转换ArrayList <Double> 对于JAVA / Android，双数组，反之亦然

Question

I've been trying to convert a Double ArrayList to a Double Array and vice-versa.

This is the code I have for converting Double ArrayList to Double Array, but the program keeps on crashing. Am I missing something? or is the logic wrong all together?

ArrayList<Double> numsy = new ArrayList<Double>();
ArrayList<Double> numsx = new ArrayList<Double>();  
for (int j = 0; j < 100; j++) {
    numsy.add((double)generator.nextFloat());
    numsx.add((double)j);
}
double[] arrayX = new double[numsx.size()];     
double[] arrayY = new double[numsy.size()];
//assigns array x and y the values in the list.
for (int k1 = 0; k1 < numsy.size(); i++) {
    double f = numsy.get(k1);
    arrayY[k1] = f;
    f = numsx.get(k1);
    arrayX[k1] = f;
}
FFT doFFT = new FFT(4);
doFFT.fft(arrayX, arrayY);
//adding the FFT numbers back to the Lists
for (int j = 0; j < 100; j++) {
    numsy.add(arrayY[j]);
    numsx.add(arrayX[j]);
}

Answer 1

You are instantiating your Arrays before adding values in your ArrayLists :

double[] arrayX = new double[numsx.size()];     
double[] arrayY = new double[numsy.size()];   

This will create arrays with length : 0 . Then you'll have a null pointer in your loop below.

UPDATE

Then you have to edit your loop:

for (int k1 = 0; k1 < numsy.size(); i++) {

to this

for (int k1 = 0; k1 < numsy.size(); k1++) {

Answer 2

ArrayList<Double> numsy = new ArrayList<Double>();
ArrayList<Double> numsx = new ArrayList<Double>();

 for (int j = 0; j < 100; j++) {
     numsy.add((double)generator.nextFloat());
     numsx.add((double)j);
 }

//Declare arrayX and arrayY after adding values to numX and numY

 double[] arrayX = new double[numsx.size()];     
 double[] arrayY = new double[numsy.size()];     
    //assigns array x and y the values in the list.
 for (int k1 = 0; k1 < numsy.size(); k1++) {
   double f = numsy.get(k1);
   arrayY[k1] = f;
   f = numsx.get(k1);
   arrayX[k1] = f;
 }

Answer 3

为什么不尝试使用内置方法：
double[] arrayX = numsx.toArray(new double[0]);
double[] arrayY = numsY.toArray(new double[0]);