What changes are necessary in the class definition to ensure its ability to use polymorphism?

To enable polymorphism in Java, you need to:

• Make methods that are intended to be overridden in subclasses non-static and non-final.
• Use the override keyword in subclasses to override base class methods.
• In the base class, methods should be declared as public or protected (so subclasses can override them).

Example:

class Animal {
    public void sound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    @Override
    public void sound() {
        System.out.println("Bark");
    }
}

// Using polymorphism
Animal animal = new Dog();
animal.sound(); // Outputs "Bark"

Thus, to support polymorphism, methods should be virtual (in Java, all non-static methods are virtual by default). The main thing is to avoid making methods static or final.