Thread: java.lang.NullPointerException

Hello guys i've got a little problem

Exception in thread "main" java.lang.NullPointerException
at TestMyUtils.InsSort(TestMyUtils.java:251)
at TestMyUtils.main(TestMyUtils.java:330)
Java Result: 1

line 251

while(position>0 && myBook[position-1].getOnoma().compareToIgnoreCase(key.getOnoma())>0){

line 330

case 2: InsSort(myBook, 'a');

whole code

import java.io.*;
class TestMyUtils{ 
   public static int metritis = 0;
   public static int metritis2 = 0;
   public static long y; 
 
   public static void EisagVivliou(Book[] myBook){      
      if(metritis<10){
        myBook[metritis]=new Book();
        System.out.println("Dose Titlo: ");
        myBook[metritis].setTitlo(UserInput.getString());
        System.out.println("Dose Suggrafea: ");
        myBook[metritis].setOnoma(UserInput.getString());
        System.out.println("Dose ISBN: ");
        System.out.println();
        boolean flag2=false;
        do{ 
          do{   
          System.out.println("***Sosto 10psifio h' 13psifio");
          myBook[metritis].setIsbn(UserInput.getString());
          y= Long.parseLong(myBook[metritis].getIsbn());
          } while( (y<999999999L || y>10000000000L) && (y<999999999999L || y>100000000000000L) ); 
          if(y<100000000000L){
          flag2 = elegxos10(y, flag2);
          } else{
          flag2 = elegxos13(y, flag2);
          }
        } while( flag2 == false);
        System.out.println("Dose Ekdotiko Oiko: ");
        myBook[metritis].setOikos(UserInput.getString());
        System.out.println("Dose Arithmo Selifon: ");
        myBook[metritis].setSelides(UserInput.getInteger());
        System.out.println("Dose Etsos Ekdosis");
        myBook[metritis].setEtos(UserInput.getInteger());
        System.out.println("Dose Timi: ");
        myBook[metritis].setTimi(UserInput.getDouble());
        ++metritis;
      }
      else System.out.println("Gemisame");
    }
   public static void ListaEpilogon(){
      System.out.println();
      System.out.println("Lista Epilogon");
      System.out.println("1. Eisagogi Stoixeion Vivliou");
      System.out.println("2. Anazitisi Vivliou");
      System.out.println("3. Emfanisi Stoixeion Vivliou me timi meta3u kapoion orion");
      System.out.println("4. Taksinomisi Biblion");
      System.out.println("5. Emfanisi stoixeion olon ton vivlion");
      System.out.println("6. Telos");
      System.out.println("Doste Epilogi (1-5): ");
  }
   public static void EpilogiPedio(){
      System.out.println();  
      System.out.println("Epilogi Pedio Anazitisis");
      System.out.println("1. ISBN");
      System.out.println("2. Etos Kukloforias");
      System.out.println("3. Epistrofi stin arxiki lista epilogon");
      System.out.println("Doste epilogi (1-3): ");
    }
   public static void EpilogiMeth(){
      System.out.println();  
      System.out.println("Epilogi Methedou Anazitisi");
      System.out.println("1. Seiriaki Anazitisi");
      System.out.println("2. Duadiki Anazitisi");
      System.out.println("3. Epistrofi Stin Epilogi Pediou Anazitisis");
      System.out.println("Doste Epilogi (1-3)");
    }
   public static void EpilogiTaks1(){
      System.out.println();  
      System.out.println("Epilogi Pediou Taksinomisis");
      System.out.println("1. Onoma Suggrafea");
      System.out.println("2. ISBN");
      System.out.println("3. Etsos Kukloforias");
      System.out.println("4. Timi Vivliou");
      System.out.println("5. Epistrofi Stin Arxiki Lista Epilogon");
      System.out.println("Doste Epilogi (1-5)");
    }
   public static void EpilogiTaks2(){
      System.out.println();  
      System.out.println("Epilogi Methodou");
      System.out.println("1. Bubblesort");
      System.out.println("2. Taksinomisi me eisagogi");
      System.out.println("3. Taksinomisi me epilogi");
      System.out.println("4. Grigori taksinomisi");
      System.out.println("5. Taksinomisi me sugxoneusi");
      System.out.println("6. Epistrofi stin epilogi pediou taksinomisi");
      System.out.println("Doste Epilogi (1-6)");
    }
   public static long seirAnazit(Book[] myBook, String key){       
        for (int i=0; i<=metritis; i++) { 
            if (myBook[i].getIsbn().equals(key)) {
                return i;
