I made a ViewPager/TabLayout app like in this video (everything same, but I don't have AppBarLayout and ImageView):   But if I want to try the app the debugger says:  "Caused by: java.lang.NullPointerException" at line 24  Here's some Code:  MainActivity.java (where the problem is)  TabLayout tablayout; ViewPager viewPager;  @Override     protected void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);     setContentView(R.layout.activity_main);     viewPager = (ViewPager)findViewById(R.id.viewPager);     ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());     adapter.addFragment(new FragmentElso(), "Elso");     adapter.addFragment(new FragmentMasodik(), "Masodik");     adapter.addFragment(new FragmentHarmadik(), "Harmadik");      viewPager.setAdapter(adapter);     tablayout.setupWithViewPager(viewPager); *(line 24)* }   }  Fragments(here's one, but all 3 are exactly the same)  View viewelso; public FragmentElso() { }  @Nullable @Override public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {     viewelso = inflater.inflate(R.layout.elso, container, false);     return viewelso; }   } And here's the ViewPagerAdapter  public final List&lt;Fragment&gt; fragmentList = new ArrayList&lt;&gt;(); public final List&lt;String&gt; FragmentListTitles = new ArrayList&lt;&gt;();  public ViewPagerAdapter(FragmentManager fm) {     super(fm); }  @Override public Fragment getItem(int position) {     return fragmentList.get(position); }  @Override public int getCount() {     return FragmentListTitles.size(); }  @Override public CharSequence getPageTitle(int position) {     return FragmentListTitles.get(position); } public void addFragment(Fragment fragment, String Title){     fragmentList.add(fragment);     FragmentListTitles.add(Title);  }   }  And I have everywhere the import: android.support.v4.app.Fragment;  In advance thanks for every help!
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
Where to spend a night in the Mumbai airport My girlfriend and I have a layover at the Mumbai airport from 19:35 to 05:55 and we would like to sleep in a cheap place near to the airport (maximum 30 minutes away). Is there any cheap hotel near there? Or any cheap lounge where to sleep well? (flight at 05:55 is an intercontinental flight and we prefer to stay "fresh"). If not possible, is the airport a safe place to sleep? How to interpret decomposition plot and check for seasonality What are the consequences of using a low-wattage USB-C power supply with a MacBook Pro? Groups of prime squared order What's the difference between a method and a function? Can someone provide a simple explanation of methods vs. functions in OOP context? Why couldn't River Song's memories bring back The Doctor in the rebooted universe? In the season 5 finale of Doctor Who, in the wedding ceremony of Amy, River Song showed up outside the door. Moments later, we discovered that she gifted Amy her blank diary which was supposed to contain information about The Doctor. It means River Song remembered The Doctor.  Memories of The Doctor was able to bring The Doctor back to existence in the rebooted Spacetime. Why couldn't River Song's memories work? Does walking fast prevent eggs hatching? I have a couple of eggs that I've picked up from some Pokestops (I'm lucky enough to live above one).   I walk a lot. I get to work through a 2 mile walk / train journey every day, so I should be picking up loads of steps for the eggs I have. However, because I'm normally always running late... for everything, I have to rush everywhere, so I walk at an above average pace.   Today I've walked around 5 miles, but the game has registered 100 metres. Is this because I walk too fast, or because of something else?  It's worth pointing out that I have the app open when walking... mostly because I have to catch them all. (This made me miss a train this morning... luckily I was early :D) More than a decade ago, I read a short story about a small group of people at the end of the world. The story was premised on some sort of mass-produced device which took ambient heat and turned it into power. The devices had been overused, and now the world was freezing.   I think the people were essentially bandits; the main character may have been a woman who lured men to her house and killed them for their body heat? There was also a strong implication that one person she had done this to was an alien who had come to warn that the devices were destroying the world.  I think the story was in a hardbound collection, and I think the collection was from the late 70's or very early 80's.  (Calling it a "short story" is just meant to distinguish it from a full novel — it may have actually been a novella or similar.) So I'm in Linux and I want to have a program accept arguments when you execute it from the command line.  