Number Class in java

Question

Number class is an abstract class in which you cannot make objects from it using new operator, but we can use the class as reference Type, I want some explaination for the following code:

public class Test {
  public static void main(String[] args) {
    Number x = 3;
    System.out.println(x.intValue());
    System.out.println(x.doubleValue());
  }
}

the output is 3 and 3.0

So, in this line Number x = 3; is there any autoboxing or unboxing(which can't happen it's the superclass) happens? or what? how the assignment happened here?

Answer 1

The method doubleValue is abstract in Number, but you are storing an Integer (through auto-boxing) when you do

Number x = 3; // This is Number x = Integer.valueOf(3);

And Integer.doubleValue() says Returns the value of this Integer as a double after a widening primitive conversion. And it then refers the reader to JLS-5.1.2. Widening Primitive Conversion. Which says, in part, A widening primitive conversion does not lose information about the overall magnitude of a numeric value in the following cases, where the numeric value is preserved exactly: and then lists (specific to this case) from int to double