Number in java how it work without a subclass?

Question

I want to know why the result of this code:

Number x = 3;
System.out.println(x.intValue());
System.out.println(x.doubleValue());

generates

3
3.0

as Number is an abstract class in Java.

So when I need to use it - I could use it with some it's subclasses like Integer, Double ...etc. Now I want to know how exactly this part of code works:

Number x = 3;

Answer 1

when you do:

Number x = 3;
System.out.println(x.intValue());
System.out.println(x.doubleValue());

Following will happen:

Number x = 3;

this will declare x as int, and will be auto boxed to an object of the class Integer turning this: Number x = 3; into Number x = new Integer(3);

then here:

System.out.println(x.intValue());

intValue is implemented as:

public int intValue() {
    return value;
}

System.out.println(x.doubleValue());

and doubleValue is implemented as:

public double doubleValue() {
    return (double)value;
}