What is the difference between an abstract class and an abstract method, as well as between an abstract method and an interface?

An abstract class is a class that cannot be instantiated directly. It can contain abstract and non-abstract methods, fields, and constructors. It is used to define common characteristics and behaviors for subclasses.

An abstract method is a method without an implementation (body). It is declared with the abstract keyword in an abstract class or interface. Subclasses of an abstract class or a class implementing an interface are required to provide implementations for all abstract methods.

Difference between an abstract method and an interface:

Characteristic	Abstract Method	Interface
Declaration location	Abstract class, Interface	Interface
Implementation requirement	Subclass of abstract class, class implementing interface	Class implementing interface
Body presence	No	No (until Java 8 - only declaration, with Java 8 - default and static methods with bodies)
Access modifiers	Public, protected, default (in abstract class); public (in interface)	Public

An interface is a fully abstract "contract," a set of abstract methods (until Java 8) and constants. A class can implement multiple interfaces. It is used to define common behavior that can be implemented by different classes and to achieve multiple inheritance of behavior.

// Example of an abstract class
abstract class Animal {
    String name;

    // Abstract method
    abstract void makeSound();

    // Non-abstract method
    void setName(String name) {
        this.name = name;
    }
}

// Example of a class inheriting an abstract class
class Dog extends Animal {
    @Override
    void makeSound() { // Implementation of abstract method
        System.out.println("Woof");
    }
}

// Example of an interface
interface Flyable {
    // Abstract method (implicitly public abstract)
    void fly();

    // Static method (from Java 8)
    static void describeFlying() {
        System.out.println("Ability to fly.");
    }

    // Default method (from Java 8)
    default void land() {
        System.out.println("Landing...");
    }
}

// Example of a class implementing the interface and inheriting an abstract class
class Bird extends Animal implements Flyable {
    @Override
    void makeSound() {
        System.out.println("Chirp");
    }

    @Override
    public void fly() { // Implementation of interface's abstract method
        System.out.println("Flying high!");
    }
}