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Can't set adapter of ListView because the ListView has a null object reference

I am trying to make a TabLayout with a ViewPager, but whenever I leave the app (Don't close it, just switch to another app and back) the fragments kind of get killed. I don't know that for sure, but the fragment's need to reCreate, and then fail.
The code for creating the TabLayout, TabLayoutAdapter and the ViewPager:
TabLayout tabLayout = (TabLayout) findViewById(R.id.tablayout);
MyTabLayoutAdapter tabLayoutAdapter;
ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
tabLayout.addTab(tabLayout.newTab().setText("-2"));
tabLayout.addTab(tabLayout.newTab().setText("-1"));
tabLayout.addTab(tabLayout.newTab().setText("0"));
tabLayout.addTab(tabLayout.newTab().setText("1"));
tabLayout.addTab(tabLayout.newTab().setText("2"));
tabLayoutAdapter = new MyTabLayoutAdapter(getSupportFragmentManager(), tabLayout.getTabCount());
viewPager.setAdapter(tabLayoutAdapter);
viewPager.setOffscreenPageLimit(3);
tabLayout.setupWithViewPager(viewPager);
viewPager.setCurrentItem(2);
The code of my TabLayoutAdapter:
public class MyTabLayoutAdapter extends FragmentStatePagerAdapter {
private MyFragment fragmentOne;
private MyFragment fragmentTwo;
private MyFragment fragmentDefault;
private MyFragment fragmentFour;
private MyFragment fragmentFive;
private int numberOfTabs;
public MyTabLayoutAdapter(FragmentManager fragmentManager, int numberOfTabs) {
super(fragmentManager);
fragmentOne = new MyFragment();
fragmentTwo = new MyFragment();
fragmentDefault = new MyFragment();
fragmentFour = new MyFragment();
fragmentFive = new MyFragment();
this.numberOfTabs = numberOfTabs;
}
#Override
public MyFragment getItem(int position) {
switch (position) {
case 0:
return fragmentOne;
case 1:
return fragmentTwo;
case 2:
return fragmentDefault;
case 3:
return fragmentFour;
case 4:
return fragmentFive;
default:
return fragmentDefault;
}
}
#Override
public int getCount() {
return numberOfTabs;
}
}
The MyFragment fragment:
public class MyFragment extends Fragment {
private View currentView;
private ListView listView;
public MyFragment() {}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
currentView = LayoutInflater.from(getContext()).inflate(R.layout.my_fragment_layout, container, false);
listView = (ListView) currentView.findViewById(R.id.listview);
return currentView;
}
//This line throws a NullPointer when I resume the app
public void populateListView(MyListAdapter myListAdapter) {
listView.setAdapter(myListAdapter);
}
}
Whenever I call
MyListAdapter myListAdapter = new MyListAdapter(this, R.layout.my_item, justSomeStringArray);
tabLayoutAdapter.getItem(0).populateListView(myListAdapter);
I get a NullPointerException saying that it can't set a adapter on a null object reference.
Anyone any idea how I can fix this?

