How to cast from List<Double> to double[] in Java?

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

With java-8, you can do it this way.

double[] arr = frameList.stream().mapToDouble(Double::doubleValue).toArray(); //via method reference double[] arr = frameList.stream().mapToDouble(d -> d).toArray(); //identity function, Java unboxes automatically to get the double value 

What it does is :

• get the Stream<Double> from the list
• map each double instance to its primitive value, resulting in a DoubleStream
• call toArray() to get the array.

Answer 2

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)  } 

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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 :-D )

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.