I have an assignment that is supposed to use three stacks to ultimately find palindromes.  Two stacks will need to hold the reverse of the input sequence.  I need to pop the items from one of those stacks and push them into the third stack.  This will obviously create two stacks with reverse items that can be compared to see if the input string is a palindrome.  I have already written a class 'ArrayStack' and 'LinkedStack'.  I am now supposed to write a class that has three ArrayStack instance variables, a constructor that takes no parameters and will create three stacks of type ArrayStack and a method that takes a string parameter and returns a boolean.  The incoming string will be used to check for Palindromes.  The individual characters of the string will need to pushed into one of the stacks.  After that is figured out, I need to do the same thing, but with using the 'LinkedStacked' class.  This is the code I have so far and have tried multiple things and have gotten multiple errors (NPE, etc).  Any pointers / explanations / advice would be appreciated.  I think I have gotten myself really confused.  Thanks in advance for any help offered.  Please note I am a beginner.    public class ArrayPalindrome {      static ArrayStack one;     static ArrayStack two;     static ArrayStack three;      public ArrayPalindrome(){         this.one = one;         this.two = two;         this.three = three;     }      public static boolean isPalindrome (String input ){         String input2 = "";         for (int i = 0; i &lt; input.length(); i++) {             char character = input.charAt(i);             one.push(character);         }         while (!one.isEmpty()) {             // add the character at the top to a string                input2 = input2 + one.pop();         }         return true;     }      public static void main(String[] args) {         if (one.equals(two)){             System.out.println("Palindrome");         }         else {             System.out.println("Not a Palindrome");         }     }
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?
How to change default window background color in Windows 10? How was this answer posted after this question was closed? Is there any point in taking the alternate exits in Bit Trip Runner 2?  I can't see any benefit to doing so in terms of unlocking a different path in the overworld, so is it just for your own benefit? How to install TP-LINK T2UH Wireless adapter Driver (Ralink mt7610u) How does the Ready action work? So from reading the Player's Handbook, it seems that the Ready action is taken instead of any other action as it is an action in its own right. However, can you move on the same turn that you ready an action? For instance, (taking the example from the Player's Handbook):     If the cultist steps on the trapdoor, I’ll pull the lever that opens it   Presumably, when you ready this action, you would want to move near to the lever so you can pull it when the time is right. Likewise, with readying a spell, you would obviously want to move away from combat so you could concentrate on the spell.  Secondly, when the trigger goes off do you have to wait for your turn, or do you in effect get a bonus turn? For instance, the cultist starts running towards you, but on his way he steps on the trap door - so you immediately pull the lever, even though it wasn't your turn. I assume you then act as normal when your actual turn comes around? If $A\subseteq\mathbb N$ and $\sum\limits_{a\in A}\frac1a$ converges then $A$ has natural density $0$ In this  to a question about a series, a theorem was stated:     If $A= \{a_i \}$ is a set such that $\sum_{i = 1}^{\infty} \frac{1}{a_i}$ converges, then $d(A) = 0$, where $d(A)$ is the  of the set.   My background in number theory is basically zero and all my attempts to prove this have been utterly unseccessful; would anyone please help me, or at least provide me with a hint to prove it? I'm trying to show that  $$\delta\big(f(x)\big) = \sum_{i}\frac{\delta(x-a_{i})}{\left|{\frac{df}{dx}(a_{i})}\right|}$$  Where $a_{i}$ are the roots of the function $f(x)$. I've tried to proceed by using a dummy function $g(x)$ and carrying out:  $$\int_{-\infty}^{\infty}dx\,\delta\big(f(x)\big)g(x)$$  Then making the coordinate substitution $u$ = $f(x)$ and integrating over $u$. This seems to be on the right track, but I'm unsure where the absolute value comes in in the denominator, and also why it becomes a sum.  $$\int_{-\infty}^{\infty}\frac{du}{\frac{df}{dx}}\delta(u)g\big(f^{-1}(u)\big) = \frac{g\big(f^{-1}(0)\big)}{\frac{df}{dx}\big(f^{-1}(0)\big)}$$  Can any one shed some light? Wikipedia just states the formula and doesn't actually show where it comes from. Which parts are from Eminems real life? In "" we saw how Bunny Rabbit got into the rapping gang and won the heart of millions of rap lovers. Now in Eminem's original life too similar thing happened. We know how Dr. Dre helped him getting into an industry which is dominated by mainly black people in USA. In fact,  called the film semi-autobiographical. So I want to know which parts are direct picturisations of his real life? Is there any known relationship between $\sum_{i=1}^{n} f(i)$ and $\sum_{i=1}^{n} \dfrac {1}{f(i)}$ If we knew $\sum_{i=1}^{n} f(i)=S$  ($n$ can be $\infty$), is there anything we could say about $\sum_{i=1}^{n} \dfrac {1}{f(i)}$ in terms of $S$?  