#Abstract Classes and Generics
####Lesson Objective
- Students will understand how to extend an Abstract Class.
- Students will understand how to read Generic notation and when to use it.
- Students will be better equipped to organize their code in an object-oriented fashion.
####Lesson
Abstract classes are a special kind of class that is just used for inheritance. That is, you cannot instantiate abstract classes, but you can create subclasses which you can then instantiate.
Here are a few differences between an abstract class and an interface:
- Abstract classes can provide default method implementations. (Interfaces can do this in JDK 8.)
- All interface methods are public, but abstract methods can be private or protected.
- Instance variables in interfaces become public static final when implemented.
- A class can implement multiple interfaces, but only inherit from one class (abstract or concrete).
- Abstract classes may have static fields and methods.
Example: MyCollection1 and MyCollection2
Example: MyNode
Why would we use an abstract class over an interface? An interface defines what an object can do, while an abstract class defines what an object is.
Generics are a way of creating a class that works for any Object, while the type isn't specified until instantiation. The syntax for Generics is angle brackets and an uppercase letter, e.g. ArrayList<E>. Some interesting things to note:
- You can add a
SubClasstoContainerClass<SuperClass>. ContainerClass<SubClass>is not a subtype ofContainerClass<SuperClass>.- The superclass of all
ContainerClass<E>isContainerClass<?>. See also: wildcards.
Example: MyNode<E>
####Assessment