ArrayList add() modifies 1st entry, adds null entries

Question

Debugged my code and all variables show up as expected:

class1 {

    callingMethod() {
        Double[] entry = new Double[size];

        while(condition) {
            for (i over entry) {
                entry[i] = Double.parseDouble(token.nextToken());
            }

            DataSet.addInput(entry); 
        }
    }
}


DataSet {
    private ArrayList<Double[]> inputsList;

    public DataSet (...) {

       list = new <Double[]>ArrayList();
    }

    public void addInput(Double[] anInputVector) {
        inputsList.add(anInputVector);
    }

}

'theEntry' is properly populated array of Doubles with each new entry. When each new entry is added to 'list', the first list entry is modified with new double values, and a null entry is added. result is 1st entry is an array of doubles (last added), followed by a long list of nulls. Any ideas?

Answer 1

It seems a case of sharing an array object. An example:

List<double[]> list = new ArrayList<>();

double[] a = new double[] { 1.0, 2.0 };
double[] b = new double[] { 3.0, 4.0 };

// Three new's: three objects.

list.add(a);
list.add(a);
list.add(b);

a[0] = 5.0;
list.get(0)[1] = 6.0;

// list: [ [5.0, 6.0]==a, [5.0, 6.0]==a [3.0, 4.0]==b ]

One solution:

public void addEntry(Double[] theEntry) {
    list.add(Arrays.copyOf(theEntry, theEntry.length));
}