Inheritance Chapter 7 1 Introduction to Inheritance Inheritance

Inheritance Chapter 7 1

Introduction to Inheritance • Inheritance allows us to define a general class and then define more specialized classes simply by adding new details to the more general class definition. • A more specialized class inherits the properties of the more general class, so that only new features need to be programmed. Chapter 7 5

Introduction to Inheritance, cont. • example – General class Vehicle might have instance variables for weight and maximum occupancy. – More specialized class Automobile might add instance variables for wheels, engine size, and license plate number. – General class Vehicle might also be used to define more specialized classes Boat and Airplane. Chapter 7 6

Programming Example: A Base Class • class Person Chapter 7 7

Derived Classes • Consider a college record-keeping system with records about students, faculty and staff. Chapter 7 8

Derived Classes, cont. Chapter 7 9

Derived Classes, cont. • Even though your program may not need any Person or Employee objects, these classes can be useful for consolidating and representing features common to all subclasses. – For example, all students, faculty, and staff have names, and these names may need to be initialized, changed, retrieved, or printed. Chapter 7 10

Derived Classes, cont. is a derived class of class Person is called the base class. • class Student and class Chapter 7 11

Derived Classes, cont. • public class Student extends Person Chapter 7 12

Derived Classes, cont. • When you define a derived class, you declare only the added instance variables and you define only the added and overridden methods. • The variables and methods of the parent class which are not declared private are inherited automatically. Chapter 7 14

Derived Classes, cont. • class Inheritance. Demo Chapter 7 15

Overriding Method Definitions • Notice that class Student has a method write. Output with no parameters, and class Person also has a method write. Output with no parameters, that class Student inherits. • When a derived class defines a method with the same name and the same number and types of parameters as a method in the base class, the method in the derived class overrides the method in the base class. Chapter 7 17

Overriding Method Definitions, cont. • When overriding a method, you can change the method definition to anything you wish, but you cannot change the method’s heading or the method’s return type. Chapter 7 18

Overriding vs. Overloading • When you override a method, the new method definition in the derived class has the same name and the same number of types of parameters as the method definition in the base class. • When the name is the same, but the number or types of the parameters differs, whether in the base class or in the derived class, the method is overloaded in the derived class. Chapter 7 19

Overriding vs. Overloading, cont. • example: public String get. Name(String title) in class Student and public String get. Name() in class Person overload method get. Name since the two methods have different parameter lists. – Both methods are available in class Student. Chapter 7 20

Private Instance Variables in the Base Class • Private instance variables inherited from a base class cannot be accessed directly. • Instead, they must be accessed using a method that is not declared private. • While this may seem inconvenient, it provides an important mechanism for controlling access and changes to instance variables in the base class. Chapter 7 23

Private Methods in the Base Class • Like private instance variables, private methods inherited from a base class cannot be accessed directly. • Instead, they, too, must be accessed using a method that is not declared private. • This, too, provides an important mechanism for controlling access to methods in the base class. Chapter 7 24

Private Methods in the Base Class • Since private methods typically serve as helping methods, their use always is limited to the class in which they are defined. Chapter 7 25

Constructors in Derived Classes • A base class has its own constructors. – Their purpose typically is to initialize the instance variables declared in the base class. • A derived class has its own constructors. – Their purpose typically is to call a constructor in the base class, and then to initialize the instance variables declared in the derived class. Chapter 7 31

Constructors in Derived Classes, cont. • To call a constructor in the base class, use super(Values_for_Instance_Variables _Declared_in_the_Base_Class); • example super(initial. Name); not Person(initial. Name); //ILLEGAL Chapter 7 32

Using super • The call to the constructor in the base class (using super) must be the first action taken in the constructor of a derived class. • When no call to the constructor in the base class is included, Java automatically includes a call to the default constructor in the base class. Chapter 7 33

