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Android Build a Self-Destructing Message Android App Using Fragments for Tabs Modifying Tabs from the Template

Posted by Carla Brooks
Carla Brooks
803 Points

NullPointerExceptionError

Im having problems with this ViewPager, im getting an error on this line: mViewPager.setAdapter(mSectionsPagerAdapter);

how can I fix this, i'm using Android Studio, please help

4 Answers

Andrei Fecioru
Andrei Fecioru
15,059 Points

You have to assign to the instance var called mSectionsPagerAdapter the result of the instantiation process for the SectionsPagerAdapter class.

From the code you posted above I see that you instantiate the SectionsPagerAdapter class, but you do not assign the result to the mSectionsPagerAdapter instance var. The result is that the mSectionsPagerAdapter instance var remains null and then you pass that null reference to the setAdapter() method.

You should do something like:

SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;

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

    // IMPORTANT: assign the newly created instance to the mSectionsPagerAdapter
    // instance var.
    mSectionsPagerAdapter = new SectionsPagerAdapter(this,getSupportFragmentManager());

    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

}

This should solve your NPE problem.

Carla Brooks
Carla Brooks
803 Points

thanks man!!! :D

Andrei Fecioru
Andrei Fecioru
15,059 Points
Andrei Fecioru
Andrei Fecioru
15,059 Points

Hello Juan,

Please make sure you actually instantiate the mViewPager instance variable before you call the setAdatper() method on it. The same applies for the mSectionsPagerAdapter instance var. Somewhere in your code (before calling the setAdapter() method you should have something like:

// I don't know the actual class names here or the constructor signature (I'm just guessing)
mViewPager = new ViewPager(...)  
mSectionsPagerAdapter = new PagerAdapter(...)

// after that you can call
mViewPager.setAdapter(mSectionsPagerAdapter)

Hope this helps.

Lance Preston
PLUS
Lance Preston
Courses Plus Student 1,389 Points
Lance Preston
Lance Preston
Courses Plus Student 1,389 Points

Can you post the LogCat please ?

Carla Brooks
Carla Brooks
803 Points
Carla Brooks
Carla Brooks
803 Points

here is part of the code that i think its giving me the error:

public class MessageActivity extends FragmentActivity {

SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;

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

    new SectionsPagerAdapter(this,getSupportFragmentManager());

    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

}