java - Null pointer exception populating a new array -


this question has answer here:

i trying implement vector of generic data type type taken input user such :

package com.example.genericvector;  import java.util.scanner;   public class genericvector <generic_type>{      private int length;     private generic_type [] vector;      genericvector(generic_type element){         this.vector[0] = element;     }       public void allocate_size(int length){      }     public void push_back(generic_type element){      }     public void display(generic_type [] vector){          for(generic_type element : vector)             system.out.print(element);          system.out.println();      }      public static void main(string[] args) {          scanner scanner = new scanner(system.in);          system.out.print("enter data type of vector : ");         string vector_type = scanner.next();          switch (vector_type){             case "integer":                 genericvector <integer> vector_int = new genericvector <integer>(new integer(10));                 break;             case "double":                 genericvector <double> vector_double = new genericvector <double>(new double(10.57));                 break;             default:                 system.out.print("invalid data type");         }     } }

upon running above code following output:

    enter data type of vector : integer exception in thread "main" java.lang.nullpointerexception     @ com.example.genericvector.genericvector.<init>(genericvector.java:12)     @ com.example.genericvector.genericvector.main(genericvector.java:40)     @ sun.reflect.nativemethodaccessorimpl.invoke0(native method)     @ sun.reflect.nativemethodaccessorimpl.invoke(nativemethodaccessorimpl.java:62)     @ sun.reflect.delegatingmethodaccessorimpl.invoke(delegatingmethodaccessorimpl.java:43)     @ java.lang.reflect.method.invoke(method.java:498)     @ com.intellij.rt.execution.application.appmain.main(appmain.java:147)  process finished exit code 1

i pretty new java, appreciated.

private generic_type [] vector;

only declares field (array) of generic type. initially, array null.

something like

private generic_type [] vector = new generic_type[10];

initializes field new vector (but please note individual array slots null @ point, too). but, uups, commentors right ... 1 of deficiencies of generics , arrays, can't use new[] generic types. on other hand, arrays have fixed size in java; have check if have grow/shrink underlying array! working solution be:

private list<t> vector = new arraylist<>();

for example (using existing java collection classes here).

final note: convention, generic types denoted using single upperchase characters. should genericvector<t> instead of generic_type.


Comments

Post a Comment

Popular posts from this blog

iOS: Performance of reloading UIImage(name:...) -

i have tableview , each row has icon. 90% of rows have same icon. in cellforrowatindexpath method load image assets folder this: var cellimage = uiimage(named: "myimage") does pose performance problem? os somehow smart enough it's same image each time? better if loaded image once , saved in class variable , assigned each time? uiimage 's named: api does kind of caching. if @ uiimage's documentation , states: use init(named:in:compatiblewith:) method (or init(named:) method) create image image asset or image file located in app’s main bundle (or other known bundle). because these methods cache image data automatically, recommended images use frequently.
Read more

How to prevent logback from emitting repeated "Empty watch file list. Disabling" messages -

i have logback configured logback.xml located here: http://pastebin.com/kuquekxy . every minute, see log message emitted of form: 18:26:21,486 |-info in reconfigureonchangetask(born:1478715201485) - empty watch file list. disabling why happening , how can rid of these? i can tell why happening, not how rid of them. the reconfigureonchangetask log line when doesn't have files watch. and doesn't have files watch because file in jar, , only watch files on filesystem . makes sense, because files within jar file don't change.
Read more

c# SetCompatibleTextRenderingDefault must be called before the first -

i tried search exception couldn't find solution on case i'am using code below invoke .net application : assembly assem = assembly.load(data); methodinfo method = assem.entrypoint; var o = activator.createinstance(method.declaringtype); method.invoke(o, null); the application invoked has form , in entrypoint of application : [stathread] static void main() { application.enablevisualstyles(); application.setcompatibletextrenderingdefault(false); //exception application.run(new form1()); } setcompatibletextrenderingdefault must called before first iwin32window object created in application. edit : assembly = assembly.load(data); methodinfo method = a.gettype().getmethod("start"); var o = activator.createinstance(method.declaringtype); method.invoke(o, null); you should create new method skips initialization...
Read more