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Thread: Array of strings Null Pointer error

  1. #1
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    Default Array of strings Null Pointer error

    My error reads
    Exception in thread "main" java.lang.NullPointerException
    at Phonebook.main(Phonebook.java:41)

    I also know this error occurs again at lines 45 and 49

    here is my code
    import java.util.Scanner;
    import java.io.File;
    import java.io.FileNotFoundException;
     
    public class Phonebook {
     
    class Entry{
    	public String Name, Number, Notes; 
    }
     
    public static Entry[] ContactList = new Entry[100];
    public static java.io.File PhoneBookEntries = new java.io.File("PhoneBookEntries.txt");
     
    public static void main(String[] args) throws FileNotFoundException{
    	Scanner input = new Scanner(System.in);
    	java.io.File PhoneBookEntries = new java.io.File("PhoneBookEntries.txt");
    //	Entry[]ContactList = new Entry [100];
    	java.io.PrintWriter output = new java.io.PrintWriter(PhoneBookEntries);
    	String tempName;
    	Character Decision;
    	int count;
    	int length;
    ///	for(int i = 0; i < 100; i++  ){//
    		System.out.println("Codes are printed as 1 to 8 characters \n use \"e\" for enter," +
    						   "\"f\" for find, \"l\" to list and \"q\" to quit.");
     
    	tempName = null;	
    	count = 0;
    	while(count < 100){
    		System.out.print("Command: ");
    		tempName = input.nextLine();
    		Decision = tempName.charAt(0);
     
    		switch(Decision){
    		case 'e' :  System.out.print("Enter Number: ");
     
     
    					ContactList[count].Name = input.nextLine();
     
     
    					System.out.print("Notes: ");
    					ContactList[count].Notes = input.nextLine();
     
    					length = tempName.length();
    					tempName = tempName.substring(2, length);
    					ContactList[count].Name = tempName;
     
    					StorePhoneBook(ContactList, count);
    					break;
     
    		case 'E' :	System.out.print("Enter Number: ");
    					ContactList[count].Number = input.nextLine();
    					System.out.print("Notes: ");
    					ContactList[count].Notes = input.nextLine();
     
    					length = tempName.length();
    					tempName = tempName.substring(2, length);
    					ContactList[count].Name = tempName;
     
    					StorePhoneBook(ContactList, count);
    					break;
     
    		case 'f' :  ReadPhoneBook(PhoneBookEntries);		
    					break;
     
    		case 'F' :  ReadPhoneBook(PhoneBookEntries);
    					break;
     
    		case 's' :  SortList(PhoneBookEntries);
    					break;
     
    		case 'S' :  SortList(PhoneBookEntries);
    					break;
     
    		case 'l' :  ListAllContacts(PhoneBookEntries);
    					break;
     
    		case 'L' :	ListAllContacts(PhoneBookEntries);
    					break;
    		}
     
    	count++;
    	}
     
    }
     
    public static void ReadPhoneBook(File PhoneBoookEntries) throws FileNotFoundException{
     
    }
     
     
    public static void StorePhoneBook(Entry ContactList[], int count) throws FileNotFoundException{
    	java.io.PrintWriter output = new java.io.PrintWriter(PhoneBookEntries);
     
    	output.print(ContactList[count]);
    }
     
     
    public static void ListAllContacts(File Phonebook)throws FileNotFoundException{
     
    }
     
    public static void SortList(File PhoneBook)throws FileNotFoundException {
     
    }
     
    }
    Last edited by copeg; April 2nd, 2011 at 08:57 PM.


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Array of strings Null Pointer error

    The first problem is that you've commented out the line declaring ContactList.
    //	Entry[] ContactList = new Entry [100];

    Secondly, in Java arrays are populated with the default value, which in the case of Objects is null. You must create new Entry's and populate your array with them before you start trying to perform operations on them.

    One thing I would strongly suggest is taking the Entry class out of the Phonebook class. It will make declaring an Entry easier to do (mostly syntactically), and allow you to work with the entries without having to deal with creating a phonebook class and operating on that class to access Entry.

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