Java - Array <init> error

Question

So, I have this class, that contains another class array, and in the constructor I want to make "n" and "nCod" equal 0.

public class ITable
{   
TableRow arr[];

class TableRow
{
    long n;
    int nCod;
    ICode cod;

}

ITable()
{
    arr = new TableRow[256];
    for(int i=0;i<256;i++)
    {
        arr[i].n = 0;
        arr[i].nCod = 0;
    }
}
}

When I run it, Eclipse console tells me:

java.lang.NullPointerException
at jhuffman.def.ITable.<init>(ITable.java:21)

That line is:

arr[i].n = 0;

Eran · Accepted Answer · 2015-03-21 17:06:06Z

3

When you create an array instance with new TableRow[256], each of its elements are initialized to null.

Therefore, each element should be initialized before being accessed :

arr = new TableRow[256];
for(int i=0;i<256;i++)
{
    arr[i] = new TableRow (); // add this
    arr[i].n = 0;
    arr[i].nCod = 0;
}

answered Mar 21, 2015 at 17:06

Eran

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Comments

Anderson Vieira · Accepted Answer · 2015-03-21 17:18:49Z

1

When you create an object array with no initial values, all the positions in the array will point to null. So, for example, arr = new TableRow[3] would initialize the array as [null, null, null].

Since you did not store any TableRow objects into arr, when you access arr[i], it returns null instead of a concrete object. If you try to access a field in null it will result in a NullPointerException as you have observed.

What you need to do is create the TableRow instances and place them into the array before you try to access them. Something like this:

arr = new TableRow[256];
for (int i = 0; i < arr.length; i++) {
    arr[i] = new TableRow();
    arr[i].n = 0;
    arr[i].nCode = 0;
}

edited Mar 21, 2015 at 17:18

answered Mar 21, 2015 at 17:06

Anderson Vieira

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