Java正确方法将对象转换/转换为Double

Question

I need to convert Object o to Double. Is the correct way to convert it to String first?

Answer 1

new Double(object.toString());

But it seems weird to me that you're going from an Object to a Double. You should have a better idea what class of object you're starting with before attempting a conversion. You might have a bit of a code quality problem there.

Note that this is a conversion, not casting.

Answer 2

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 3

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.

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 (eg Integer) and then coerce them into a double manually if desired.

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

Answer 5

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 6

Also worth mentioning -- if you were forced to use an older Java version prior to 1.5, and you are trying to use Collections, you won't be able to parameterize the collection with a type such as Double .

You'll have to manually " box " to the class Double when adding new items, and " unbox " to the primitive double by parsing and casting, doing something like this:

LinkedList lameOldList = new LinkedList();
lameOldList.add( new Double(1.2) );
lameOldList.add( new Double(3.4) );
lameOldList.add( new Double(5.6) );

double total = 0.0;
for (int i = 0, len = lameOldList.size(); i < len; i++) {
  total += Double.valueOf( (Double)lameOldList.get(i) );
}

The old-school list will contain only type Object and so has to be cast to Double .

Also, you won't be able to iterate through the list with an enhanced-for-loop in early Java versions -- only with a for-loop.

Answer 7

I tried this and it worked:

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

Answer 8

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
}