java null pointer exception with static array

Question

I got a null pointer exception when accessing a static array from a static member method. The exception is thrown when i call setData(x, y, z) from a thread. When I debugged it I found out data[0] is null when i try to write to it. I just don't understand how it can be null

public class dataContainer 
{
    private static final short nrOfDataElements = ids.total_ids;
    private static regularDataElement[] data = new regularDataElement[nrOfDataElements];


    public static synchronized void getData(final short i, regularDataElement r)
    {
        if ( (i >= 0) && (i < nrOfDataElements) )
            r.set(data[i].getTimestamp(), data[i].getValue());  
    }          

    public static synchronized void setData(short i, double ts, long val)
    {
        if ( (i >= 0) && (i < nrOfDataElements) )
            data[i].set(ts, val); //<<-------null pointer exception, debugging showed data[i] == null, (with i = 0 and nrOfDataElements = 12)
    }
}

and

public class regularDataElement 
{
    regularDataElement()
    {
        set(0, 0);
    }

    public void set(double _ts, long _val)
    {
        System.out.println(this.ts + " " + _ts + " " + this.val + " " + _val); System.out.flush();
        this.ts = _ts;
        this.val = _val;    
    }

    public double getTimestamp()
    {
        return this.ts;
    }

    public long getValue()
    {
        return this.val;
    }

    private double ts;
    private long val;

}

Answer 1

The statement private static regularDataElement[] data = new regularDataElement[nrOfDataElements]; initializes data with an array the size of nrOfDataElements . It does not initialize each element in this array. Ie, all the elements are null.

If you wish to initialize the elements, you should do so yourself. Eg, you could add a static block to handle this:

static regularDataElement[] data = new regularDataElement[nrOfDataElements];
static {
    for (int i = 0; i < nrOfDataElements; ++i) {
        data[i] = new regularDataElement();
    }
}

Answer 2

Did you ever initialize the data array?

private static regularDataElement[] data = 
    new regularDataElement[nrOfDataElements];

will create an array full of null objects of size nrOfDataElements . It won't actually initialize any elements in the array.

Answer 3

You don't appear to be allocating memory for data[i] , which is why you're getting the NPE.

Allocating memory for the array itself is not enough. You need to allocate memory for each element:

for (int i = 0; i < nrOfDataElements; ++i) {
   data[i] = new regularDataElement(...);
}

(Replace ... with the actual arguments.)

Answer 4

You never actually create any objects. You must add somewhere:

for (int i = 0; i < data.length; i++) {
    data[i] = new regularDataElement();
}

Answer 5

When you initialize an Object array in Java , like data in your code, all elements are by default set to null .

So, you need to populate the data array first, before being able to call any methods against its elements.

Assuming that the regularDataElement class has a no-args (ie, no parameters) constructor, you could do

static regularDataElement[] data = new regularDataElement[nrOfDataElements];

static
{
    for (int i=0; i<nrOfDataElements; i++)
    {
        data[i] = new regularDataElement();
    }
}

Of course, you could have a separate method to initialize the array, eg

static regularDataElement[] initialize(int nrOfDataElements)
{
    regularDataElement[] elements = new regularDataElement[nrOfDataElements];

    for (int i=0; i<nrOfDataElements; i++)
    {
        elements[i] = new regularDataElement();
    }

    return elements;
}

and then call that method to create and initialize the data array, replacing the statement

static regularDataElement[] data = new regularDataElement[nrOfDataElements];

with

static regularDataElement[] data = initialize(nrOfDataElements);

Also, as a matter of following established coding conventions, you should name your classes starting with a capital letter, ie use RegularDataElement instead of regularDataElement .