How do you change the type of an object in Java?

In Java, you cannot change the class (the type) of an object once it has been created. That is not possible.

When an object is instantiated using the new keyword, it is created as an instance of a specific class. This class defines the object's structure (its fields) and behavior (its methods). The type of the object is fixed for its entire lifetime.

Understanding Object Types vs. Variable Types

It's crucial to distinguish between the type of an object and the type of a variable.

• Object Type: This is the actual class the object belongs to at the moment it's created. This type is immutable.
• Variable Type: This is the declared type of the variable that holds a reference to an object. A variable can refer to objects of its declared type or any subclass of that type.

As the provided reference states: "You may assign objects of different classes to the same variable. Naturally, you can only assign objects of the same class of the variable or any of its subclasses."

What You Can Do Instead

While you cannot change an object's type, you can perform operations related to types and variables:

1. Assign Objects to Variables of Compatible Types:

   • You can assign an object of a subclass to a variable whose type is a superclass. This is often called upcasting and is implicitly allowed.
   • You can assign an object of a class to a variable of the same class.

   // Example: Assigning objects to compatible variables
   class Animal { /* ... */ }
   class Dog extends Animal { /* ... */ }
   class Cat extends Animal { /* ... */ }
   
   Animal myAnimal = new Dog(); // Valid: Assigning Dog (subclass) to Animal (superclass) variable
   Dog myDog = new Dog();       // Valid: Assigning Dog to Dog variable
   // Dog anotherDog = new Animal(); // Invalid: Cannot assign Animal (superclass) to Dog (subclass) variable directly

2. Type Casting:

   • You can explicitly cast an object reference to a different type. This doesn't change the object itself, but it tells the compiler how you intend to treat the object reference at that point in the code.
   • Casting is typically used for downcasting (casting a superclass reference to a subclass type). This requires an explicit cast and will result in a ClassCastException if the object being referenced is not actually an instance of the target subclass or one of its subclasses.

   // Example: Type Casting (Downcasting)
   Animal genericAnimal = new Dog(); // genericAnimal variable refers to a Dog object
   
   // To access Dog-specific methods, you need to cast
   if (genericAnimal instanceof Dog) { // Check the actual type before casting
       Dog specificDog = (Dog) genericAnimal; // Valid cast
       specificDog.bark(); // Access Dog-specific method
   }
   
   Animal anotherAnimal = new Cat(); // anotherAnimal variable refers to a Cat object
   // Dog tryToCast = (Dog) anotherAnimal; // This would compile but throw ClassCastException at runtime

Summary Table

Feature	Object Type	Variable Type
Mutability	Cannot be changed after creation	Can refer to different objects over time
Defined By	The class used with new	The type declared when the variable is created
Assignment	N/A	Can hold references to its type or subclasses

In essence, while you can change which object a variable points to (potentially an object of a different, compatible type), you cannot alter the fundamental type of an object itself once it exists.