Abstract classes and methods (abstract)

An abstract class cannot be instantiated. It may contain abstract methods (without a body) and concrete methods.

Abstract method:

• Declared with the abstract keyword.
• Has no body (ends with semicolon).
• Must be implemented by the first concrete subclass.

Syntax:

abstract class Animal {
    abstract void makeSound(); // no body
    void sleep() {             // concrete method
        System.out.println("Sleeping");
    }
}

Concrete subclass must implement abstract methods:

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

Key points:

• Abstract classes can have constructors (called when subclass is instantiated).
• They can have fields, concrete methods, static methods, etc.
• Cannot be instantiated: new Animal(); // error.
• If a subclass does not implement all abstract methods, it must also be declared abstract.

Why use abstract classes?

• To provide a common base with shared code and enforce certain methods.
• To define a template for subclasses.

Abstract vs Interface (brief): Abstract classes can hold state (fields) and provide partial implementation; interfaces (before Java 8) were purely abstract. Java 8+ interfaces can have default/static methods.