java hashmap array to double array

I think the problem is that there are two steps here that you are trying to conflate into one. First, you want to create an array of arrays large enough to hold all of the individual arrays: double[][] data = new double[fData.size()][]; Next, you'll want to iterate over all the entries, building an array large enough to hold the value: double[][] data = new double[fData.size()][]; int index = 0; for (float[] entry: fData.values()) { /* ... */ ++index; } At each of these loop iterations, you'll want to allocate an array to hold all of the floats: double[][] data = new double[fData.size()][]; int index = 0; for (float[] entry: fData.values()) { data[index] = new double[entry.length]; /* ... */ ++index; } And finally, you'll want to copy the data in: double[][] data = new double[fData.size()][]; int index = 0; for (float[] entry: fData.values()) { data[index] = new double[entry.length]; for (int i = 0; i < entry.length; ++i) data[index][i] = entry[i]; ++index; } And you should be golden!

You need: double data[][] = new double[fData.size()][]; Then later in the loop, you need: data[i] = new double[tValue.length];

double data[][] = new data[fData.size()][]; data[i] = new double[tValue.length];