Question

Just trying to understand the behavior of java.lang.Object class.

public class TypeCheck{
    static void printMethod(Object obj)
    {
        System.out.println(obj);
    }

    public static void main(String[] args)
    {
        Object obj       = new Object();
        Integer intObj   = new Integer(12);
        String stringObj = new String("Hello");

        printMethod(obj); //---> java.lang.Object@78b5f53a
        printMethod(intObj); //---> 12
        printMethod(stringObj); // ---> Hello
    }
}

My questions are:

1. When should we use Object class?
2. But, when I only pass printMethod(intObj) and in printMethod(Object obj) I do an addition: System.out.println(obj+1) it does not work. If obj recognizes that it’s an Integer, why can’t I do operations on it?
   Exception: TypeCheck.java:5: operator + cannot be applied to java.lang.Object,int System.out.println(obj+1);

3. Now, if I do this:

   public class TypeCheck
   {
       static void printMethod(Object obj)
       {
           System.out.println(obj.getClass().getName());
       }
       public static void main(String[] args)
       {
           int i = 666;
           printMethod(i);
       }
   }
   

   It returns java.lang.Integer, but it’s defined as int (primitive type). Why does Java convert a primitive type to it’s wrapper class when passed to an Object.

Answer 1

1. Almost never. You should use as specific an object you can.
2. That should work. See below
3. Because primitives, i.e. int, are not objects. Therefore it needs to be made an object (boxed) to be passed as one.

EDIT in response to question edit:

(Number 2)
Inside of printMethod, the type of obj is not int, it is Object. You can’t add objects! You would need to cast it back to Integer or int before you could perform such operations on it. What would the + do if you passed an instance of TypeCheck into TypeCheck.printMethod?