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Java Forum / General / April 2006

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simple java help

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laclac01@gmail.com - 27 Apr 2006 14:45 GMT
I have attached code and was wondering if someone can help me though
it.  I am just trying to make a simple database type program so i can
learn java.  I know C++ well so i think i still need to get out of the
c++ mindset.
I get this error in eclipse "Exception in thread "main"
java.lang.NullPointerException
    at allLists.AddName(allLists.java:12)
    at server.main(server.java:20)"
I havent a clue what it means  But i get the error when doing the
command
list[len].name = name;
in my addName method.
Why can't just do this and it work?

public static void main(String[] args)
    {

        ServerSocket s = null;
        allLists list = new allLists();
        list.AddName("jack","was","here");
        try
        {
           s = new ServerSocket(SERVERPORT);
        }
        catch(IOException e)
        {
           System.out.println(e);
           System.exit(1);
        }

        //while (true)
        //{
           Socket incoming = null;
           try
           {
               incoming = s.accept();
           }
           catch(IOException e)
           {
               System.out.println(e);
         //      continue;
           }

           new inSocket(incoming, list).start();

        //}

    }

}

public class allLists
{
    ResourceList[] list = new ResourceList[1000];
    int len=0;

    void AddName(String name, String IP, String reliability)
    {
        System.out.println(name);
        System.out.println(len);
        list[len].name = name;
        len++;

    }
    void PrintName()
    {
        System.out.println(list[0].name);
    }

public class ResourceList
{
    public String name;
    public String IP;
    public String Reliability;
   
}
Josef Kieser - 27 Apr 2006 15:13 GMT
Hello,

> I get this error in eclipse "Exception in thread "main"
> java.lang.NullPointerException
[quoted text clipped - 4 lines]
> list[len].name = name;
> in my addName method.

You try to do something on an NULL-object.

This:

>     ResourceList[] list = new ResourceList[1000];

allocates memory for an array of ResourceList instances, but it does not
create those instances.
You need do something like this:

list[len] = new ResourceList();
list[len].name = name;
...

HTH,
Josef
Mark Thomas - 27 Apr 2006 16:57 GMT
>> I get this error in eclipse "Exception in thread "main"
>> java.lang.NullPointerException
[quoted text clipped - 21 lines]
> HTH,
> Josef

And it would make it easier for java-types to understand your code if
you remembered that class names start with capital letters & variable
names start with lowercase letters.
Thanks

Mark
Trung Chinh Nguyen - 27 Apr 2006 15:22 GMT
You forgot to call the constructor list[len] = new ResourceList();

> I have attached code and was wondering if someone can help me though
> it.  I am just trying to make a simple database type program so i can
[quoted text clipped - 72 lines]
>    
> }


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