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Java Forum / General / September 2007An easy one but where am I going wrong
Dear group This is driving me nuts - I thought I had it working and then not. I have 2 classes - "tester" and "support" support has two instance variables: private int[] x private int[] y and a constructor something like this: public support(int[] c, int[] d) { for (int i=0; i<x.length; i++) { x[i] = c[i]; } for (int i=0; i<y.length; i++) { y[i] = d[i]; } } and support compiles fine. Trouble is, when I come to pass the arguments to support from tester I get a NullPointerException E.g. in tester I have int[] j = {1,2,3,4,5,6}; int[] k = {0,1,2,3,4,5}; support fred = new support(j,k) and it's the creation of the fred object that's giving me the runtime error. This must be a simple thing but I can't spot it anywhere. Sun ... >This is driving me nuts - I thought I had it working and then not. > >I have 2 classes - "tester" and "support" ... I am unable to spot the error in your snippets, but then, I generally will not spend time looking at code snippets. Usually, an SSCCE will describe the problem much more clearly. <http://www.physci.org/codes/sscce.html>  SignatureAndrew Thompson http://www.athompson.info/andrew/ > and a constructor something like this: > [quoted text clipped - 22 lines] > and it's the creation of the fred object that's giving me the runtime > error. This must be a simple thing but I can't spot it anywhere. Where does the NullPointException occur? Is it somewhere inside the constructor or is it the constructor call itself? I would try using the length of your input arrays as the limit in your for loops. I'm not positive without trying it, but this would avoid the possibility of trying to assign to an element within x that doesn't exist. Since you didn't show how you initialize x and y this might be a problem. So: for (int i=0; i<c.length; i++) for the first one and for (int i=0; i<y.length; i++) for the second one might help. -marty > > and a constructor something like this: > [quoted text clipped - 40 lines] > > - Show quoted text - Thanks - but I've found the error now - needed to create a new instance of the variables within the constructor - then worked fine. >>> public support(int[] c, int[] d) >>> { [quoted text clipped - 7 lines] >>> } >>> } > Thanks - but I've found the error now - needed to create a new > instance of the variables within the constructor - then worked fine. Another approach that might help: <http://java.sun.com/javase/6/docs/api/java/util/Arrays.html#copyOf(int[],%20int)> import java.util.Arrays; public class Support { /* Consider following the source-code convention: * name classes with an initial upper-case letter. */ private final int [] x; private final int [] y; /** Constructor. * @param c <code>int []</code> source array. * @param d <code>int []</code> source array. * @throws NullPointerException * if <code>c</code> or <code>d</code> is <code>null</code>. */ public Support( int [] c, int [] d ) { x = Arrays.copyOf( c, c.length ); y = Arrays.copyOf( d, d.length ); } // ... the rest of the class } SignatureLew ... > I would try using the length of your input arrays as the limit in your > for loops. I'm not positive without trying it, but this would avoid the > possibility of trying to assign to an element within x that doesn't > exist. ... That's always something to consider when things are going wrong, but it has its own exception, ArrayIndexOutOfBoundsException, and so would not cause a NullPointerException. Patricia ... > private int[] x > private int[] y [quoted text clipped - 12 lines] > } > } "private int[] x" leaves x null. If x is null, referencing x.length will cause a null pointer exception. You need to do something like "x = new int[c.length];" before treating x as though it referenced an array, rather than being null. Patricia >I have 2 classes - "tester" and "support" Classes begin with a capital letter. See http://mindprod.com/jgloss/codingconventions.html Always post your actual code. The devil is in the details. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com sunstormrider@hotmail.com wrote, >> I have 2 classes - "tester" and "support" > Classes begin with a capital letter. See > http://mindprod.com/jgloss/codingconventions.html Now that is useful, direct, and helpful advice. To the OP: Take Roedy's advice for what it is, something that will make you a better programmer. Some might think that he should have couched his comment in sugar - "I hope you don't mind, but in my response I'll recast the class name to an initial upper-case letter in order to conform to widely-accepted Java coding conventions, blah blah." The message would have been identical - here's what should be, here's the link that explains it in detail. The conventions in question, guidelines really, were published by Sun in 1999 and are mostly followed by just about everybody. They should be mostly followed by everybody. > Always post your actual code. The devil is in the details. Roedy is one of those with great wisdom in these matters. SignatureLew Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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