Abstract Classes and Interfaces

Abstract Classes

• Abstract classes are defined using the abstract keyword.
• Any class containing one or more abstract methods must be declared as abstract.
• Instances of abstract classes cannot be created directly.
• Abstract classes are meant to be extended.

Abstract Methods

• Abstract methods do not have an implementation (body).
• Abstract methods must be overridden by subclasses.

Concrete Classes

• Concrete classes can be instantiated.
• They do not contain abstract methods.

Concrete Methods

• Concrete methods have an implementation (body).

Interfaces

• An interface is a completely abstract class.
• It contains only abstract methods.

Abstract Data Type (ADT)

• ADT is a class conceptualized without implementation details.
• ADTs can be concrete or abstract.

Interface Concept

• Interfaces describe how things interact.
• In Java, interfaces are classes with 100% abstract methods.
• Interfaces in Java 8 can include default and static methods.
• Focus will be on interfaces with only abstract methods.

Interface Purpose

• Interfaces enhance abstraction by separating "what to do" from "how to do it."
• Coding to an interface increases code flexibility and makes code easier to extend.
• Interfaces establish a contract between classes.
• Implementing an interface means a class agrees to implement all its methods.
• Interfaces support functionality of multiple inheritance, avoiding the diamond problem.

Inheritance

• Classes B and C inherit from Class A, each creating their version of method someMethod().
• Class D inherits from both B and C presenting the multiple inheritance problem.
• Java does not directly support multiple inheritance of implementation.
• Interfaces provide multiple conformance, avoiding conflicts.
• A class can extend one class and implement multiple interfaces.

Defining Interfaces

• Use the interface keyword instead of class.
• General form: public interface InterfaceName { /* constant declarations; abstract method signatures; */ }
• Interface methods are implicitly public and abstract, so these keywords are optional.

Using Interfaces

• Interfaces are used similarly to abstract classes, as a data type.
• Subclasses implement interfaces instead of extends.
• The implements keyword is used on subclasses.

Interface vs. Abstract Class

• Interfaces: constructors are none, variables are constants (public static final), methods have no implementation (public abstract).
• Abstract Classes: constructors are invoked by subclasses, variables can be of any type, methods can be concrete and abstract.

Interface Characteristics

• An interface describes "what" an object can do, specifying method names, parameter lists, and return types.
• Implementation describes "how" an object does it, providing the particular details necessary to make a workable object.

Using Interfaces

• Strong IS-A relationships are modeled using classes.
• If related classes need to share code, use an abstract class that will be extended.
• Weak IS-A relationships can be modeled using interfaces.

More Rules

• When all methods will be implemented by classes that implement the interface, it is useful.
• To circumvent single inheritance restrictions, design one class as a regular or abstract superclass and the other classes as interfaces.
• If it's possible to create your superclass without any method definition or member variables, prefer interfaces to abstract classes.

Interface Example Code

• It does not include a constructor in interfaces.
• Abstract classes still have constructors.
• Abstract method ends with semicolon.
• All interface methods must end with a semicolon because they have no body.
• Bulldog IS-A Canine which means IS-A Animal. Also, Bulldog IS-A Pet.
• Bulldog must provide implementation for abstract methods.

Additional Notes

• Interfaces cannot be instantiated and cannot use new to do so.
• The implement keyword is used by classes to implement an interface.
• Interfaces provides full abstraction, its abstract methods do not have a body.
• Interface modifiers can be omitted.
• Interface variables are implicitly public final static.
• Interface methods are implicitly public abstract.
• Interfaces extend other interfaces.
• A class can extend one class and implement any number of interfaces.

Core Usage

• To be able to upcast to more than one base type is the main purpose of it.
• It uses polymorphism to decide which implementation to call at runtime.
• Interfaces are preferred over abstract classes.
• It can define a common supertype for unrelated classes.
• It is more flexible than the classes.