Java convert Arraylist to float[]

问题:

How I can do that?

I have an arraylist, with float elements. (Arraylist <Float>)

(float[]) Floats_arraylist.toArray()

it is not working.

cannot cast from Object[] to float[]

回答1:

Loop over it yourself.

List<Float> floatList = getItSomehow();
float[] floatArray = new float[floatList.size()];
int i = 0;

for (Float f : floatList) {
    floatArray[i++] = (f != null ? f : Float.NaN); // Or whatever default you want.
}

The nullcheck is mandatory to avoid NullPointerException because a Float (an object) can be null while a float (a primitive) cannot be null at all.

In case you're on Java 8 already and it's no problem to end up with double[] instead of float[], consider Stream#mapToDouble() (no there's no such method as mapToFloat()).

List<Float> floatList = getItSomehow();
double[] doubleArray = floatList.stream()
    .mapToDouble(f -> f != null ? f : Float.NaN) // Or whatever default you want.
    .toArray();

回答2:

You can use Apache Commons ArrayUtils.toPrimitive():

List<Float> list = new ArrayList<Float>();
float[] floatArray = ArrayUtils.toPrimitive(list.toArray(new Float[0]), 0.0F);

回答3:

Apache Commons Lang to the rescue.