Returning array in Java

Question

When I run this code,

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */


class Posting
{
    String title;
}

public class Test
{
    Posting[] dew()
    {
        Posting[] p = new Posting[100];
        for(int i = 0; i <p.length; i++)
        {
            p[i].title  = "this is " + i;
        }
        return p;
    }

    public static void main(String args[])
    {
        Test t = new Test();
        Posting[] out = t.dew();

        for(int i = 0; i < out.length; i ++)
        {
            System.out.println(out[i].title);
        }
    }
}

I get this error, run:

Exception in thread "main" java.lang.NullPointerException
    at mistAcademic.javaProject.newsBot.core.Test.dew(Test.java:20)
    at mistAcademic.javaProject.newsBot.core.Test.main(Test.java:29)
Java Result: 1
BUILD SUCCESSFUL (total time: 1 second)

Could you have any idea?

Array is empty.. Posting object is not created how can you access its title field — KDjava
– KDjava, Commented Jan 6, 2013 at 9:18

bobbymcr · Accepted Answer · 2011-08-30 05:56:02Z

11

You have to initialize the array element before setting fields on it.

p[i] = new Posting(/* ... */);
// THEN set the fields
p[i].title = /* ... */;

answered Aug 30, 2011 at 5:56

bobbymcr

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Comments

Thilo · Accepted Answer · 2011-08-30 05:55:57Z

9

Posting[] p = new Posting[100];

This will only create the array itself, all entries are set to null.

So you need to create the instances and put them into the array as well.

    for( int i = 0; i <p.length ; i++ )
    {
        p[i] = new Posting();    // <=  create instances
        p[i].title  = "this is " + i ;
    }

answered Aug 30, 2011 at 5:55

Thilo

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Comments

Peter · Accepted Answer · 2011-08-30 05:56:47Z

4

you have to init your postings

Posting[] dew()
    {
        Posting[] p = new Posting[100];

        for( int i = 0; i <p.length ; i++ )
        {
            p[i] = new Posting();
            p[i].title  = "this is " + i ;
        }

        return p ;
    }

answered Aug 30, 2011 at 5:56

Peter

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Comments

Nikhil Mahajan · Accepted Answer · 2013-02-11 11:55:27Z

1

You need to initialize each object of the array. Add following lines before

p[i] = new Posting();
p[i].title  = "this is " + i ; in the for loop.

edited Feb 11, 2013 at 11:55

answered Aug 30, 2011 at 7:27

Nikhil Mahajan

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Comments

Swagatika · Accepted Answer · 2011-08-30 08:57:44Z

0

By doing : Posting[] p = new Posting[100];

will create an array of 100 null objects. p[0], p[1], p[2]..... p[99] = null so when you do :

 p[i].title

its actually same as : null.title and hence you get NullPOinterException.

answered Aug 30, 2011 at 8:57

Swagatika

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