Here's an approach which lets the Fragment instance drive the data retrieval by using an AsyncTask implementation (GetDataForListViewTask).
But, to follow Commonsware's advice, GetDataForListViewTask merely obtains the data. It does not set up the adapter for use on the ListViews. Instead, setting up the ListAdapter is done in the Fragment itself.
GetDataForListViewTask passes the data back to the Fragment instance using EventBus. This keeps GetDataForListViewTask from having to maintain a reference to the ListView, Activity, Context or any of that risky stuff.
And, by not "reaching in" to a Fragment from the outside to change its widgets or even hand it data, this code also tries (very broadly) to follow another piece of CommonsWare's guidance:
In general, having code outside of a fragment attempt to manipulate
the widgets inside that fragment — as you are doing here — is a bad
idea.
In any case, if you use this approach, which is a bit different from your original approach, you'll want to modify the GetDataForListViewTask so that it obtains the data from elsewhere in your app or the network, etc.
GetDataForListViewTask.java
import android.os.AsyncTask;
import org.greenrobot.eventbus.EventBus;
import java.util.ArrayList;
class GetDataForListViewTask extends AsyncTask<Void, Void, ArrayList<String>> {
#Override
protected ArrayList<String> doInBackground(Void... ignored) {
// since this method runs on a worker thread, you may
// replace this code with a network call or a background computation
// from another part of the app is ready
ArrayList<String> listItems = new ArrayList<>();
for (int i=0; i<10; i++) {
listItems.add("item " + i);
}
return listItems;
}
#Override
protected void onPostExecute(ArrayList<String> result) {
super.onPostExecute(result);
// post event so that Fragment can use the data to populate its ListView
EventBus.getDefault().post(new MyFragment.ListViewDataReadyEvent(result));
}
}
MainActivity.java
import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tablayout);
ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
MyAdapter tabLayoutAdapter = new MyAdapter(getSupportFragmentManager(), 5);
viewPager.setAdapter(tabLayoutAdapter);
viewPager.setOffscreenPageLimit(3);
tabLayout.setupWithViewPager(viewPager);
viewPager.setCurrentItem(2);
}
class MyAdapter extends FragmentStatePagerAdapter {
private final int count;
public MyAdapter(FragmentManager fm, int count) {
super(fm);
this.count = count;
}
#Override
public MyFragment getItem(int position) {
return MyFragment.newInstance();
}
#Override
public int getCount() {
return this.count;
}
#Override
public CharSequence getPageTitle(int position) {
return "Tab " + position;
}
}
}
MyFragment.java
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import org.greenrobot.eventbus.EventBus;
import org.greenrobot.eventbus.Subscribe;
import org.greenrobot.eventbus.ThreadMode;
import java.util.ArrayList;
public class MyFragment extends Fragment {
private ListView listView;
private GetDataForListViewTask getDataForListViewTask;
public static MyFragment newInstance() {
return new MyFragment();
}
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setRetainInstance(true); // to support use of GetDataForListViewTask
EventBus.getDefault().register(this);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
listView = (ListView) LayoutInflater.from(getContext()).inflate(
R.layout.my_fragment_layout, container, false);
getDataForListViewTask = new GetDataForListViewTask();
getDataForListViewTask.execute();
return listView;
}
#Override
public void onDestroy() {
if (getDataForListViewTask != null) {
getDataForListViewTask.cancel(false);
}
EventBus.getDefault().unregister(this);
super.onDestroy();
}
static class ListViewDataReadyEvent {
ArrayList<String> data;
ListViewDataReadyEvent(ArrayList<String> data) {
this.data = data;
}
}
#SuppressWarnings("unused")
#Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(ListViewDataReadyEvent event) {
if (listView != null) {
ArrayAdapter<String> listAdapter = new ArrayAdapter<>(listView.getContext(),
android.R.layout.simple_list_item_1, event.data.toArray(new String[]{}));
listView.setAdapter(listAdapter);
}
}
}
activity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="vertical" android:padding="4dip"
android:gravity="center_horizontal"
android:layout_width="match_parent" android:layout_height="match_parent">
<android.support.design.widget.TabLayout
android:id="#+id/tablayout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:tabMode="fixed"
app:tabGravity="fill" />
<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/viewpager"
android:layout_width="match_parent"
android:layout_height="match_parent">
</android.support.v4.view.ViewPager>
</LinearLayout>
my_fragment_layout.xml
<?xml version="1.0" encoding="utf-8"?>
<ListView android:id="#+id/listview"
android:layout_alignParentTop="true"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
</ListView>
Selected compile statements for build.gradle
compile 'org.greenrobot:eventbus:3.0.0'
compile 'com.android.support:design:25.3.1'
compile 'com.android.support:appcompat-v7:25.3.1'

onCreateView() is not called on a fragment until the fragment is being added to a FragmentManager. Your final code snippet will only work, at best, if it is called sometime after fragmentOne is added to a FragmentManager. Otherwise, the fragment's ListView will not exist yet.
In general, having code outside of a fragment attempt to manipulate the widgets inside that fragment — as you are doing here — is a bad idea.

How to add swipe tab feature with existing FragmentTabHost?

