopup.java:398)
02-08 07:26:46.082: E/AndroidRuntime(6790): at com.nutrifit1.Activities.WorkoutPlannerActivity.getChoiceIndex(WorkoutPlannerActivity.java:198)
02-08 07:26:46.082: E/AndroidRuntime(6790): at com.nutrifit.Fragments.QuantityDialogFragment$1.onClick(QuantityDialogFragment.java:44)

Thanks so much in advance for your help!

Answer 1

The Null Pointer Exception was coming from the overridden getItem method in the TabsAdapter class. The problem was that the my implementation of FragmentPagerAdapter was not keeping a reference/ references to the Fragment object(s) that was being instantiated.

For future reference: if using a TabHost and ViewPager, add a means to keep object references to Fragments being instantiated by the PagerAdapter to avoid Null Pointer Exceptions.