java - I cannot figure out why my collection is null at some points -


i adding dvds collection. , trying sort every time dvd added. keep getting error message: 

exception in thread "main" java.lang.nullpointerexception     @ dvdcollection.adddvd(dvdcollection.java:44)

could guys show me how fix it?

    public void adddvd (string title, string director, int year,   double cost, boolean bluray)    {     if (count == collection.length)     increasesize();     collection[count] = new dvd (title, director, year, cost, bluray);     totalcost += cost;     count++;      if (count > 0)     {         int min;         (int = 0; < collection.length - 1; i++)         {             min = i;             (int j = + 1; j < collection.length; j ++)             {                if(collection[j].getdirector().compareto(collection[i].getdirector()) <   0)                min = j;                 temp[min] = collection[min];                collection[min] = collection[j];                collection[j] = temp[min];             }         }     }   } public class movies  {     //-----------------------------------------------------------------     // creates dvdcollection object , adds dvds it. prints         // reports on status of movies.      //----------------------------------------------------------------- public static void main (string[] args) {     dvdcollection movies = new dvdcollection();     movies.adddvd ("the godfather", "francis ford coppola", 1972, 24.95, true);     movies.adddvd ("district 9", "neill blomkamp", 2009, 19.95, false);     movies.adddvd ("iron man", "jon favreau", 2008, 15.95, false);     movies.adddvd ("all eve", "joseph mankiewicz", 1950, 17.50, false);     movies.adddvd ("the matrix", "andy & lana wachowski", 1999, 19.95, true);     system.out.println (movies);     movies.adddvd ("iron man 2", "jon favreau", 2010, 22.99, false);     movies.adddvd ("casablanca", "michael curtiz", 1942, 19.95, false);     system.out.println (movies);        system.out.println(movies);       }    }

java comes lot of handy libraries. don't have reinvent wheel , sorting hand (except learning).

hence, make dvd implement comparable comparison based on director, , use treeset.

this set keeps elements sorted natural order, using tree structure : more efficient sorting collection @ each insertion.


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