Using Double.toString(), if the number is too small or too large, you will get a scientific notation like this: 3.4875546345347673E-6. There are several ways to have more control of output string format.

Use with AIdouble num = 0.000074635638;
// use Double.toString()
System.out.println(Double.toString(num));
// result: 7.4635638E-5

// use String.format
System.out.println(String.format ("%f", num));
// result: 0.000075
System.out.println(String.format ("%.9f", num));
// result: 0.000074636

// use DecimalFormat
DecimalFormat decimalFormat = new DecimalFormat("#,##0.000000");
String numberAsString = decimalFormat.format(num);
System.out.println(numberAsString);
// result: 0.000075

Use String.format() will be the best convenient way.