Filling a Java HashMap from a database table

I have a table that has two columns, one is string and other is double type. for example,

+========+=============+
| Center | Efficiency  |
+========+=============+
| A1     | 0.603576904 |
+--------+-------------+
| A2     | 0.557877389 |
+--------+-------------+
| A3     |  0.70911818 |
+--------+-------------+
| A4     | 0.048944132 |
+--------+-------------+
| A1     | 0.064781491 |
+--------+-------------+
| A2     | 0.192317935 |
+--------+-------------+
| A3     | 0.316078683 |
+--------+-------------+
| A4     | 0.121362541 |
+--------+-------------+
| A1     | 0.815996499 |
+--------+-------------+
| A2     | 0.032305255 |
+--------+-------------+
| A3     | 0.750355015 |
+--------+-------------+
| A4     | 0.071286058 |
+--------+-------------+

I'm trying to read all the values and load them into a Java HashMap<String, Double[]>

I don't have any problem with reading table data but can not place them into the HashMap as I wanted.

I wrote a sample code to populate a HashMap from two different arrays of string and double but still confused to work with the value part of the Hashmap which is an array.

  String[] names = { "8A", "8C", "8J", "8A", "8C", "8J", "8A", "8C", "8J", "8A", "8C", "8J", };

  Double[] metrics = { 0.60, 0.55, 0.70, 0.04, 0.06, 0.19, 0.31, 0.12, 0.81, 0.03, 0.75, 0.07 };

  for (int i = 0; i < 12; i++)
  {
     if (values.get(names[i]) == null)
     {
        values.put(names[i], new ArrayList<Double>(Arrays.asList(metrics[i])));
     }
     else
        values.get(names[i]).add(metrics[i]);
  }

  System.out.println(values);

Any suggestion how can I handle with the value part of the HashMap which is an array and the size will depend on the table row numbers.

Solution

Nowadays Map has some convenience methods and you can use computeIfAbsent:

Map<String, List<Double>> values = new HashMap<>();
        for (int i = 0; i < 12; i++) {
            List<Double> items = values.computeIfAbsent(names[i], k -> new ArrayList<>());
            items.add(metrics[i]);
        }

There's a more elaborate post about that here.

But if you want to have an Map<String, double[]> then that's very inconvenient during loading, since arrays have fixed size and you'd need to copy them over and over. You could arrive at double[] using the list version for loading and eventually convert it

Map<String, double[]> dvalues = new HashMap<>();
values.forEach((k, v) -> dvalues.put(k, v.stream().mapToDouble(i -> i).toArray()));

Or if it should be a Double[]

Map<String, Double[]> dvalues = new HashMap<>();
values.forEach((k, v) -> dvalues.put(k, v.toArray(new Double[v.size()])));

The only alternative I see is to iterate over the raw data twice, first calculating the number of values for each key, create a map from that with properly sized arrays, iterate again over the raw data keeping track of the maxIdx in each value array as they fill up ... very complicated.