I am using FragmentTabHost in my app. It is working perfectly well. I have two tabs, which are opened by clicking on the top. Now, I want to add swipe feature in this. I am wondering whether I should use a different approach for the same.
MainActivity:
package com.example.shiza.callhistorycontrol;
import android.os.Bundle;
import android.support.v4.app.FragmentTabHost;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
public class MainActivity extends AppCompatActivity {
private FragmentTabHost mTabHost;
Toolbar toolbar;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Setting up a toolbar for the navigation purpose.
toolbar = (Toolbar)findViewById(R.id.app_bar);
toolbar.setTitle(" Call History Control");
toolbar.setLogo(R.mipmap.ic_launcher);
setSupportActionBar(toolbar);
// The fragments management is done here
mTabHost = (FragmentTabHost)findViewById(android.R.id.tabhost);
mTabHost.setup(this, getSupportFragmentManager(), R.id.realtabcontent);
mTabHost.addTab(mTabHost.newTabSpec("simple").setIndicator("HOME"),
Home.class, null);
mTabHost.addTab(mTabHost.newTabSpec("contacts").setIndicator("ABOUT APP"),
Home.class, null);
}
}
main xml:
<android.support.v4.app.FragmentTabHost
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#android:id/tabhost"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- the app bar customized-->
<include
android:id="#+id/app_bar"
layout="#layout/app_bar"
/>
<TabWidget
android:id="#android:id/tabs"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="0"/>
<FrameLayout
android:id="#android:id/tabcontent"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_weight="0"/>
<FrameLayout
android:id="#+id/realtabcontent"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"/>
</LinearLayout>
<android.support.v4.view.ViewPager
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/pager"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
</android.support.v4.app.FragmentTabHost>
Please provide me any links or any suggestions?

Here is an example that I wrote creates TabHost with swipe feature:
First the main fragment that holds the adapter and add tabs with (two childes fragments).
public class MyFragment extends Fragment
{
private TabHost mTabHost;
private ViewPager mViewPager;
private TabsAdapter mTabsAdapter;
public MyFragment() {
}
#Override
public void onCreate(Bundle instance)
{
super.onCreate(instance);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_main, container, false);
mTabHost = (TabHost) v.findViewById(android.R.id.tabhost);
mTabHost.setup();
mViewPager = (ViewPager) v.findViewById(R.id.pager);
mTabsAdapter = new TabsAdapter(getActivity(), mTabHost, mViewPager);
// Here we load the content for each tab.
mTabsAdapter.addTab(mTabHost.newTabSpec("simple").setIndicator("HOME"), FirstPage.class, null);
mTabsAdapter.addTab(mTabHost.newTabSpec("simple").setIndicator("HOME"), SecondPage.class, null);
return v;
}
#Override
public void onResume() {
super.onResume();
}
public static class TabsAdapter extends FragmentStatePagerAdapter implements TabHost.OnTabChangeListener, ViewPager.OnPageChangeListener
{
private final Context mContext;
private final TabHost mTabHost;
private final ViewPager mViewPager;
private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();
static final class TabInfo
{
private final String tag;
private final Class<?> clss;
private final Bundle args;
TabInfo(String _tag, Class<?> _class, Bundle _args)
{
tag = _tag;
clss = _class;
args = _args;
}
}
static class DummyTabFactory implements TabHost.TabContentFactory
{
private final Context mContext;
public DummyTabFactory(Context context)
{
mContext = context;
}
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);
}
public void onTabChanged(String tabId)
{
int position = mTabHost.getCurrentTab();
mViewPager.setCurrentItem(position);
}
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels)
{
}
public void onPageSelected(int position)
{
// Unfortunately when TabHost changes the current tab, it kindly
// also takes care of putting focus on it when not in touch mode.
// The jerk.
// This hack tries to prevent this from pulling focus out of our
// ViewPager.
TabWidget widget = mTabHost.getTabWidget();
int oldFocusability = widget.getDescendantFocusability();
widget.setDescendantFocusability(ViewGroup.FOCUS_BLOCK_DESCENDANTS);
mTabHost.setCurrentTab(position);
widget.setDescendantFocusability(oldFocusability);
}
public void onPageScrollStateChanged(int state)
{
}
}
Then create another two fragments that represent each tab and call them:
FirstPage
SecondPage
link those to xml files, call new MyFragment and you are good to go.

How to add sliding tabs on an existing navigation bar?

