You do know that you can create arrays of variable size by using a variable as the array length?

 public float[] newFloatArray(int size) {
     float[] array = new float[size];
     return array;
 }

Filling the array can be done with a loop or using the JRE supplied Arrays class helper methods. You will need to handle the first and last index in the array separately:

 public float[] newGolfArray(int size) {
     float[] array = new float[size];
     Arrays.fill(array, 1F);
     array[0] = 4F;
     array[size - 1] = 2F;
     return array;
 }

There is no easy way to handle this in Java. You can:

• Use Wrapper types (Integer[], Float[], Double[]) and have a function taking Number[] as an argument. Works since arrays are covariant in Java:

public static void main(String[] args) {
    f(new Integer[]{1,2,3});
    f(new Float[]{1,2,3});
    f(new Double[]{1,2,3});
}

private static void f(Number[] numbers) {
    f[0].doubleValue();
}

Note that this approach increases memory consumption significantly.

• Convert int[] and float[] arrays to double[] and work with doubles all along. Preferably create overloaded versions of your method where the ones taking int[] and float[] are only doing the conversion and delegate to actual double[] implementation.

• I believe Scala can handle this seamlessly as Java primitive types are semantically objects in Scala.

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();