Using super, cont. • equivalent definitions: public Student() { super(); student. Number= 0; } and public Student() { student. Number= 0; } Chapter 7 34

The this Method • Within the definition of one constructor, it can be appropriate to call another constructor in the same class. • The keyword this is used to call another constructor in the same class. • example this(initial. Name, 0) Chapter 7 35

The this Method, cont. • Any use of this must be the first action in the constructor definition. – Thus, a constructor definition cannot contain a call using super and a call using this. • To use both super and this, include a call using this in one constructor and a call using super in the constructor called using this. Chapter 7 36

Calling an Overridden Method can be used to call a method in the base class that has been overridden in the derived class. • example • super. write. Output(); • However, you cannot repeat the use of super to invoke a method in some ancestor class other than the immediate base (parent) class. Chapter 7 37

An Object Can Have More than One Type • If class Undergraduate is derived from class Student and class Student is derived from class Person, then every object of class Undergraduate is also an object of class Student and an object of class Person. • A reference to an object of class Undergraduate can be substituted for a reference to an object of class Student or a reference to an object of class Person. Chapter 7 45

An Object Can Have More than One Type, cont. • However, a reference to an object of class person cannot be substituted for a reference to an object of class Student or an object of class Undergraduate. • A reference to an object of an ancestor cannot be substituted for a reference to an object of a derived class. Chapter 7 47

The Class Object • In Java, every class descends from (and inherits features from) the Object class. • Therefore, every object of every class is of type Object. • Unless a class is declared explicitly to be a descendant of some other class, it is an immediate descendant of the class Object. Chapter 7 50

The Class Object, cont. • An object of any class can substituted when a parameter of type Object is expected. • Every class inherits some methods from the class Object: – equals() – to. String() but usually these methods are overridden by the derived class or by an intermediate ancestor class. Chapter 7 51

Method to. String • Inherited method to. String takes no arguments. • Typically, method to. String is coded to produce and return a string which contains everything of interest about the object. Chapter 7 52

Method to. String, cont. • example public String to. String(); { return (“Name: “ + get. Name()+ n + “Number: “ + get. Number()); } • Whenever a new class is created, a suitable to. String method should be defined. Chapter 7 53

Method to. String, cont. • Method to. String can be called my the conventional means, Object_Name. to. String, or by using only Object_Name. • example System. out. println(s. to. String()); or System. out. println(s); Chapter 7 54

Abstract Classes • An abstract class is not intended to be used to create objects. Chapter 7 70

Abstract Classes, cont. • By declaring one or more methods to be abstract and by omitting the method body, only objects of derived classes which override the method(s) can be instantiated. • example public abstract void draw. Here(); • A class that has at least one abstract method must be declared abstract. Chapter 7 71

Abstract Classes, cont. • class Figure Chapter 7 73

Abstract Classes, cont. • An abstract class serves as a placeholder. • An abstract class makes it simpler to define derived classes. • An abstract class assures that all its derived classes implement its abstract method(s), or they too will be abstract. Chapter 7 74

Interfaces • An interface specifies the headings for methods that must be defined for any class that implements the interface. Chapter 7 75

Interfaces, cont. • Interface Writeable Chapter 7 76

Interfaces, cont. • To implement an interface, a class must – include the phrase implements Interface_Name at the start of the class definition • example implements My. Interface, Your. Interface – implement all the method headings listed in the definition of the interface. Chapter 7 77

Interfaces, cont. • class Writeable. Undergraduate Chapter 7 78

Interfaces, cont. • An interface is a type. • A method may have a parameter of an interface type. • Any class that implements the interface can be substitute for the parameter. • A class can implement any number of interfaces by providing implementations for all the methods of all the interfaces it claims to implement. Chapter 7 79

Dynamic Binding • Different objects can invoke different method definitions using the same method name. • For example, if b references a Box and t references a Triangle, b and t invoke different definitions of method draw. At even if b and t are declared to be objects of type Figure. Chapter 7 81

