Java array extension/reduction

I work with the Oracle JGeometry method which returns (or takes as method argument) 2D segments as double array (x1,y1,x2,y2 ... xn,yn). I need to extend this array to contain 3D segments as double array (x1,y1,z1,x2,y2,z2 ... xn,yn,zn) by default Z value or reduce to 2D (by removing of all Z coordinate). I wrote simple utility methods for making this. Is there any easier or smarter way to do this?

The conversion from 2D to 3D:

public static double[] to3D(double z, double[] inputArray) {
List<Double> convertedItems = new ArrayList<>();
for (int i = 0; i < inputArray.length; i++) {
if ((i + 1) % 2 == 0) {
}
}
return convertedItems.stream().mapToDouble(Double::doubleValue).toArray();
}


The conversion from 3D to 2D:

public static double[] to2D(double[] inputArray) {
List<Double> convertedItems = new ArrayList<>();
for (int i = 0; i < inputArray.length; i++) {
if ((i + 1) % 3 == 0) {
continue;
}
}
return convertedItems.stream().mapToDouble(Double::doubleValue).toArray();
}


I would go with primitive Arrays, instead of using Collections and Streams in this case.

public static double[] to3D(double z, double[] in) {
// Pre-allocated array 1.5 size of the original array
double[] out = new double[in.length * 3 / 2];
//loop with a step 2
for (int i = 0, j = 0; i < in.length; i += 2) {
out[j++] = in[i];
out[j++] = in[i + 1];
out[j++] = z;
}
return out;
}

public static double[] to2D(double[] in) {
// Pre-allocated array 2/3 size of the original array
double[] out = new double[in.length * 2 / 3];
//loop with a step 3
for (int i = 0, j = 0; i < in.length; i += 3) {
out[j++] = in[i];
out[j++] = in[i + 1];
}
return out;
}


Reasons for using arrays instead of lists: Input and output both are arrays, the target size is known, we only need basic operations and they would be faster in this case.

I hope it helps.

• Just a small remark: Starting with j = -1 seems a bit ugly. Using post increment (j++) and starting with j = 0 seems "nicer". Mar 8, 2021 at 11:59
• You are right, I just do it to avoid last unused post-increment, If you think it still should have that then you can edit it or tell me if I must, Thanks Mar 8, 2021 at 14:04
• You can include int j = 0 directly inside the loop and you could explain why you are using arrays instead of lists (fixed cardinality of arrays). Mar 8, 2021 at 15:49
• In this case int j = 0 cannot be initialised inside loop because it needs incremented value in each iteration. Mar 9, 2021 at 7:49
• I badly explained it, I meant for (int i = 0, j = 0; i < in.length; i += 3). Mar 9, 2021 at 8:27