I have created my own test JSON and try to incorporate it into my Android app using Retrofit but for some reason I can´t seem to understand I get a Null Point Exception when trying to fetch the JSON.   I have created JSON that looks something like this:  [{"date":"2019/01/01","availability":"available"},{"date":"2019/01/02","availability":"closed"},{"date":"2019/01/03","availability":"closed"}]   And so on...  Then I have set up both the Retrofit code and the Room database code.  Here is the Retrofit code from the main class:  public class MainActivity extends AppCompatActivity {  MainRepository mainRepository; Call&lt;List&lt;RFVariables&gt;&gt; getRFPosts;  @Override protected void onCreate(Bundle savedInstanceState) {     super.onCreate( savedInstanceState );     setContentView( R.layout.activity_main );      getRFPosts = mainRepository.getGetPosts();      getRFPosts.enqueue( new Callback&lt;List&lt;RFVariables&gt;&gt;() {         @Override         public void onResponse(Call&lt;List&lt;RFVariables&gt;&gt; call, Response&lt;List&lt;RFVariables&gt;&gt; response) {             List&lt;RFVariables&gt; myResponse = response.body();              if(myResponse != null) {                 for(RFVariables item: myResponse) {                     Log.d("TAG", item.getDate());                  }             }          }          @Override         public void onFailure(Call&lt;List&lt;RFVariables&gt;&gt; call, Throwable t) {          }     } );   It crashes on the getRFPosts = mainRepository.getGetPosts(); line with the error Attempt to invoke interface method 'void retrofit2.Call.enqueue(retrofit2.Callback)' on a null object reference         at com.slothmode.sunnyatsea.MainActivity.onCreate(MainActivity.java:25)  The repository code looks like this:  private Retrofit retrofitObject; private RFCalls retrofitRepository; private Call&lt;List&lt;RFVariables&gt;&gt; getPosts;  public MainRepository(Application application) {     this.retrofitObject = new Retrofit.Builder()             .baseUrl( "https://api.myjson.com/bins/" )             .addConverterFactory( GsonConverterFactory.create() )             .build();      this.retrofitRepository = this.retrofitObject.create( RFCalls.class );     this.getPosts = this.retrofitRepository.findPosts(); }  public Call&lt;List&lt;RFVariables&gt;&gt; getGetPosts() {     return getPosts; }   I have done this successfully before, so not sure what's mising this time around. Seems like an exact replica of my other successful projects. What am I missing? Let me know if there is more code I should show. Thanks in advance. Really stuck here.
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?
Multiple logistic regression power analysis I have a logistic regression model and output an $R^2$ value. I then go and add another predictor variable to fit a second model. I can output a new $R^2$ value associated with the second model. When I run an ANOVA test, I see no significant improvement in the second model, but I want to assess the power associated with including the additional variable in model 2.  I have found  for linear regression that uses an $F$-Test. I want to do something similar for a logistic regression using G*Power.   But there appears to be very little documentation on multiple logistic regression models like my situation. .  From what I understand, in G*Power I set Test Family == z tests and statistical test == logisitic regression. But I am not sure what to set R² other X equal to. Is that the improvement in $R^2$?  Reading the tutorial in 27.4 from the  makes no variation of $R^2$, whereas , does not discuss the improvements made from $R^2$. I want to install a portable Tex Live in windows 10. But after searching online I just get this from .  The -portable installer option (or V command in the text installer or corresponding GUI option) creates a completely self-contained TEX Live installation under a common root and forgoes system integration. You can create such an installation directly on a USB stick, or copy it to a USB stick afterwards. To run TEX using this portable installation, you need to add the appropriate binary directory to the search path during your terminal session, as usual. On Windows, you can double-click tl-tray-menu at the root of the installation and create a temporary ‘tray menu’ offering a choice of a few common tasks, as shown in this . The ‘More…’ entry explains how you can customize this menu.  Any more instructions in detail? What should I do? Is there a C++ decompiler? Is there a way to display a player name in /tellraw command? like the title says, I'm looking for a way to display a player name in a /tellraw command which is run by a command block. Example:  /tellraw @a {"text":"","extra":[{"text":"A player has won the game!","color":"blue"}]}   And the output is something like this:  A player has won the game!   