如何在 Java 中将 List < Double > 转换为 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);

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[]

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 :-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.

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.

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.

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.

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()]));