Crash when implementing tabs and swipeable views

Question

I’m starting in android development and want to put tabs with swipeable views, I followed a tutorial from Androidhive why of google tutorial It didn’t make any sense to me. The point is that after doing Swipe my application is crashing, I tried to understand the logcat but did not find the error. Follow code and logcat.

Logcat

07-11 13:46:24.218: E/AndroidRuntime(10550): FATAL EXCEPTION: main
07-11 13:46:24.218: E/AndroidRuntime(10550): Process: com.myeventsgw2, PID: 10550
07-11 13:46:24.218: E/AndroidRuntime(10550): java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.view.ViewPager.populate(ViewPager.java:1053)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:555)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:514)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:495)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.myeventsgw2.MainActivity.onTabSelected(MainActivity.java:72)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.android.internal.app.ActionBarImpl.selectTab(ActionBarImpl.java:577)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.android.internal.app.ActionBarImpl$TabImpl.select(ActionBarImpl.java:1105)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.android.internal.widget.ScrollingTabContainerView$TabClickListener.onClick(ScrollingTabContainerView.java:547)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.view.View.performClick(View.java:4445)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.view.View$PerformClick.run(View.java:18429)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.os.Handler.handleCallback(Handler.java:733)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.os.Handler.dispatchMessage(Handler.java:95)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.os.Looper.loop(Looper.java:136)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at android.app.ActivityThread.main(ActivityThread.java:5001)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at java.lang.reflect.Method.invoke(Native Method)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
07-11 13:46:24.218: E/AndroidRuntime(10550):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)

Mainactivity

package com.myeventsgw2;

import com.myeventsgw2.adapter.TabsPagerAdapter;
import com.myeventsgw2.R;
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;
    // Tab titles
    private String[] tabs = { "Top Rated", "Games", "Movies" };

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

        // Initilization
        viewPager = (ViewPager) findViewById(R.id.pager);
        actionBar = getActionBar();
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Adding Tabs
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }

        /**
         * on swiping the viewpager make respective tab selected
         * */
        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // on changing the page
                // make respected tab selected
                actionBar.setSelectedNavigationItem(position);
            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
            }
        });
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        // on tab selected
        // show respected fragment view
        viewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    }

}

Tabspagera pter

package com.myeventsgw2.adapter;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

import com.myeventsgw2.GamesFragment;
import com.myeventsgw2.MoviesFragment;
import com.myeventsgw2.TopRatedFragment;

public class TabsPagerAdapter extends FragmentPagerAdapter {

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {
        switch (index) {
        case 0:
            // Top rated
            return new TopRatedFragment();
        case 1:
            // second
            return new GamesFragment();
        case 3:
            // third
            return new MoviesFragment();
        }
        return null;
    }

    @Override
    public int getCount() {
        //number of tabs
        return 3;
    }
}

Topratedfragment (example of tab)

package com.myeventsgw2;

import com.myeventsgw2.R;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class TopRatedFragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_top_rated,
                container, false);

        return rootView;
    }
}

Answer 1

Error is in your method getItem. Your Adapter has 3 Fragments correct? But by doing the switch, is skipping index 2 and going to the 3 that does not exist.

Logo on index 2 it returns null generating the Exception.

A small hit should fix this problem:

@Override
public Fragment getItem(int index) {
    switch (index) {
        case 0:
            // Top rated
            return new TopRatedFragment();
        case 1:
            // second
            return new GamesFragment();
        case 2:
            // third
            return new MoviesFragment();
    }

    return null;
}