• After defining an interface, one or more classes can implement it.

• To implement an interface, use the implements clause in the class definition.

• The class must implement all the methods required by the interface.

• The general form of a class that includes the implements clause is:

class classname extends superclass implements interface {
// class-body
}

• To implement more than one interface, the interfaces are separated by commas.

• The extends clause is optional when implementing interfaces.

• Methods that implement an interface must be declared as public.

• The signature of the implementing method must exactly match the signature specified in the interface.

• Example implementing the Series interface:

• In this example, the ByTwos class implements the Series interface, generating a series of numbers that increases by two with each call to the getNext() method.

• The getNext(), reset(), and setStart() methods in the ByTwos class are declared with the public access specifier.

• This is necessary because when implementing a method defined by an interface, it must be declared as public.

• All members of an interface are implicitly public, so the implementation in the class must respect this access modifier.

• Here is a class that demonstrates ByTwos:

• It is allowed and common for classes that implement interfaces to define additional members.

• The inclusion of the getPrevious() method did not require any changes to the Series interface.

• The flexibility of modifying implementations without affecting the interface is one of the main advantages of using interfaces in object-oriented programming.

• The ByThrees class also implements the Series interface.

Abstract classes:

• If a class includes an interface but does not implement all the methods defined by it, it must be declared abstract.

• Abstract classes cannot be instantiated directly (that is, objects cannot be created from them).

• They can be used as abstract superclasses, allowing subclasses to provide the full implementation of the interface's methods.