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If you don't mind using a 3rd party library, commons-lang has the ArrayUtils type with various methods for manipulation.
Double[] doubles;
...
double[] d = ArrayUtils.toPrimitive(doubles);
There is also the complementary method
doubles = ArrayUtils.toObject(d);
Edit: To answer the rest of the question. There will be some overhead to doing this, but unless the array is really big you shouldn't worry about it. Test it first to see if it is a problem before refactoring.
Implementing the method you had actually asked about would give something like this.
double[] getDoubles(int columnIndex) {
return ArrayUtils.toPrimitive(data[columnIndex]);
}
In Java 8, this is one-liner:
Double[] boxed = new Double[] { 1.0, 2.0, 3.0 };
double[] unboxed = Stream.of(boxed).mapToDouble(Double::doubleValue).toArray();
Note that this still iterates over the original array and creates a new one.
Double is a wrapper class,
The Double class wraps a value of the primitive type double in an object. An object of type Double contains a single field whose type is double.
In addition, this class provides several methods for converting a double to a String and a String to a double, as well as other constants and methods useful when dealing with a double.
The double data type,
The double data type is a double-precision 64-bit IEEE 754 floating point. Its range of values is 4.94065645841246544e-324d to 1.79769313486231570e+308d (positive or negative). For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.
Check each datatype with their ranges : Java's Primitive Data Types.
Important Note : If you'r thinking to use double for precise values, you need to re-think before using it. Java Traps: double
In a comment on @paxdiablo's answer, you asked:
"So basically, is it better to use Double than Float?"
That is a complicated question. I will deal with it in two parts
Deciding between double versus float
On the one hand, a double occupies 8 bytes versus 4 bytes for a float. If you have many of them, this may be significant, though it may also have no impact. (Consider the case where the values are in fields or local variables on a 64bit machine, and the JVM aligns them on 64 bit boundaries.) Additionally, floating point arithmetic with double values is typically slower than with float values ... though once again this is hardware dependent.
On the other hand, a double can represent larger (and smaller) numbers than a float and can represent them with more than twice the precision. For the details, refer to Wikipedia.
The tricky question is knowing whether you actually need the extra range and precision of a double. In some cases it is obvious that you need it. In others it is not so obvious. For instance if you are doing calculations such as inverting a matrix or calculating a standard deviation, the extra precision may be critical. On the other hand, in some cases not even double is going to give you enough precision. (And beware of the trap of expecting float and double to give you an exact representation. They won't and they can't!)
There is a branch of mathematics called Numerical Analysis that deals with the effects of rounding error, etc in practical numerical calculations. It used to be a standard part of computer science courses ... back in the 1970's.
Deciding between Double versus Float
For the Double versus Float case, the issues of precision and range are the same as for double versus float, but the relative performance measures will be slightly different.
A
Double(on a 32 bit machine) typically takes 16 bytes + 4 bytes for the reference, compared with 12 + 4 bytes for aFloat. Compare this to 8 bytes versus 4 bytes for thedoubleversusfloatcase. So the ratio is 5 to 4 versus 2 to 1.Arithmetic involving
DoubleandFloattypically involves dereferencing the pointer and creating a new object to hold the result (depending on the circumstances). These extra overheads also affect the ratios in favor of theDoublecase.
Correctness
Having said all that, the most important thing is correctness, and this typically means getting the most accurate answer. And even if accuracy is not critical, it is usually not wrong to be "too accurate". So, the simple "rule of thumb" is to use double in preference to float, UNLESS there is an overriding performance requirement, AND you have solid evidence that using float will make a difference with respect to that requirement.