I have a navigation drawer on my current application. However, it seems like I'm having more modules coming in, I have to make sure that my application is still organized. So I was thinking to add a sliding tab on one of my navigation drawer fragment.
During my attempt, something went wrong and the log shows the following error
NullPointerException: Attempt to invoke virtual method 'boolean android.content.Intent.migrateExtraStreamToClipData()' on a null object reference
I tried googling for the issue with no success in getting the solution. Can anyone tell me what is actually wrong with my code?
My following files are.
MainActivity.java
package com.epsilon.rfcalculator;
import android.content.Intent;
import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBarDrawerToggle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
public class MainActivity extends ActionBarActivity {
private ListView mDrawerList;
private DrawerLayout mDrawerLayout;
private ArrayAdapter<String> mAdapter;
private ActionBarDrawerToggle mDrawerToggle;
private String mActivityTitle;
private MyFragment fragment;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mDrawerList = (ListView) findViewById(R.id.navList);
mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mActivityTitle = getTitle().toString();
addDrawerItems();
setupDrawer();
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
}
private void addDrawerItems() {
String[] osArray = {"Free Space Propagation Model", "Two Ray Ground Propagation Model", "Power Conversion", "Wavelength Conversion", "Link Budget"};
mAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, osArray);
mDrawerList.setAdapter(mAdapter);
mDrawerList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
#Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
//Toast.makeText(MainActivity.this, "Time for an upgrade!", Toast.LENGTH_SHORT).show();
displayView(position);
}
});
}
private void displayView(int position) {
// update the main content by replacing fragments
Intent intent = null;
switch (position) {
case 0:
fragment = new MyFragment();
break;
//intent = new Intent(this, FreeSpaceActivity.class);
//break;
case 1:
intent = new Intent(this, TransmissionActivity.class);
break;
case 2:
intent = new Intent(this, PowerActivity.class);
break;
case 3:
intent = new Intent(this, WavelengthActivity.class);
break;
case 4:
intent = new Intent(this, LinkBudgetActivity.class);
break;
//case 5: intent = new Intent(this,MapActivity.class);break;
}
startActivity(intent);
}
private void setupDrawer() {
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, R.string.drawer_open, R.string.drawer_close) {
/** Called when a drawer has settled in a completely open state. */
public void onDrawerOpened(View drawerView) {
super.onDrawerOpened(drawerView);
getSupportActionBar().setTitle("Menu");
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
/** Called when a drawer has settled in a completely closed state. */
public void onDrawerClosed(View view) {
super.onDrawerClosed(view);
getSupportActionBar().setTitle(mActivityTitle);
invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
}
};
mDrawerToggle.setDrawerIndicatorEnabled(true);
mDrawerLayout.setDrawerListener(mDrawerToggle);
}
#Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
mDrawerToggle.syncState();
}
#Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
mDrawerToggle.onConfigurationChanged(newConfig);
}
#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 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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
// Activate the navigation drawer toggle
if (mDrawerToggle.onOptionsItemSelected(item)) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
MyFragment.java //This is a new java file added corresponding to my attempt to add the sliding tab.
package com.epsilon.rfcalculator;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TabHost;
import android.widget.TabWidget;
import java.util.ArrayList;
public class MyFragment extends Fragment
{
private TabHost mTabHost;
private ViewPager mViewPager;
private TabsAdapter mTabsAdapter;
public MyFragment() {
}
#Override
public void onCreate(Bundle instance)
{
super.onCreate(instance);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_main, container, false);
mTabHost = (TabHost) v.findViewById(android.R.id.tabhost);
mTabHost.setup();
mViewPager = (ViewPager) v.findViewById(R.id.pager);
mTabsAdapter = new TabsAdapter(getActivity(), mTabHost, mViewPager);
// Here we load the content for each tab.
mTabsAdapter.addTab(mTabHost.newTabSpec("one").setIndicator("One"), FreeSpaceActivity.class, null);
mTabsAdapter.addTab(mTabHost.newTabSpec("two").setIndicator("Two"), TransmissionActivity.class, null);
return v;
}
public static class TabsAdapter extends FragmentPagerAdapter implements TabHost.OnTabChangeListener, ViewPager.OnPageChangeListener
{
private final Context mContext;
private final TabHost mTabHost;
private final ViewPager mViewPager;
private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();
static final class TabInfo
{
private final String tag;
private final Class<?> clss;
private final Bundle args;
TabInfo(String _tag, Class<?> _class, Bundle _args)
{
tag = _tag;
clss = _class;
args = _args;
}
}
static class DummyTabFactory implements TabHost.TabContentFactory
{
private final Context mContext;
public DummyTabFactory(Context context)
{
mContext = context;
}
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);
}
public void onTabChanged(String tabId)
{
int position = mTabHost.getCurrentTab();
mViewPager.setCurrentItem(position);
}
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels)
{
}
public void onPageSelected(int position)
{
// Unfortunately when TabHost changes the current tab, it kindly
// also takes care of putting focus on it when not in touch mode.
// The jerk.
// This hack tries to prevent this from pulling focus out of our
// ViewPager.
TabWidget widget = mTabHost.getTabWidget();
int oldFocusability = widget.getDescendantFocusability();
widget.setDescendantFocusability(ViewGroup.FOCUS_BLOCK_DESCENDANTS);
mTabHost.setCurrentTab(position);
widget.setDescendantFocusability(oldFocusability);
}
public void onPageScrollStateChanged(int state)
{
}
}
}
activity_main.xml
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- The first child in the layout is for the main Activity UI-->
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin"
tools:context=".MainActivity"
android:background="#ffffffff">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:text="Holy Operating Systems, Batdroid!"
android:textSize="24sp"
android:gravity="center"
android:layout_marginTop="100dp"/>
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"/>
</RelativeLayout>
<!-- Side navigation drawer UI -->
<ListView
android:id="#+id/navList"
android:layout_width="200dp"
android:layout_height="match_parent"
android:layout_gravity="left|start"
android:background="#ffeeeeee"/>
</android.support.v4.widget.DrawerLayout>
fragment_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<TabHost
android:id="#android:id/tabhost"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TabWidget
android:id="#android:id/tabs"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="0"/>
<FrameLayout
android:id="#android:id/tabcontent"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_weight="0"/>
<android.support.v4.view.ViewPager
android:id="#+id/pager"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"/>
</LinearLayout>
</TabHost>
</LinearLayout>