For example,   ./myprogram 42 -b -s  So then the program would store that number 42 as an int and execute certain parts of code depending on what arguments it gets like -b or -s. Derivative of the product of operators and Derivative of exponential I'm asked to show that  $$\frac{d(\hat{A}\hat{B})}{d\lambda} ~=~ \frac{d\hat{A}}{d\lambda}\hat{B} + \hat{A}\frac{d\hat{b}}{d\lambda}$$ With $\lambda$ a continuous parameter. Should I use the definition $$\frac{d\hat{A}}{d\lambda} ~=~ \lim_{\epsilon \to 0} \frac{\hat{A}(\lambda + \epsilon) - \hat{A}(\lambda)}{\epsilon}$$ applied to $\hat{A}\hat{B}$ like $$\frac{d(\hat{A}\hat{B})}{d\lambda} ~=~ \lim_{\epsilon \to 0} \frac{\hat{A}(\lambda + \epsilon)\hat{B}(\lambda + \epsilon) - \hat{A}(\lambda)\hat{B}(\lambda)}{\epsilon}$$ and do some algebra to get the RHS of the first equation, or I'm missing something? Another interesting derivative to pay attention to is: $$\frac{d}{d\lambda}\exp(\hat{A}(\lambda) )~?$$ System settings won't open on Ubuntu 20.04 LTS after adding System76/pop PPA How does Python evaluate the expression 1+++2?  How many ever + I put in between, it is printing 3 as the answer. Please can anyone explain this behavior  And for 1--2 it is printing 3 and for 1---2 it is printing -1 Applications of ultrafilters I'm looking for some interesting applications of ultrafilters and also everything of interest involving ultrafilters. Do you know some applications or interesting things involving ultrafilters?  I'm at the beginning, so I'd prefer some applications that also a beginner could read. It isn't any of the words on this  and I believe it started with the letter 'e' (though please offer your word even if it doesn't).  Example sentence: Because he researched what he had heard from his friends, he was able to refute many of their _________________.  Factoid seems to be the closest one, but the word I am looking for only refers to a belief that is false. Opt in "mentor" privilege at 7.5k I am suggesting a Mentor privilege at 7.5k. It would be on a per-user per-site opt in basis.  At some reputation level (perhaps 7.5k) there could be an opt in "mentor" privilege.  If someone opts in then when a new user signs up, they become their Mentor. The new user is the Mentee.  They will get notified of any posts or suggested edits they make, and can help and support them.  While this would by no means help everyone (I'm sure there are more new people signing up than there are 7.5k people willing to help them), it could help mitigate the "new user's bad posts" and such like.  By no means would being someone's mentor mean you are the only person who can edit and help them, and you aren't obliged to do so - but if you have opted in then you should help.  How would this work?   Any Mentor would have no more than 5 active Mentees at once. Once the Mentee gets to 125 rep, the downvote privilege (or any other level deemed sensible) they are no longer a Mentee. If they aren't active for a month, they are no longer a Mentee. If the Mentor has 5 Mentees (i.e. cannot mentor anyone else) then any Mentee not active for 2 weeks is no longer a Mentee. The Mentee can opt out at any time, the Mentor can opt out, but if they opt out a lot (say 5 people in 2 months) they are blocked from the privilege for 2 weeks. The Mentor gets no extra rep. They can still get rep from answering their Mentee's questions and similar. Once the new user gets 20 rep on the site, a chat room is created for them to talk. No user with the association bonus will become a Mentee. This would (maybe) be opt in for sites. For example, this might not be at all useful for this site, or for , but on  or  I imagine this could be quite good.    appears to be asking the same thing, based on the title, but that is asking about people who know each other offline mentoring each other.    Every single one of these suggestions (especially the numbers) is open to discussion. Maybe 125 is to low to stop being mentored, or maybe it's too high. What do you think? bounding a $C^1$ function that upholds $f'(t)\leq 1-f(t)^2$ Where do I change my shell? Is there such a thing as a line/vertices symbology? If not a workaround? In some situations, particularly when editing, I would like to see my lines as well as all the vertices similar to the effect I get when using the Edit Vertices tool but on all features in the feature class not just the selected feature.  The solution should be dynamic, without having to create a separate point layer but having one line features class loaded twice in the TOC is acceptable.  Programmatic solution using an example (addin, ArcObjects &amp; VB.Net) is acceptable.  I have the ArcEditor license in case there is a trick to use with representations. Can you use twinned spell on the same target? So me and my DM have been arguing on how Twinned spell works. Twinned spell:  When you cast a spell that targets only one creature and  doesn't have a range of self, you can spend a number of  sorcery points equal to the spell's level to target a second  creature in range with the same spell (1 sorcery point if  the spell is a cantrip).  I argued that &quot;a second creature&quot; can be a the same target as it is not worded as &quot;another creature&quot; For this example counter spell with twinned spell.  A wizard casts fireball on me I use CounterSpell#1 with Twinned Spell Wizard casts CounterSpell on my CounterSpell#1 I use him as the same target for my twinned Spell CounterSpell#2  Result countered Is this possible? Several ways to prove that $\sum\limits^\infty_{n=1}\left(1-\frac1{\sqrt{n}}\right)^n$ converges