I have a Person class, and Object class is just the Object class of Java. I have a Student class that extends the Person Class as well. Can somebody explain why in these different scenarios I get errors when casting and some work?
Person p = (Person) new Object(); // error
Person p = (Person) new Student(“Steve”, 21, 12345); // works
Object o = new Person(“Steve”, 21); // works
Person p = (Person) o; // works
Casting doesn't magically turn an object into something else. It just tells the compiler to attempt to access a piece of data as a different type.
In the first example, you create an Object, which can't be cast into Person. Casting does not magically transform things.
In the second example, you create a Student object. Since Student inherits from Person, you can also consider it of type person.
Line 3: Again, you create a Person object but you hold it in a variable of type object. That doesn't change what the object actually is, which is why the downcasting to Person works.
Object class is Base Class of all java classes ,That's why reference of Object capable to hold object of any java class .
But if any class extends another class then child class not directly extends Object class in this case multilevel Inheritance concept implements implicitly .
In Your case
Person class is super class ,
Student class is sub class.
That's means Student class acquire properties of Person class , Casting is possible in this case as wall as Student extends Object class through Person Class.
It's OK to cast a child(sub) class object to a parent(super) class because child class already know and has everything(ie. variables and methods) from the inheritance of parent class, but not the other way around.
Example:
Parent class has method a1().
Child class added a method a2().
Child automatically gets a1() through inheritance.
Parent knows nothing about a2().