App force close on viewPager.setAdapter(adapter)

I have an application page with viewpager containing two fragments.
When my activity pops up , a server request is made for loading current data and then adapter is set.
But the app crashes with the following error.
Attempt to invoke virtual method 'void android.support.v4.app.Fragment.setMenuVisibility(boolean)' on a null object reference
While debugging, viewPager.setAdapter(adapter) showed null pointer exception. Neither my viewPager is null nor my Adapter.
I am not able to figure out why this is happening.
Please help!!
Thanks in advance!!

Found the problem.. In my fragmentPagerAdapter, getItem() was returning null for a fragment.

Try this code
create a class file TabOne.java
package com.example.tabfragment;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class TabOne extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.tab_one, null);
return rootView;
}
}
create a class file TabTwo.java
package com.example.tabfragment;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class TabTwo extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.tab_two, null);
return rootView;
}
}
**create xml file tab_one.xml and tab_two.xml **
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="#+id/textview"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:gravity="center"
android:text="#string/tab_2" />
</RelativeLayout>
Create a class TabFragmentPageAdapter.java
package com.example.tabfragment;
import java.util.List;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
public class TabFragmentPageAdapter extends FragmentPagerAdapter {
private Bundle args;
private List<Fragment> fragments;
public TabFragmentPageAdapter(FragmentManager fm, List<Fragment> fragments,
Bundle args) {
super(fm);
this.fragments = fragments;
this.args = args;
}
#Override
public Fragment getItem(int position) {
Fragment fragment = fragments.get(position);
fragment.setArguments(args);
return fragment;
}
#Override
public int getCount() {
return fragments.size();
}
#Override
public int getItemPosition(Object object) {
return POSITION_NONE;
}
}
Create a classs file TabFragments.java
public class TabFragments extends Fragment implements OnPageChangeListener,
OnTabChangeListener {
public static final int TAB_LOGIN = 0;
public static final int TAB_REG = 1;
private TabHost tabHost;
private int currentTab = TAB_LOGIN;
private ViewPager viewPager;
private TabFragmentPageAdapter pageAdapter;
private List<Fragment> fragments;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_home, null);
tabHost = (TabHost) rootView.findViewById(android.R.id.tabhost);
viewPager = (ViewPager) rootView.findViewById(R.id.viewpager);
viewPager.setOnPageChangeListener(this);
fragments = new ArrayList<Fragment>();
fragments.add(new TabOne());
fragments.add(new TabTwo());
return rootView;
}
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
setRetainInstance(true);
pageAdapter = new TabFragmentPageAdapter(getChildFragmentManager(),
fragments, getArguments());
pageAdapter.notifyDataSetChanged();
viewPager.setAdapter(pageAdapter);
setupTabs();
}
private void setupTabs() {
tabHost.setup();
tabHost.addTab(newTab(R.string.tab_1));
tabHost.addTab(newTab(R.string.tab_2));
for (int i = 0; i < tabHost.getTabWidget().getChildCount(); i++) {
tabHost.getTabWidget().getChildAt(i)
.setBackgroundColor(Color.parseColor("#304c58"));
// tabHost.setBackgroundResource(R.drawable.tab_selector);
final View view = tabHost.getTabWidget().getChildTabViewAt(i);
final View textView = view.findViewById(android.R.id.title);
((TextView) textView).setTextColor(Color.parseColor("#e2ebf0"));
((TextView) textView).setSingleLine(true);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
