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Thread: I get the error Exception in thread "main" java.lang.NullPointerException?

  1. #1
    Junior Member
    Join Date
    Sep 2013
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    Default I get the error Exception in thread "main" java.lang.NullPointerException?

    Hi all;

    I'm new to java.

    I have a program that is intented to get a text file (in here,ScanTest.txt) containing some names and values such as:

    #This is a comment

    The line starting with "#" is considered as a comment.

    Then, I have a method called "getString" so that if I give a name to it, it should return corresponding value as following:


    To implement this program, I've used "hashtable" class. Here is my code:

    package io;

    import java.io.*;
    import java.util.*;

    public class HashTableDemo {
    static String fileName;
    static Hashtable hash;
    static Enumeration names;

    public HashTableDemo() {
    fileName = "D:\\Documents and Settings\\Bahareh\\Desktop\\ScanText.txt";
    hash = new Hashtable();
    public static String getString(String st)
    return (String)hash.get(st);

    public static void main(String args[]) throws Exception{
    // Create a hash map
    //Hashtable hash = new Hashtable();
    String st;
    InputStream in = new FileInputStream(fileName);
    Scanner sc = new Scanner(in).useDelimiter("=");
    while (sc.hasNext()) {
    hash.put(sc.next(), sc.next());
    // Show all hashes in hash table.
    names = hash.keys();
    while (names.hasMoreElements()) {
    str = (String) names.nextElement();
    System.out.println(str + ": "+ /*hash.get(str)*/getString(str));
    catch(IOException e)

    When I run it, I get the error:
    Exception in thread "main" java.lang.NullPointerException
    at java.io.FileInputStream.<init>(FileInputStream.jav a:134)
    at java.io.FileInputStream.<init>(FileInputStream.jav a:97)
    at io.HashTableDemo.main(HashTableDemo.java:29)
    Java Result: 1
    BUILD SUCCESSFUL (total time: 2 seconds)

    What's the problem?

    Besides, I want to ignore the lines starting with "#" sign and not to add them to the hash table.

    How should I seperate these lines?
    How does java recognize that first character of a line is "#" sign?

    Please help...


  2. #2
    Super Moderator
    Join Date
    Jun 2013
    So. Maryland, USA
    My Mood
    Thanked 891 Times in 874 Posts

    Default Re: I get the error Exception in thread "main" java.lang.NullPointerException?

    Welcome to the forum! Please read the announcement topic at the top of most sub forums to learn how to properly post code ans ask questions.

  3. #3
    Grand Poobah
    Join Date
    Mar 2011
    My Mood
    Thanked 169 Times in 160 Posts

    Default Re: I get the error Exception in thread "main" java.lang.NullPointerException?

    Error occurs on line 29 of your code. Look at that line and work what is null. Trying printing out stuff on the line above. When you know what is null then work out why it is null. Then fix it!
    Improving the world one idiot at a time!

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