            } else return -1;
        }       
        return -1;
    }
   public static int seirAnazit(Book[] myBook, int key){        
        int pos = -20;
        for (int i=0; i<metritis; i++) {            
            if (myBook[i].getEtos()==key) {
                pos = i;
            }  else pos = -1;
        }        
        return pos;
    }
   public static int diadAmazit(Book[] myBook, String key){
       int left = 0;
       int right = metritis-1;
       return diadAnazit2(myBook, key, left, right);
   }
   public static int diadAnazit2(Book[] myBook, String key, int left, int right){
     int mid,pos=-1;
     while(pos==-1 && left<=right){
       mid = (left+right)/2;
       if(myBook[mid].getIsbn().compareTo(key)>0){
         right = mid-1;
       } else if(myBook[mid].getIsbn().compareTo(key)<0){
         left = mid+1;
       } else {
         pos = mid;
       }
      }
      return pos;
   }
   public static int diadAmazit1(Book[] myBook, int key){
       int left = 0;
       int right = metritis-1;
       return diadAnazit2(myBook, key, left, right);
   }
   public static int diadAnazit2(Book[] myBook, int key, int left, int right){
     int mid,pos=-1;
     while(pos==-1 && left<=right){
       mid = (left+right)/2;
       if(myBook[mid].getEtos()>key){
         right = mid-1;
       } else if(myBook[mid].getEtos()<key){
         left = mid+1;
       } else {
         pos = mid;
       }
      }
      return pos;
   }
   public static void emfanise(Book[] myBook){
    for (int i=0; i<metritis; i++)
         System.out.println(myBook[i]);
        }
   public static void valueSearch(int min, int max,Book[] myBook){
      for(int i=0; i<metritis; i++){
       if(myBook[i].getTimi()>=min && myBook[i].getTimi()<=max )
			System.out.println(myBook[i]);
		}
    }    
   public static boolean elegxos10(long x, boolean z){
      long ar10 = (x%10)/(1)*1;
      long ar9 = (x%100)/(10)*2;
      long ar8 = (x%1000)/(100)*3;
      long ar7 = (x%10000)/(1000)*4;
      long ar6 = (x%100000)/(10000)*5;
      long ar5 = (x%1000000)/(100000)*6;
      long ar4 = (x%10000000)/(1000000)*7;
      long ar3 = (x%100000000)/(10000000)*8;
      long ar2 = (x%1000000000)/(100000000)*9;
      long ar1 = x/(1000000000)*10;
      long kfz = ar1+ar2+ar3+ar4+ar5+ar6+ar7+ar8+ar9+ar10;
      if(kfz%11==0) {
            return true;
      }       
             return false;        
        }     
   public static boolean elegxos13(long x, boolean z){
    long ar13 = (x%10)/1*1;    
    long ar12 = (x%100)/10*3;
    long ar11 = (x%1000)/100*1;
    long ar10 = (x%10000)/1000*3;
    long ar9 = (x%100000)/10000*1;
    long ar8 = (x%1000000)/100000*3;
    long ar7 = (x%10000000)/1000000*1;
    long ar6 = (x%100000000)/10000000*3;
    long ar5 = (x%1000000000)/100000000*1;
    long ar4 = (x%10000000000L)/1000000000*3;
    long ar3 = (x%100000000000L)/10000000000L*1;
    long ar2 = (x%1000000000000L)/100000000000L*3;
    long ar1 = x/1000000000000L*1;
    long kfz = ar1+ar2+ar3+ar4+ar5+ar6+ar7+ar8+ar9+ar10+ar11+ar12+ar13;
    System.out.println(kfz);
    if(kfz%10==0) {
      return true;
      }               
    return false;
    } 
   public static void swap (Book myBook[], int x, int y) {
        Book temp = myBook[x];
        myBook[x] = myBook[y];
        myBook[y] = temp;
    }
   public static void BubbleSort(Book[] myBook, char a){
     int i;
     int j;
     boolean flag;
     if(a =='a'){
       for(i = 1; i<metritis; i++){
         flag = true;
         for(j=0; j<metritis-i; j++){
           if(myBook[j].getOnoma().compareToIgnoreCase(myBook[j+1].getOnoma())>0){
           swap(myBook, j, j+1);
           flag = false;
           }
         }
       }
     } else if(a  == 'b'){
       for(i = 1; i<metritis; i++){
         flag = true;
         for(j=0; j<metritis-i; j++){
           if(myBook[j].getIsbn().compareToIgnoreCase(myBook[j+1].getIsbn()) >0){
           swap(myBook, j, j+1);
           flag = false;
           }
         }
       }     
     } else if(a  == 'c'){
       for(i = 1; i<metritis; i++){
         flag = true;
         for(j=0; j<metritis-i; j++){
           if(myBook[j].getEtos()>myBook[j+1].getEtos()){
           swap(myBook, j, j+1);
           flag = false;
           }
         }
       }
 