tabHost.getTabWidget().getChildAt(i)
.findViewById(android.R.id.icon);
tabHost.getTabWidget().getChildAt(i).getLayoutParams().height = 75;
} else {
if (view != null) {
// reduce height of the tab
view.getLayoutParams().height *= 0.77;
if (textView instanceof TextView) {
((TextView) textView).setGravity(Gravity.CENTER);
textView.getLayoutParams().height = ViewGroup.LayoutParams.MATCH_PARENT;
textView.getLayoutParams().width = ViewGroup.LayoutParams.WRAP_CONTENT;
}
}
}
}
tabHost.setOnTabChangedListener(TabFragments.this);
tabHost.setCurrentTab(currentTab);
}
private TabSpec newTab(int titleId) {
TabSpec tabSpec = tabHost.newTabSpec(getString(titleId));
tabSpec.setIndicator(getString(titleId));
tabSpec.setContent(new TabFactory(getActivity()));
return tabSpec;
}
#Override
public void onPageScrollStateChanged(int position) {
}
#Override
public void onPageScrolled(int position, float arg1, int arg2) {
}
#Override
public void onPageSelected(int position) {
tabHost.setCurrentTab(position);
}
#Override
public void onTabChanged(String tabId) {
currentTab = tabHost.getCurrentTab();
viewPager.setCurrentItem(currentTab);
updateTab();
}
#SuppressWarnings("unused")
private void updateTab() {
switch (currentTab) {
case TAB_LOGIN:
TabOne login = (TabOne) fragments.get(currentTab);
break;
case TAB_REG:
TabTwo register = (TabTwo) fragments
.get(currentTab);
break;
}
}
class TabFactory implements TabContentFactory {
private final Context context;
public TabFactory(Context context) {
this.context = context;
}
#Override
public View createTabContent(String tag) {
View v = new View(context);
v.setMinimumHeight(0);
v.setMinimumWidth(0);
return v;
}
}
}
Create you activity class
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
public class MainActivity extends ActionBarActivity {
Fragment fragment = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fragment = new TabFragments();
Log.i("fragment", "" + fragment.getId());
if (fragment != null) {
FragmentManager fm = getSupportFragmentManager();
fm.beginTransaction().replace(R.id.frame_container, fragment)
.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE)
.commit();
}
}
}
Create mainactivity.xml file
<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<FrameLayout
android:id="#+id/frame_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
/>
</android.support.v4.widget.DrawerLayout>
create tab fragment_home.xml file
<?xml version="1.0" encoding="utf-8"?>
<TabHost xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:ts="http://schemas.android.com/apk/res-auto"
android:id="#android:id/tabhost"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<TabWidget
android:id="#android:id/tabs"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:fadingEdge="none"
android:background="#394c58"
android:tabStripEnabled="false" />
<FrameLayout
android:id="#android:id/tabcontent"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<android.support.v4.view.ViewPager
android:id="#+id/viewpager"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</FrameLayout>
</LinearLayout>
</TabHost>

Tutorial to implement the use of TabHost in Android 2.2 + ViewPager and Fragments

A short tutorial for people like me who had some trouble finding a way to implement TabHost and ViewPager, including page swiping with fingers and tab click to change pages. The shown solution is compatible with Android versions 2.2+.
It includes Tabs initialization, ViewPager connected with Tabs and Page Scrolling management.
Its main peculiarity is the optimization for earlier versions of Android (Android 2.2 (Froyo), API version 8) and the simple implementation for different purposes.

