Copy some elements from an array of double to an array list of double

You need to do a few steps to achieve this

// Construct a temporary list.
List<Double> ell = new ArrayList<Double>();
// iterate our input array 'a'.
for (double d : a) {
  // test if the value 'd' is greater then the threshold 'x'.
  if (d > x) {
    // if it is, add it to the temporary list.
    ell.add(d);
  }
}
// construct an array of double(s).
double[] r = new double[ell.size()];
for (int i = 0; i < r.length; i++) {
  // Add the element(s) to the output array.
  r[i] = ell.get(i);
}
return r; // Return the Array.

public static Double[] threshold(double[] a, double x){
    List<Double> l = new ArrayList<Double>(a.length);
    for( double d : a)
        if( d > x )
            l.add(d);

    return l.toArray(new Double[0]);
}

A couple things:

1. Generics (<>) only except classes not primitives so you need to use Double not double.
2. You need a for loop to iterate through each item in a
3. You can use toArray() to convert to type Double[] (assuming you can change your return type). Otherwise you need to do what @Elliott suggests, and create a new double[] with a for-loop.

I would do it this way

public static double[] threshold(double[] a, double x) {
    int n = 0;
    for (double e : a) {
        if (e > x) {
            n++;
        }
    }
    double[] a2 = new double[n];
    n = 0;
    for (double e : a) {
        if (e > x) {
            a2[n++] = e;
        }
    }
    return a2;
}