The OO in JAVA Part 2

1. The OO in JAVA Part 2

2. Contents: • Inheritance • Final Methods and Classes • Abstract Methods and Classes • Interfaces • Polymorphism • Generating UML Diagrams using NETBEANS

4. Inheritance

5. Inheritance • Inheritance refers to the ability to define new classes from existing class definitions. • Sub-classing. • Parent class – child class • Child classes inherit all the inheritable properties, events and methods of the parent class. • Where it is permissible may override the parent class implementation

6. Inheritance • Override – is the act of using the same method from the parent class and implementing it differently. The argument list and the return type are the same. • Inheritance makes objects highly reusable as existing functionality is extended and not duplicated across modules. • Base class, superclass or parent class • Subclass, derived class or child class

7. Inheritance • Inheritance is on of the foundation principles of object-oriented programming because it allows the creation of hierarchical classifications. • Using inheritance, you can create a general class that defines traits common to a set of related items. • This class can then be inherited by other, more specific classes, each adding those things that are unique to it.

8. Inheritance • A class that is inherited is called the superclass, base class or parent class • The class that does the inheriting is called a sub class, child class or derived class

9. INHERITANCE • a way to form new classes using classes that have already been defined • In Java, all classes, including the classes that make up the Java API, are subclassed from the Object superclass.

10. INHERITANCE • Superclass – Any class above a specific reference class in the class hierarchy. • Subclass – Any class below a specific reference class in the class hierarchy. • Class members are inherited FROM a superclass BY a subclass

11. INHERITANCE Benefits of Inheritance • Once a behavior (method) is defined in a superclass, that behavior is automatically inherited by all subclasses. • A method can be encoded only once and it can be used by all subclasses. • A subclass only needs to implement the differences between itself and the parent.

12. INHERITANCE Rules of Inheritance: MEMBERS • A subclass inherits ALL the members (fields, methods, and nested classes) from its superclass. However, only the public and protected can be accessed directly from within the subclass. • Constructors are not members, so they are not inherited by subclasses. • The constructor of the superclass can be invoked from the subclass.

13. INHERITANCE Rules of Inheritance: PRIVATE MEMBERS • A subclass does not inherit the private members of its parent class, at least directly. • Use the inherited public or protected methods to access the private members.

14. INHERITANCE Rules of Inheritance: EXTENSION • A class can extend only one other class. 

15. INHERITANCE Inheritance example:

16. INHERITANCE Rules of Inheritance: FIELDS • Inherited non-private fields can be used directly, just like any other field. • A field in the subclass declared with the same name as the one in the superclass hides the superclass’s (not recommended). (Field Hiding) • New fields that are not in the superclass can be declared in the subclass.

17. INHERITANCE Rules of Inheritance: METHODS • Inherited methods can be used directly as they are. • A new instancemethod in the subclass that has the samesignature as the one in the superclass overrides the superclass’s. Method Overriding • A new staticmethod in the subclass that has the samesignature as the one in the superclass hides the superclass’s. Method Hiding • The access specifier for an overriding method can allow more, but not less, access than the overridden method. • New methods that are not in the superclass can be declared in the subclass.

18. Progress Check • When creating a subclass, what keyword is used to include a superclass? • Does a subclass include the members of its superclass? • Does a subclass have access to private members of its superclass?

19. “ ”

20. “The super Keyword”

21. super KEYWORD When to use your super? • Invoke overridden method. • Refer to a hidden field. • Invoke the superclass constructor. • Refer to members of the superclass. NOTE: • If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass.

22. super KEYWORD RULES for superclass constructor calls: • The super() call must occur as the first statement in a constructor. • The super() call can only be used in a constructor definition. • This implies that the this() construct and the super() calls cannot both occur in the same constructor.

23. super KEYWORD When are superclass constructor calls usually used? • You want to call a parent constructor which has parameters (the automatically generated super constructor call has no parameters). • There is no parameterless parent constructor because only constructors with parameters are defined in the parent class.

24. super KEYWORD Key concepts behind super • When a subclass calls super(), it is calling the constructor of its immediate super class. • Super() always refers to the superclass immediately above the calling class. • This is true in a multilevel heirarchy. • Super() must always be the first statement executed inside a subclass constructor.

25. Super

26. Super

27. “___ lap, ___ countdown, ___ destination”

28. Final Methods & Classes

29. FINAL Methods & Classes Final Methods: • Methods which cannot be overridden • In class Monster: • FireMonster:

30. FINAL Methods & Classes Final Classes: • Classes which cannot be extended

31. FINAL Methods & Classes When to make your method orclass final? • Implementation of the method is critical to the consistent state of the object and should not be changed. • Methods called from constructors should generally be declared final.

33. Abstract Classes & Methods

34. Abstract Classes & Methods Abstract Class: • class that is declared abstract • may or may not include abstract methods

35. Abstract Classes & Methods Abstract Method: • method that is declared without an implementation (without braces, and followed by a semicolon)

36. Abstract Classes & Methods Rules: • If a class includes abstract methods, the class itself must be declared abstract. • An abstract class cannot be instantiated, but they can be subclassed. • You cannot declare an instance of an abstract class. • An abstract class must be inherited by a subclass in order for its data and behavior to be used in an application. • When a non-abstract class extends an abstract class, the former should implement all abstract methods of the latter. • When an abstractclass extends an abstract class, the subclass may implement all, some, or none of the abstract methods of the superclass.

37. Abstract Classes & Methods

38. Abstract Classes & Methods

39. Abstract Classes & Methods

40. Abstract Classes & Methods Question: • Why use abstract classes and methods? 

41. Abstract Classes & Methods

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