Download Abstract Classes and Interfaces

Interfaces
Week 4
Main concepts to be covered
•Interfaces
•Multiple inheritance
Abstract classes and methods
• Abstract methods have abstract in the
signature.
• Abstract methods have no body.
• The presence of at least one abstract
method makes the class abstract.
• Abstract classes cannot be instantiated.
• Concrete (i.e. non-abstract) subclasses
complete the implementation.
The Animal class
public abstract class Animal
{
fields omitted
/**
* Make this animal act - that is: make it do
* whatever it wants/needs to do.
*/
abstract public void act(List<Animal> newAnimals);
other methods omitted
}
Further abstraction
Selective drawing
(multiple inheritance)
Multiple inheritance
• Having a class inherit directly from multiple
ancestors.
• Each language has its own rules.
– How to resolve competing definitions?
• Java forbids it for classes.
• Java permits it for interfaces.
– No competing implementation.
Interface
• interface
– A Java programming language keyword used
to define a collection of method definitions
and constant values. It can later be
implemented by classes by means of the
"implements" keyword.
Interfaces as method specifications
• Interfaces specify method signatures only
• Each method signature is followed by a
semi-colon (;)
An Actor interface
public interface Actor
{
fields omitted
/**
* Perform the actor's daily behavior.
*/
void act(List<Actor> newActors);
other methods omitted
}
Features of interfaces
•
•
•
•
All methods are abstract.
There are no constructors.
All methods are public.
All fields are public, static and final.
Multiple interfaces
• Because interfaces simply specify method
signatures, a single class can implement
several different interfaces in order to
ensure more methods can be
implemented.
Classes implement an interface
public class Fox extends Animal implements Drawable
{
...
}
public class Hunter implements Actor, Drawable
{
...
}
Implementing an Interface
• When a class implements an interface, it is
essentially signing a contract.
– Either the class must implement all the
methods declared in the interface and its
superinterfaces, or the class must be declared
abstract.
– The method signature in the class must match
the method signature as it appears in the
interface. A class that implements the
ActionListener interface must contain the
method ActionPerformed
Interfaces as types
• Implementing classes do not inherit code,
but ...
• ... implementing classes are subtypes of
the interface type.
• So, polymorphism is available with
interfaces as well as classes.
Interfaces are useful for the
following:
• Capturing similarities among unrelated
classes without artificially forcing a class
relationship
• Declaring methods that one or more
classes are expected to implement
• Modelling multiple inheritance, a feature of
some object-oriented languages that
allows a class to have more than one
superclass