The only thing I was able to find on the web is . If there is no known relationship, is it possible for there to exist one that we just haven't discovered? Can't login with Stack Exchange OpenID when HTTPS-Everywhere plugin is active - apparent protocol mismatch Cookie path and its accessibility to subfolder pages This is an example from Munkres's Topology (Example 4 in Section 22 titled &quot;The Quotient Topology&quot;, 2nd edition).  Example 4: Let $X$ be the closed unit ball $$\{ x \times y \mid x^2 + y^2 \le 1\}$$ in $\mathbb{R}^2$, and let $X^{\ast}$ be the partition of $X$ consisting of all the one-point sets $\{ x \times y \}$ for which $x^2 + y^2 &lt; 1$, along with the set $S^1= \{ x \times y \mid x^2 + y^2 = 1 \}$. Typical saturated open sets in $X$ are pictured by the shaded regions in the figure below. One can show that $X^{\ast}$ is homeomorphic with the subspace of $\mathbb{R}^3$ called the unit 2-sphere, defined by $$S^2 = \{ x \times y \times z \mid x^2 + y^2 + z^2 =1 \}.$$  I am confused about two points in the example.  Problem 1: About the saturated open sets in $X$. In the example, the typical ones are pictured by the shaded regions ($U, V$) in the figure. However, I am not sure whether the boundaries (visually, by picture) of $U$ and $V$ are included. Particularly, there are two boundaries for $U$. Are they both contained in $U$? And why? Problem 2: How to show that $X^{\ast}$ is homeomorphic with $S^2$? What is the mapping between them?  The following is my understanding:  Solution to problem 1: For $V$, the boundary is not included. For $U$, the outer boundary is included, while the inner one not. Is this solution right? Why are so many forces explainable using inverse squares when space is three dimensional? Short sci-fi story of plan devised by marooned spaceship pilot to save paradise world from development Spaceship crashes on beautiful, remote planet. The pilot survives and lives quite well on the beautiful "island paradise" world amongst friendly, good-natured people... but, one day (while relaxing in a hammock near a beach) he "realized he was dying" (actually, that he would eventually die, then being unable to help the people he had come to love). He fears future "discovery" of his adopted world and the inevitable plundering of the resources and people by "developers"... so he devises... "a plan" "convinced island elders to support his plan" "his students taught others" "achieved 80% literacy... a remarkable feat" "established world government" "more generous than developer expected" "successfully argued for independent status" "established 100% taxation rate" "finished resort buildings converted to school" "developer hired to build resort in isolated area"  ...'not having luck finding it... my recollection of the story is good, but apparently not EXACT/searchable phrases Consider the following figure from Faraway's Linear Models with R (2005, p. 59).    The first plot seems to indicate that the residuals and the fitted values are uncorrelated, as they should be in a homoscedastic linear model with normally distributed errors. Therefore, the second and third plots, which seem to indicate dependency between the residuals and the fitted values, suggest a different model.  But why does the second plot suggest, as Faraway notes, a heteroscedastic linear model, while the third plot suggest a non-linear model?  The second plot seems to indicate that the absolute value of the residuals is strongly positively correlated with the fitted values, whereas no such trend is evident in the third plot. So if it were the case that, theoretically speaking, in a heteroscedastic linear model with normally distributed errors  $$ \mbox{Cor}\left(\mathbf{e},\hat{\mathbf{y}}\right) = \left[\begin{array}{ccc}1 &amp; \cdots &amp; 1 \\ \vdots &amp; \ddots &amp; \vdots \\ 1 &amp; \cdots &amp; 1\end{array}\right] $$  (where the expression on the left is the variance-covariance matrix between the residuals and the fitted values) this would explain why the second and third plots agree with Faraway's interpretations.  But is this the case? If not, how else can Faraway's interpretations of the second and third plots be justified? Also, why does the third plot necessarily indicate non-linearity? Isn't it possible that it is linear, but that the errors are either not normally distributed, or else that they are normally distributed, but do not center around zero? Closing jFrame after clicking JButton Using some matlab (I know it's cheating) I found that: $$k=0 =&gt; Result=1$$ $$k=1 =&gt; Result=1$$ $$k=2 =&gt; Result=2$$ $$k=3 =&gt; Result=5$$ $$k=4 =&gt; Result=15$$ $$k=5 =&gt; Result=52$$ $$k=6 =&gt; Result=203$$  This sequence is puzzling... How can I do a brute force (ciphertext only) attack on an CBC-encrypted message? Given a CBC ciphertext and IV, how can I find the encryption key?  We are limited with an 8 chars key, each char in the range of [a..h], so I can generate every possible key (these are only $8^8 = 2^{24}$ (about 4 million) different possible keys).  How would I go about finding the correct one though? How to integrate $\int_0^{\pi/2}\frac{\sin^nx}{\sin^nx+\cos^nx}dx$? Possible Duplicate:          How can we integrate $$\int_0^\frac{\pi}2\frac{\sin^nx}{\sin^nx+\cos^nx}\,\mathrm dx , \,\,\,\,\,\,\,\,\, n\in N \quad?$$ Thanks for any hint. Why is setTimeout(fn, 0) sometimes useful?