How Do I Fix Nullpinterexception ?

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Java

exceptions

I am trying to get string input in a variable answer, convert it to int and then find corresponding value in arraylist. But i am getting NullPointerException for line answer=keyboard.nextLine();

public void printCustomer(){
     int iSize=customerList.size();
     boolean loopControl=true;
     String answer="";
     while(loopControl){
         int loop;
         Customer cusTemp;
         System.out.printf("Which Customer is Ordering ?\n");
         for(loop=0;loop<iSize;loop++){
             cusTemp=customerList.get(loop);
             System.out.printf("% 2d. %-20s \n",loop+1,cusTemp.getName());
             
         }
         answer=keyboard.nextLine();
         int index=Integer.parseInt(answer)-1;
         if(index<iSize){
             tempCustomer=customerList.get(index);
             loopControl=false; 
         }
         else{
         System.out.printf("\n Sorry, wrong option.");
         }
     }    
     
   }