java-Cast String Arraylist to double ArrayList

Question

I have a String arraylist and i am going to convert it to a double arraylist. Is there any way except using loops like for,While to convert it?

ArrayList<String>  S=new ArrayList<String>();

S=FillTheList();

ArrayList<Double>  D=new ArrayList<Double>();

Answer 1

In the JDK proper, up to version 7, no. JDK 8 will have functional programming support, though (see comment by @JBNizet below for the syntax).

You can use Guava to achieve this however:

final Function<String, Double> fn = new Function<String, Double>()
{
    @Override
    public Double apply(final String input)
    {
        return Double.parseDouble(input);
    }
};

// with S the original ArrayList<String>

final List<Double> D = Lists.transform(S, fn);

Note that while there is no loop in this code, internally the code will use loops anyway.

More to the point of your original question however, you cannot cast a String to a Double, since these are two different classes. You have to go through a parsing method like shown above.

Answer 2

Assuming you actually want your ArrayList to contain Doubles when you're done, no, there's no alternative, you're going to have to loop through, converting each String to a Double and adding the Double to the new ArrayList.

Note that at runtime, Java doesn't actually know the type that a List contains - it doesn't know that it's an ArrayList<Double>, it just knows that it's an ArrayList. There's therefore no way for the JVM to know what operation needs to happen to convert the contents of your ArrayList<String> (which it also sees as ArrayList) and add them to your ArrayList<Double>.