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

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?

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.

Guava has a method to do this for you: double[] Doubles.toArray(Collection<Double>)

This isn't necessarily going to be any faster than just looping through the Collection and adding each Double object to the array, but it's a lot less for you to write.

You can convert to a Double[] by calling frameList.toArray(new Double[frameList.size()]), but you'll need to iterate the list/array to convert to double[]

You can use the ArrayUtils class from commons-lang to obtain a double[] from a Double[].

Double[] ds = frameList.toArray(new Double[frameList.size()]);
...
double[] d = ArrayUtils.toPrimitive(ds);

As per your question,

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

1. First you have to convert List<Double> to Double[] by using

   Double[] array = frameList.toArray(new Double[frameList.size()]);
   

2. Next you can convert Double[] to double[] using

   double[] doubleArray = ArrayUtils.toPrimitive(array);
   

You can directly use it in one line:

double[] array = ArrayUtils.toPrimitive(frameList.toArray(new Double[frameList.size()]));

You can use primitive collections from Eclipse Collections and avoid boxing altogether.

DoubleList frameList = DoubleLists.mutable.empty();
double[] arr = frameList.toArray();

If you can't or don't want to initialize a DoubleList:

List<Double> frames = new ArrayList<>();
double[] arr = ListAdapter.adapt(frames).asLazy().collectDouble(each -> each).toArray();

Note: I am a contributor to Eclipse Collections.