Object Casting Java

Object Casting Java

What is casting?

Assigning one data type to another or one object to another is known as casting. Java supports two types of casting – data type casting and object casting.

Conditions of assigning objects of different classes one to another
  1. Same class objects can be assigned one to another and it is what we have done with Officer1 class.
  2. Subclass object can be assigned to a super class object and this casting is done implicitly. This is known as upcasting (upwards in the hierarchy from subclass to super class).
  3. strong>Java does not permit to assign a super class object to a subclass object (implicitly) and still to do so, we need explicit casting. This is known as downcasting (super class to subclass). Downcasting requires explicit conversion.
Program explaining the above rules of Java Object Casting


Object Casting Java
Output screen of Java Object Casting

The super class Flower includes only one method smell() and it is overridden by subclass Rose.

In the above two statements, nothing is new as both object are calling their own smell() methods. Observe the following two statements.

In the first statement, subclass object r is assigned (casted) to super class object f. It is a perfect assignment accepted by JVM and casting is done implicitly. In the super class object f, the subclass object reference (address) r exists. Calling f.smell(), f will call Rose smell() method.

From the above observation, a rule can be made as follows.

"If a subclass object is assigned to a super class object, the super class object will call subclass overridden method". This is an important rule which leads to dynamic polymorphism.

The above statement does not compile as super class object is assigned to subclass object. To do it, we require explicit casting.

In the above statement, Flower object f is explicitly casted to Rose object r. In the earlier statement f = r, the f includes Rose reference. Now, f returns the Rose reference to Rose itself. For this reason f.smell() prints the subclass Rose smell(). That is, r is calling its own smell(). What happens if f = r does not exist earlier. It will be a an exception (not compilation error). That is, "before doing explicit casting, implicit casting must be done".

28 thoughts on “Object Casting Java

  1. yours

    r = (Rose) f;
    f.smell();
    How could it run fine? This is not possible. Run it yourself and try once.
    I am getting this exception.

    Exception in thread “main” java.lang.ClassCastException: Flower cannot be cast t
    o Rose
    at Rose.main(Rose.java:17)

  2. vithu

    in 27th line why did you put f.smell(); instead of r. smell.
    because you assigned super class object to subclass object in 26th line then why did you call smell() method on super class object?
    please could you clearly explain that?

    Thank you

    1. S. Nageswara Rao, Corporate Trainer Post author

      In Java explicit casting does not come free of cost. Before doing explicit casting, you must store the subclass object in the super class object. Once stored, super class object returns sub class object (which you have stored earlier).

      Imagine, I want you to give me a red marker. Unless I gave earlier, how you will give to me? The same thing here also.

    1. S. Nageswara Rao, Corporate Trainer Post author

      many methods Java API do not know what to run until runtime. These methods know what to return dynamically. For example, the firstElement() method of Vector returns the fist element stored. How to design the method firstElement(); Vector do not know what the element the user stores. To overcome this problem, the designers preferred to return Object class like this:

      public Object firstElement();

      Suppose, if the user stores an Integer object as the first element, then what happens let us see.
      The JVM does like this:

      Integer i1 = new Integer(10);
      Object obj1 = i1; // implicit casting
      return obj1;

      i1 subclass object is stored in super class object obj1 of Object class and then returned.

      While retrieving what you will do?

      Object obj2 = vect.firstElement();
      Integer i1 = (Integer) obj2; // explicit casting

    1. S. Nageswara Rao, Corporate Trainer Post author

      In implicit casting, we place the reference of a subclass object in super class object. In explicit casting, we get back the subclass reference object from super class object. Why all this twist? Observe, many methods of Java API, return an object of Object class. JVM keeps a subclass object in object of Object class and then returns; because it does not know what object it gets at runtime to return. We do explicit casting and get back the object. For example, the elementAt(int) method of Vector class.

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