     } else{
       for(i = 1; i<metritis; i++){
         flag = true;
         for(j=0; j<metritis-i; j++){
           if(myBook[j].getTimi()>myBook[j+1].getEtos()){
           swap(myBook, j, j+1);
           flag = false;
           }
         }
       }     
     }
   }
   public static void InsSort(Book[] myBook, char a){
     int position;
     Book key;
     if(a == 'a'){
       for(int i=1; i<myBook.length; i++){
         key = myBook[i];
         position = i;
         while(position>0 && myBook[position-1].getOnoma().compareToIgnoreCase(key.getOnoma())>0){
           myBook[position] = myBook[position-1];
           position--;
         }
       myBook[position] = key;  
       }
     }
   }
   public static void main(String args[]){
    int epilogi = 0;
    int pos;
    long pos2;
    Book[] myBook = new Book[10];
    do{
      ListaEpilogon();
      epilogi=UserInput.getInteger();
      switch(epilogi){
          case 1: EisagVivliou(myBook);
              break;
          case 2: if(metritis>0){ EpilogiPedio();
          epilogi=UserInput.getInteger();
          switch(epilogi){
              case 1: EpilogiMeth();
              epilogi=UserInput.getInteger();
              switch(epilogi){
                  case 1: 
                      System.out.println("Dose ISBN gia anazitisi");
                      String key = UserInput.getString();
                      pos2 = seirAnazit(myBook, key);
                      System.out.println(pos2);
                      break;
                  case 2:
                      System.out.println("Dose ISBN gia anazitisi");
                      key = UserInput.getString();
                      pos2 = diadAmazit(myBook, key);
                      System.out.println(pos2);
                      break;
                  case 3: break;
              }
                  break;
              case 2: EpilogiMeth();
              epilogi=UserInput.getInteger();
              switch(epilogi){
                  case 1: 
                      System.out.println("Dose Etsos gia anazitisi");
                      int key = UserInput.getInteger();
                      pos = seirAnazit(myBook, key);
                      System.out.println(pos);
                      break;
                  case 2:System.out.println("Dose Etos gia anazitisi");
                      key = UserInput.getInteger();
                      pos = diadAmazit1(myBook, key);
                      System.out.println(pos); 
                      break;
                  case 3: break;    
             }
          } break;
          } else {
            System.out.println("\n ***Adeios pinakas*** \n");
            break;
          }
          case 3: if(metritis>0){
              System.out.println("Dose thn mikroteri 8etiki timi");
              int min = UserInput.getInteger();
              System.out.println("Dose thn megaluteri 8etiki timi");
              int max = UserInput.getInteger();
              valueSearch(min, max, myBook);
              break;} 
                 else {
                 System.out.println("\n ***Adeios pinakas*** \n");
                 break;}
 
          case 4: EpilogiTaks1();
          epilogi = UserInput.getInteger();
          switch(epilogi){
              case 1: EpilogiTaks2();
              epilogi = UserInput.getInteger();
              switch(epilogi){
                  case 1: BubbleSort(myBook, 'a');
                  case 2: InsSort(myBook, 'a');
                  case 3:
                  case 4:
                  case 5:
                  case 6: break;                      
              }
                  break;
              case 2: EpilogiTaks2();
              epilogi = UserInput.getInteger();
              switch(epilogi){
                  case 1: BubbleSort(myBook, 'b'); 
                  case 2: InsSort(myBook, 'b');
                  case 3: 
                  case 4:
                  case 5:
                  case 6: break;
              }
                  break;
              case 3: EpilogiTaks2();
              epilogi = UserInput.getInteger();
              switch(epilogi){
                  case 1: BubbleSort(myBook, 'c');
                  case 2: InsSort(myBook, 'c');
                  case 3:
                  case 4:
                  case 5:
                  case 6: break;
              }
                  break;
              case 4: EpilogiTaks2();
              epilogi = UserInput.getInteger();
              switch(epilogi){
                  case 1: BubbleSort(myBook, 'd');
                  case 2: InsSort(myBook, 'd');
                  case 3:
                  case 4:
                  case 5:
                  case 6: break;
              }
                  break;
              case 5: break;
          }
              break;              
 
          case 5:if(metritis>0){ emfanise(myBook);
          System.out.println();
          break;} else {
          System.out.println("\n ***Adeios pinakas*** \n"); 
          break;
          }
          case 6:System.out.println("\n\t ... TELOS PROGRAMMATOS ...\n");
          break;
          default : System.out.println("\tLathos Epilogi!!!Eisagete ari8mo 1-6\n");
    }
    }while(epilogi!=6);
  }
}

Thanks a lot!!

Try using a println to find out which variable(s) have a null value. Put the println on the line before the line that throws the exception. When you find the variable with a null value, back trace the program and see where it should have been initialized and initialize it.
If you can not locate the problem with the printlns added, post the code with the printlns and your question.