Question

I have a variable like that:

List<Double> frameList =  new ArrayList<Double>();

/* Double elements has added to frameList */

How can I have a new variable has a type of double[] from that variable in Java with high performance?

Answer 1

High performance – every Double object wraps a single double value. If you want to store all these values into a double[] array, then you have to iterate over the collection of Double instances. A O(1) mapping is not possible, this should be the fastest you can get:

 double[] target = new double[doubles.size()];
 for (int i = 0; i < target.length; i++) {
    target[i] = doubles.get(i).doubleValue();  // java 1.4 style
    // or:
    target[i] = doubles.get(i);                // java 1.5+ style (outboxing)
 }

---

Thanks for the additional question in the comments 😉 Here’s the sourcecode of the fitting ArrayUtils#toPrimitive method:

public static double[] toPrimitive(Double[] array) {
  if (array == null) {
    return null;
  } else if (array.length == 0) {
    return EMPTY_DOUBLE_ARRAY;
  }
  final double[] result = new double[array.length];
  for (int i = 0; i < array.length; i++) {
    result[i] = array[i].doubleValue();
  }
  return result;
}

(And trust me, I didn’t use it for my first answer – even though it looks … pretty similiar 😀 )

By the way, the complexity of Marcelos answer is O(2n), because it iterates twice (behind the scenes): first to make a Double[] from the list, then to unwrap the double values.