The tutorial includes 4 classes and 2 layouts. It has been tested with an Android phone 2.2, and you can just copy & paste to try it.
MainActivity.java (the main activity):
package samples.tabhost.andreaoid.net;
import java.util.ArrayList;
import java.util.List;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.support.v4.view.ViewPager.OnPageChangeListener;
import android.widget.TabHost;
import android.widget.TabHost.OnTabChangeListener;
public class MainActivity extends FragmentActivity implements OnTabChangeListener, OnPageChangeListener {
MyPageAdapter pageAdapter;
private ViewPager mViewPager;
private TabHost mTabHost;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mViewPager = (ViewPager) findViewById(R.id.viewpager);
// Tab Initialization
initialiseTabHost();
// Fragments and ViewPager Initialization
List<Fragment> fragments = getFragments();
pageAdapter = new MyPageAdapter(getSupportFragmentManager(), fragments);
mViewPager.setAdapter(pageAdapter);
mViewPager.setOnPageChangeListener(MainActivity.this);
}
// Method to add a TabHost
private static void AddTab(MainActivity activity, TabHost tabHost, TabHost.TabSpec tabSpec) {
tabSpec.setContent(new MyTabFactory(activity));
tabHost.addTab(tabSpec);
}
// Manages the Tab changes, synchronizing it with Pages
public void onTabChanged(String tag) {
int pos = this.mTabHost.getCurrentTab();
this.mViewPager.setCurrentItem(pos);
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
// Manages the Page changes, synchronizing it with Tabs
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
int pos = this.mViewPager.getCurrentItem();
this.mTabHost.setCurrentTab(pos);
}
#Override
public void onPageSelected(int arg0) {
}
private List<Fragment> getFragments(){
List<Fragment> fList = new ArrayList<Fragment>();
// TODO Put here your Fragments
MySampleFragment f1 = MySampleFragment.newInstance("Sample Fragment 1");
MySampleFragment f2 = MySampleFragment.newInstance("Sample Fragment 2");
MySampleFragment f3 = MySampleFragment.newInstance("Sample Fragment 3");
fList.add(f1);
fList.add(f2);
fList.add(f3);
return fList;
}
// Tabs Creation
private void initialiseTabHost() {
mTabHost = (TabHost) findViewById(android.R.id.tabhost);
mTabHost.setup();
// TODO Put here your Tabs
MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab1").setIndicator("Tab1"));
MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab2").setIndicator("Tab2"));
MainActivity.AddTab(this, this.mTabHost, this.mTabHost.newTabSpec("Tab3").setIndicator("Tab3"));
mTabHost.setOnTabChangedListener(this);
}
}
MyPageAdapter.java (fragment manager):
package samples.tabhost.andreaoid.net;
import java.util.List;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
public class MyPageAdapter extends FragmentPagerAdapter {
private List<Fragment> fragments;
public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {
super(fm);
this.fragments = fragments;
}
#Override
public Fragment getItem(int position) {
return this.fragments.get(position);
}
#Override
public int getCount() {
return this.fragments.size();
}
}
MyTabFactory (tab manager):
package samples.tabhost.andreaoid.net;
import android.content.Context;
import android.view.View;
import android.widget.TabHost.TabContentFactory;
public class MyTabFactory implements TabContentFactory {
private final Context mContext;
public MyTabFactory(Context context) {
mContext = context;
}
public View createTabContent(String tag) {
View v = new View(mContext);
v.setMinimumWidth(0);
v.setMinimumHeight(0);
return v;
}
}
MySampleFragment.java (the single fragment - included for demonstration purposes):
package samples.tabhost.andreaoid.net;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
public class MySampleFragment extends Fragment {
private static View mView;
public static final MySampleFragment newInstance(String sampleText) {
MySampleFragment f = new MySampleFragment();
Bundle b = new Bundle();
b.putString("bString", sampleText);
f.setArguments(b);
return f;
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
mView = inflater.inflate(R.layout.sample_fragment, container, false);
String sampleText = getArguments().getString("bString");
TextView txtSampleText = (TextView) mView.findViewById(R.id.txtViewSample);
txtSampleText.setText(sampleText);
return mView;
}
}
activity_main.xml:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context=".MainActivity" >
<TabHost
android:id="#android:id/tabhost"
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
<TabWidget
android:id="#android:id/tabs"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_weight="0" />
<FrameLayout
android:id="#android:id/tabcontent"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_weight="0" />
<android.support.v4.view.ViewPager
android:id="#+id/viewpager"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom" />
</LinearLayout>
</TabHost>
</RelativeLayout>
sample_fragment.xml (you can put here your fragment layout):
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<TextView
android:id="#+id/txtViewSample"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentRight="true"
android:layout_alignParentTop="true"
android:text="" />
</RelativeLayout>