Read this: Your going to run into some problems later in this class

package com.joinedit.ribbitpractice;

import java.util.Locale;

import android.support.v4.app.FragmentActivity;

import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.app.FragmentPagerAdapter;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

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


public class MainActivity extends ActionBarActivity implements ActionBar.TabListener { // 3.4 implements interface: ActionBar.TabListener means that this MainActivity class is acting as the listener for the tabs

    // create TAG variable for Log.e(TAG, currentUser.getUsername());
    public static final String TAG = MainActivity.class.getSimpleName();

    /**
     * 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.v4.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;

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

        ParseAnalytics.trackAppOpenedInBackground(getIntent()); //Lesson 2.3 copy/pasted in code from Parse.com analytics to enable analytic tracking

        //Check to see if anyone is logged in before we start these intent
        //so we don't redirect users to the LogIn screen after the Logged in and arrived at the MainActivity
        //Whenever you use any signup or login methods, the user is cached on disk. You can treat this
        //cache as a session and automatically assume the user is logged in:
        ParseUser currentUser = ParseUser.getCurrentUser();
        if(currentUser == null) {
            //Intent launch LogInActivity
            navigateToLogin(); //refactor>Extract>Method  all the intent stuff to navigateToLogin()
        }
        else {
            Log.i(TAG,currentUser.getUsername());
        }

        // Set up the action bar. Action bar displays the tabs and allows us to tap on them. Simple.
        final ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // 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);

        // When swiping between different sections, select the corresponding
        // tab. We can also use ActionBar.Tab#select() to do this if we have
        // a reference to the Tab.
        mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
        //^3.7 .setOnPageChangeListener == when we Swipe. Swipe triggers a page change thus the name of the method
            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);
                //^3.7 sets tab to this navigational item when swiped.
                // this makes sure ViewPager and ActionBar tab stay in sync.
                // So we won't be at a Inbox Fragment but on the ActionBar show "Friends" tab
            }
        });

        // we add the tabs and titles to the ActionBar.
        // And then we're also setting the tab listener to use this activity.
        // For each of the sections in the app, add a tab to the action bar.
        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {         // Notice that we call the getCount method of the adapter to see how many tabs we need. // And for each tab that we add, we are calling the getPageTitle method from the adapter to get the titles we set earlier.


            // Create a tab with text corresponding to the page title defined by
            // the adapter. Also specify this Activity object, which implements
            // the TabListener interface, as the callback (listener) for when
            // this tab is selected.
            actionBar.addTab(
                    actionBar.newTab()
                            .setText(mSectionsPagerAdapter.getPageTitle(i))
                            .setTabListener(this)); //*simple to implement: from implements ActionBar.TabListener up at the top
                                                   // and a few methods down below*
        }
    }

    private void navigateToLogin() {
        Intent intent = new Intent(this, LoginActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); //add Flags so when you hit the back key on your phone it won't send you to the MainActivity
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK); //basically: logging in should be a new task and the old task should be cleared so we can't go back to it.
        startActivity(intent);
    }

    // The menu [...] // Lesson 2.10 Logging out: Log out, clear the cache, and show login screen.
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu); // R.menu.menu_main == (Res> Menu > menu_main.xml)
        return true;
    }

    //Lesson 2.10 onOptionsItemSelected
    @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.

        //*One thing we check is the id of the item:
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_logout) { //if (user taps on logout option)
            ParseUser.logOut(); //<-- this from Parse User documentation
            navigateToLogin(); //push the user back to LogIn screen
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in
        // the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
    }
}

package com.joinedit.ribbitpractice;

import java.util.Locale;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.content.Context;


/**
 * Created by Fu on 8/12/2015.
 */
/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */

public class SectionsPagerAdapter extends FragmentPagerAdapter { //Adapts fragments for a ViewPager

    protected Context mContext; //3.4 getString method below* needs the Context, thus create Context here

    public SectionsPagerAdapter(Context context, FragmentManager fm) {
        super(fm);
        mContext = context;
    }

    @Override
    //3.4
    // The ViewPager in MainActivity uses this adapter and it will
    // call this getItem method each time it needs a fragment.
    // This will be each time the user taps on a tab or swipes to the next one.
    //That's why this method returns a Fragment     (accessModifier ReturnDataType methodName)
    public Fragment getItem(int position) { //3.4 the 'int' parameter is used for indexing the fragment in the 'ViewPager'. It's just like an array that starts at zero. (Section 1, Section 2, etc)

        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).

        //Simple: switch statement to return a different fragment based on the position: ie: in "Inbox" or "Friends"
        switch(position){
            case 0:
                return new InboxFragment();
            case 1:
                return new FriendsFragment();
        }

        return null;
    }

    @Override
    public int getCount() { //simply returns the "number of tabs in the ViewPager"
        // Show 2 total pages.
        return 2;
    }

    @Override
    //Change the title of the tabs. Change from "Section 1" and "Section 2" to "Inbox" and "Friends".
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
            case 0:
                return mContext.getString(R.string.title_section1).toUpperCase(l); //getString method* needs the Context, thus create Context here
            case 1:
                return mContext.getString(R.string.title_section2).toUpperCase(l);
        }
        return null;
    }
}