App crashes when set Swipe Views with Tabs

Question

I am pasting all the code bellow because i am not sure where i went wrong anymore. The app works fine until i let //mViewPager.setCurrentItem(tab.getPosition()); go in action, the app wont start anymore, i tried to follow the steps on the developers site. An explanation for the problem would also be appreciated. Thank you!

package com.basel.ribbit;

import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;

import com.parse.ParseAnalytics;
import com.parse.ParseUser;

public class MainActivity extends FragmentActivity {



    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a {@link FragmentPagerAdapter}
     * derivative, which will keep every loaded fragment in memory. If this
     * becomes too memory intensive, it may be best to switch to a
     * {@link android.support.v13.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;
    public static final String TAG = MainActivity.class.getSimpleName();

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

        //createActionBar();

        ParseAnalytics.trackAppOpened(getIntent());
        createActionBar();

        ParseUser currentUser = ParseUser.getCurrentUser();

        if(currentUser == null){
            navigateToLogin();}else{
                Log.i(TAG,"A NEW USER HAS LOGGED IN    " +  currentUser.getUsername());
            }


        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
        setPagerOnItemSelected();


    }

    private void setPagerOnItemSelected() {
        mViewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

            @Override
            public void onPageSelected(int position) {
                // TODO Auto-generated method stub
            //  getActionBar().setSelectedNavigationItem(position);

            }

            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onPageScrollStateChanged(int arg0) {
                // TODO Auto-generated method stub

            }
        });
    }

private void createActionBar() {
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        ActionBar.TabListener tabListener = new ActionBar.TabListener() {

            @Override
            public void onTabUnselected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }

            @Override
            public void onTabSelected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub
            //mViewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabReselected(Tab tab, FragmentTransaction ft) {
                // TODO Auto-generated method stub

            }
        };
        actionBar.addTab(actionBar.newTab().setText("Hello").setTabListener(tabListener));
        actionBar.addTab(actionBar.newTab().setText("Hi").setTabListener(tabListener));



    }

    private void navigateToLogin() {
        Intent intent = new Intent (this, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(intent);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;


    }


    @Override
    public boolean onMenuItemSelected(int featureId, MenuItem item) {
        // TODO Auto-generated method stub
        int id = item.getItemId();
        if(id == R.id.action_logout){
            ParseUser.logOut();
            navigateToLogin();

        }
        return super.onMenuItemSelected(featureId, item);


    enter code here
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

}

Answer 1

Move this lines before createActionBar();

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

You are crashing because you haven't initialized mViewPager yet, and you are trying to use it. If you wait until after you've set mViewPager, it should work.

The other possibility is that you don't have a ViewPager in activity_main.xml with id pager.