Java correct way convert/cast object to Double

Question

I try to found, but all seems vague. I need convert Object o to Double. Is correct way first convert to String? Thanks.

Answer 1

In Java version prior to 1.7 you cannot cast object to primitive type

double d = (double) obj;

You can cast an Object to a Double just fine

Double d = (Double) obj;

Beware, it can throw a ClassCastException if your object isn't a Double

Answer 2

Tried all these methods for conversion ->

    public static void main(String[] args) {

    Object myObj = 10.101;
    System.out.println("Cast to Double: "+((Double)myObj)+10.99);   //concates

    Double d1 = new Double(myObj.toString());
    System.out.println("new Object String - Cast to Double: "+(d1+10.99));  //works

    double d3 = (double) myObj;
    System.out.println("new Object - Cast to Double: "+(d3+10.99));     //works

    double d4 = Double.valueOf((Double)myObj);
    System.out.println("Double.valueOf(): "+(d4+10.99));        //works

    double d5 = ((Number) myObj).doubleValue();
    System.out.println("Cast to Number and call doubleValue(): "+(d5+10.99));       //works

    double d2= Double.parseDouble((String) myObj);
    System.out.println("Cast to String to cast to Double: "+(d2+10));       //works
}

Answer 3

If your Object represents a number, eg, such as an Integer, you can cast it to a Number then call the doubleValue() method.

Double asDouble(Object o) {
    Double val = null;
    if (o instanceof Number) {
        val = ((Number) o).doubleValue();
    }
    return val;
}

Answer 4

You can use the instanceof operator to test to see if it is a double prior to casting. You can then safely cast it to a double. In addition you can test it against other known types (e.g. Integer) and then coerce them into a double manually if desired.

    Double d = null;
    if (obj instanceof Double) {
        d = (Double) obj;
    }

Answer 5

I tried this and it worked:

Object obj = 10;
String str = obj.toString(); 
double d = Double.valueOf(str).doubleValue();

Answer 6

You can't cast an object to a Double if the object is not a Double.

Check out the API.

particularly note

valueOf(double d);

and

valueOf(String s);

Those methods give you a way of getting a Double instance from a String or double primitive. (Also not the constructors; read the documentation to see how they work) The object you are trying to convert naturally has to give you something that can be transformed into a double.

Finally, keep in mind that Double instances are immutable -- once created you can't change them.