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Thread: Hey! can anyone help me . i cant find an error (beginner)

  1. #1
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    Default Hey! can anyone help me with array of object . i cant find an error (beginner)

    hi, i m receiving error

    Please help me. and do tell me where i was wrong
    Thank you in advance

    Welcome to National Database
    Exception in thread "main" java.lang.NullPointerException
    at general.main(general.java:46)
    import java.util.Scanner;
    public class general {
     
    	String name;
    	String fname;
    	int dob;
    	char sex;
    	public general()
    	{
    		this("no","no",0,'n');
    	}
     
    	public general(String string, String string2, int i, char c) {
    		// TODO Auto-generated constructor stub
    	}
     
    	protected void addgeninfo()
    	{
    		Scanner inp = new Scanner(System.in);
     
    		System.out.println("Please Enter your name:");
    		name=inp.next();
     
    		System.out.println("Please Enter your Father's name:");
    		fname=inp.next();
     
    		System.out.println("Please Enter your dob(format:DDMMYYYY):");
    		dob=inp.nextInt();
     
    		String temp;
    		System.out.println("Please Enter your sex(m/f):");
    		temp=inp.next();
    		sex =temp.charAt(0);
     
    	}
     
    	public static void main(String[] asgc)
    	{
    		int totnationals=10;
    		System.out.println("Welcome to National Database");
    		general nationals[] = new general[totnationals];
     
     
    		for( int i =0; i <10; i++)
    		{
    			nationals[i].addgeninfo();
    		}
     
     
    	}
    }
    Last edited by fredsilvester93; July 18th, 2012 at 07:54 PM.


  2. #2
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    Default Re: Hey! can anyone help me . i cant find an error (beginner)

    A NullPointerException (NPE) means that you are trying to use a variable that has not been initialized yet. The exception will tell you which line of your code is causing the error, and so you'll need to inspect that line, find the offending variable, search in your code and see where you think you might be initializing it but in fact are not.

    Edit: I'd look at that constructor of yours where you think you might be initializing class fields, but in fact are not. You might want to fill in the body of that constructor with a little bit of code.

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    Default Re: Hey! can anyone help me . i cant find an error (beginner)

    i cant find any errr when i remove array it works properly.. bt it is nt working properly whn i create array of object

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    Default Re: Hey! can anyone help me . i cant find an error (beginner)

    In Java (unlike C++) when you "create an array of objects," it actually just creates an array of reference types for that kind of object; it doesn't actually create the objects. (That concept takes a little getting used to. At least it did for me.)

    Anyhow...

    Try something like the following with your original code. (The error has nothing to do with the constructor.)
            general nationals[] = new general[...whatever...];
     
            for( int i = 0; i < nationals.length; i++) //<---It's better not to hard-code the size of the array as some constant.  Use the length member
            {
                nationals[i] = new general();
                // Now you have an object to deal with
                nationals[i].addgeninfo(); // Or whatever...
    .
    .
    .

    Once you get past the problem with null pointer exception, you can go back and flesh out the constructor so that it does what I think you had in mind.



    Cheers!

    Z
    Last edited by Zaphod_b; July 19th, 2012 at 01:47 PM.

  5. The Following User Says Thank You to Zaphod_b For This Useful Post:

    fredsilvester93 (July 19th, 2012)

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