Java将对象用作双重对象而无需显式强制转换

Question

Say I have this:

Object obj = new Double(3.14);

Is there a way to use obj like a Double without explicitly casting it to Double ? For instance, if I wanted to use the .doubleValue() method of Double for calculations.

Answer 1

No, there is no way to do this. obj is of type Object (even though it is a Double instance ), and the Object class does not have such methods as doubleValue() . The proper way would indeed be to cast:

Double d = (Double) obj;

Answer 2

The only way to do it if you can not cast is to use reflection :

Object obj = new Double(3.14);

Method m1 = obj.getClass().getMethod("doubleValue");
System.out.println("Double value: " + m1.invoke(obj));

Method m2 = obj.getClass().getMethod("intValue");
System.out.println("Int value: " + m2.invoke(obj));

Double value: 3.14
Int value: 3

This is usually only useful in some limited corner cases - normally, casting, generics, or using some supertype is the right approach.

Answer 3

You cannot.

Since your reference is to an Object , you will only have the methods which Object has at its disposal.

Note that you can use Number , which is the superclass of Integer , Short , etc and which defines .doubleValue() :

final Number n = new Double(2.0);
n.doubleValue(); // works

Answer 4

The closest you can do is this

Number num = new Double(3.14);
double d= num.doubleValue();

You can only call methods that the compiler knows is available, not based on the runtime type of the objects.

In short Object doesn't have a doubleValue() method so you cannot call it. You have to have a reference type which has the method you want to call.

Answer 5

No, it's not possible. The instance obj is a reference to an Object and can see only the methods of the Object class. You must cast it to Double to use specific methods of the Double class.

Answer 6

No! The only alternative is to use:

obj.toString()

Or, to use as double:

Double.parseDouble(obj.toString())

But it is not a good practice. Certainly has some another good alternative to your case. Post your code