How can new features be added to an existing class?

In Java, adding new capabilities to an existing class can be done in several ways:

• Inheritance: create a subclass that extends the functionality of the base class.
• Composition: include an instance of an existing class in a new class and add new methods.
• Using interfaces and default methods: starting from Java 8, interfaces can contain methods with implementations, allowing to add behavior.
• Decorator Pattern: wrap an object of an existing class in another class that adds new capabilities.

Example of inheritance:

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

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

    void fetch() {
        System.out.println("Fetching ball");
    }
}

Thus, the Dog class extends the Animal class, adding new methods and changing behavior.