But I want the output to be something like  JasonSPQR has won the game!   Using the @p parameter How would you evaluate the following limit:   $$\lim_{x\to \infty}\sqrt[6]{x^{6}+x^{5}}-\sqrt[6]{x^{6}-x^{5}}$$   I tried to use this formula: $a^{3}-b^{3}=(a-b)(a^{2}+ab+b^{2})$,  It didn't work.  Any hints? Determining user's reputation as of particular date? I would like to programmatically - or with the API determine what a user's reputation is on a particular day. As such, I have found this link:   However, it does not outline a programmatic method to do so. Solve $p_4(x) = x^4 −(2m + 4)x^2 + (m−2)^2 $such that $p_4$ is a product of two non-constant integer-coeficient polynomials I'm having trouble getting the starting idea for a problem I've been presented with:  I need to find values for m (integer) such that the following polynomial     $p_4(x) = x^4 −(2m + 4)x^2 + (m−2)^2$; $m$ is integer   can be described as the product of two non-constant integer-coeficient polynomials.  Any help/pointers/tips on how to go about this are greatly appreciated!  Edit: I meant polynomials with integer coeficients, but for some reason typed positive. Prove that every number ending in a $3$ has a multiple which consists only of ones. Prove that every number ending in a $3$ has a multiple which consists only of ones.      Eg. $3$ has $111$, $13$ has $111111$.   Also, is their any direct way (without repetitive multiplication and checking) of obtaining such  multiple of any given number( ending with $3$) ? Back in Europe I have a N64 console. As I am currently living in Japan, I find myself tempted to buy some old N64 games in Japanese. However, the articles about the N64's region locking usually assume Japanese games on an American console...  My question thus is: Can you play a Japanese N64 Game on a European console? What harm can be done with a copy of one's passport? In one of the questions on this site, it was suggested the use of  to save a backup copy of your passport, in case you need to proof who you are if your original document is lost.   I like the idea, but what if someone gets access to your Dropbox folders and is able to acquire the copy of your passport.  The same goes for when you leave a copy of your passport at a hotel receptions desk. Once someone has the info printed on that copy, would that make it easier to steal your identity? Let $A\in M_{n\times n}(\Bbb R)$ so that $I\notin span(A,A^2,...,A^n)$. Prove that $\det(A)=0$.   I was thinking of showing $A$ is not invertible, meaning it has an eigenvalue of $0$. Since no matter the power you give to $A$, it is still not the identity, you can deduce that at least one eigenvalue is indeed $0$. However this doesn't work if you have different eigenvalues in the Matrix, so I got stuck.  Help will be most appreciated. I used to be able to color code each folder in Finder.   I know I can use tags in Mavericks, but is there a way to restore the ability to actually color code folders?  Thanks. How does printf with multiple arguments separated by space work I have the following printf function:  $ printf '%s %s %s\t%s\n' 100644 blob 8e1e f1.txt 100644 blob 9de7 f2.txt | git mktree   Can anyone please explain what it produces and why? I expected to have equal number of formatting options %s for each argument string but I have a lot more arguments strings here. I want to georeference jpg map without any digitizing data Is there a way to take a screenshot of an Android device and save it as an image file? Finding $\lim_{n\to\infty}\binom{2n}{n}$ I found a limit problem which is $\lim_{n\to\infty}{2n \choose n}$ as we know that $\sum_{i=0}^{n}{n \choose i}^2={2n \choose n}$ now we could transform the given problem as $\lim_{n\to\infty} \sum_{i=0}^{n}{n \choose i}^2$ and as $\sum_{i=0}^{n}{n \choose i}^2$ is a polynomial in $n$, therefore, $\lim_{n\to\infty}\sum_{i=0}^{n}{n \choose i}^2=\infty$ Please tell me if my solution is correct or not,and if it is incorrect then please provide me a solution.  Thanks For $G$ group and $H$ subgroup of finite index, prove that $N \subset H$ normal subgroup of $G$ of finite index exists Story about a future with races How will multiple GPUs improve Cycles' rendering performance? I have an NVIDIA Titan GPU and I'm able to add another card, but I wonder whether  Blender will benefit from more than one GPUs?  Should I do it as SLI or should I just do it as an added card? I just need some help in understanding how Blender will use either SLI or a second GPU.  Note that I only use Cycles. example of not surjective isometry on a bounded not closed set. Is there an example of a bounded not closed set $A\subset \mathbb R^n$ for some $n\in \mathbb N$ and an isometry $f:A\rightarrow A$ ($\forall x,y\in A\space;\space d(x,y)=d(f(x),f(y))$ where $d$ is the euclidean metric) such that $f$ isn't surjective?