Returns a {@code Double} instance representing the specified
{@code double} value.
If a new {@code Double} instance is not required, this method
should generally be used in preference to the constructor
{@link #Double(double)}, as this method is likely to yield
significantly better space and time performance by caching
frequently requested values.
@param d a double value.
@return a {@code Double} instance representing {@code d}.
Returns a {@code Double} instance representing the specified {@code double} value. If a new {@code Double} instance is not required, this method should generally be used in preference to the constructor {@link #Double(double)}, as this method is likely to yield significantly better space and time performance by caching frequently requested values.
@param d a double value. @return a {@code Double} instance representing {@code d}.