Re: How to cast an Object to Double?

RedGrittyBrick <>
Wed, 17 Oct 2007 23:58:12 +0100
www wrote:

OK. My full testing program is:

public class HelloWorld

    public static void main( String[] args )
        Map<String, Object> map = new HashMap<String, Object>(10);
        Properties states = new Properties();
            states.load(new FileInputStream("property_file.txt"));

This loads the values as type String.

        catch(IOException e) {
        //copy entries from states file to the map
        for (Map.Entry value : states.entrySet())
            map.put((String)value.getKey(), value.getValue());

I'm not sure why you are copying a HashTable backed Properties object
into a new HashMap. Surely you could just use "states" where you later
use "map"?

        Object obj = map.get("HAT_SIZE");
        double value = (Double)obj; //causing error!!!!!!!! I don't
understand why.

Because obj is a String, you can't cast a String to a Double.

        //following is ok, but why need to cast to String first?
        double value = Double.parse((String)obj);

Because the compiler doesn't know the type of obj, you told the compiler
that obj was an Object, it doesn't know that obj will hold a String at



The property file("property_file.txt") has the content:

Note that 5.999 is a String even if it doesn't look like it.

Thank you for your help.

I'd rethink the approach to avoid all mention of type "Object".

e.g. one step along that path ...

public class PropertyDouble {
     public static void main(String[] args) throws FileNotFoundException,
             IOException {

         Properties states = new Properties();
         states.load(new FileInputStream("property_file.txt"));

         // copy entries from states file to the map
         Map<String, Double> map = new HashMap<String, Double>(10);
         for (Map.Entry entry : states.entrySet()) {
             String key = (String) entry.getKey();
             Double value = Double.parseDouble(
                     (String) entry.getValue());
             map.put(key, value);

         double value = map.get("HAT_SIZE");

         System.out.println("Double : " + Double.toString(value));

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