Error Reading Data into Arrays in Java

Question

I created a method that puts the read numbers into a NumberArray and in total 2 NumberArrays are created per input file. I have created an array of the object NumberRow on the line I marked with "!!!!". I put the read double into the array. However, when I read those arrays, numberRow[0] is not correct; all the values belonging in numberRow[1] are in there, and the values in numberRow[1] are correct. There is probably a simple solution, but I really don't see what is going wrong here.

Unit[] unitArray = new Unit[dataset.numberOfRecords];
    double[] emptyDoubleArray = new double[dataset.numberOfRecords];
    for(int x = 0; x<dataset.numberOfVariables; x++){
        numberRow[x] = new NumberRow(emptyDoubleArray);
    }
    for(int i = 0; i<dataset.numberOfRecords; i++){
        String label = in.next();
        double[] elementsPerUnit = new double[dataset.numberOfVariables];
        for(int k = 0; k<dataset.numberOfVariables; k++){
            double misc = in.nextDouble();
        !!!!!   numberRow[k].NumberArray[i] = misc;
            elementsPerUnit[k] = misc;
        }
        unit = new Unit(label, elementsPerUnit);
        unitArray[i] = unit;
    }
    unitRow = new UnitRow(unitArray);
    out.print(Arrays.toString(numberRow[0].NumberArray));
}

The above code is virtually impossible to follow. You've left out far too much context (and also left out all the comments). — Hot Licks
– Hot Licks, Commented Jun 5, 2013 at 1:31

Patashu · Accepted Answer · 2013-06-05 01:28:42Z

1

Arrays are objects in Java. That is, they are not copied and passed by value (like int, etc), they are passed by reference (like Object, String...)

If you create an array with new and pass it to two objects, there is still only one array (you only used new once, think about it this way). When one object edits the array, the single copy of the array, having been edited, 's edits are seen by the other object.

The solution is, create a new array if it should be distinct from all other arrays.

EDIT: You create one array here (note the new)

double[] emptyDoubleArray = new double[dataset.numberOfRecords];

this one array is passed to all NumberRows (note, no new)

numberRow[x] = new NumberRow(emptyDoubleArray);

therefore if I edit any NumberRow's array it is seen in all NumberRows.

edited Jun 5, 2013 at 1:28

answered Jun 5, 2013 at 1:21

Patashu

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3 Comments

tortilla Over a year ago

But am I not doing this here? for(int x = 0; x<dataset.numberOfVariables; x++){ numberRow[x] = new NumberRow(emptyDoubleArray); }

Hot Licks Over a year ago

That isn't creating an array, it's creating a NumberRow object.

tortilla Over a year ago

Oh wait, I see, thank you very much ! I put the empty double array in the for loop, creating two seperate ones.

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