Dynamic Binding, cont. • Handling the invocation of a method that may be overridden later is called dynamic binding or late binding. • The type of object being referenced at the time of the method call, not the type of reference that was declared, determines which method is invoked. Chapter 7 82

Dynamic Binding, cont. • Consider Figure f; Box b = new Box(1, 4, 4); f = b; f. draw. At(2); Triangle t = new Triangle(1, 2); f = t; f. draw. At(2); Chapter 7 83

Dynamic Binding, cont. • Method draw. At is inherited from class Figure and is not overridden. • But, method draw. Here is invoked within the definition of method draw. At, and method draw. Here is overridden. • The type of object referred to by f determines which method draw. Here is invoked. Chapter 7 84

Dynamic Binding with to. String • Recall method to. String typically is used to prepare and return a string, describing an object, for output to the screen. • The name of this method can be omitted, thanks to dynamic binding, because one definition of method println expects a single argument of type Object which it uses to invoke the method to. String associated with the object. Chapter 7 90

Polymorphism • Polymorphism comes from Greek meaning “many forms. ” • In Java, polymorphism refers to the dynamic binding mechanism that determines which method definition will be used when a method name has been overridden. • Thus, polymorphism refers to dynamic binding. Chapter 7 91

Subtle Difference • Dynamic binding refers to the process carried out by the computer. • Polymorphism can be thought of as something objects do. • Polymorphism, encapsulation, and inheritance, and considered to be the main features of object-oriented programming. Chapter 7 92

The Class JApplet • An applet is a derived class from the class JApplet public class Label. Demo extends JApplet • Class JApplet has methods named init and paint. • When you define methods init or paint in an applet, you are overriding inherited methods. Chapter 7 98

The Class JApplet, cont. • Methods init and/or paint can be invoked without having to (re)define them. • Thanks to polymorphism, methods init and/or paint defined in an applet will be invoked by library class methods and other methods when you run the applet with the applet (of type JApplet) as the parameter to the method. Chapter 7 99

The Class JApplet, cont. • The class JApplet is used as a base class to derive applets to run from a webpage. Chapter 7 100

The Class JFrame • A Graphical User Interface or GUI is simply a windowing interface for some kind of program. • The class JFrame, rather than the class JApplet, is used to produce GUIs that can run as regular Java applications. Chapter 7 101

The Class Jframe, cont. • class Button. Demo Chapter 7 102

The Class Jframe, cont class Show. Button. Demo Chapter 7 103

The Class JFrame, cont. Chapter 7 104

The Class JFrame, cont. • The class JFrame, and every class derived from it, has a method named set. Visible. – When its argument has the value true, the GUI is visible. • Labels, buttons, JFrames, and other components inherit method set. Visible from a common ancestor. Chapter 7 105

The Class JFrame, cont. • A class derived from JFrame has no init method, but it does use a constructor. • Code that would be in the init method of an applet is placed in the constructor of the class derived from JFrame. • A GUI derived from a JFrame sets an initial size set. Size(WIDTH, HEIGHT); Chapter 7 106

Window Events and Window Listeners • With a JFrame, a GUI’s close-window button needs to be programmed. • A close-window button generates a window event which is handled by a window listener. • Class Window. Adapter is a window listener, so every class derived from class Window. Adapter is a window listener. Chapter 7 107

Window Events and Window Listeners, cont. • A window listener is registered with a JFrame GUI using method add. Window. Listener. Chapter 7 108

Window Events and Window Listeners, cont. • class Window. Destroyer Chapter 7 109

Window Events and Window Listeners, cont. • A Window. Destroyer object is created and registered as a window listener for our JFrame GUI using Window. Destroyer listener = new Window. Destroyer(); add. Window. Listener(listener); • When the window-close button is clicked, the JFrame GUI ends. Chapter 7 110

The Action. Listener Interface • The Action. Listener interface has only one method heading that must be implemented. public void action. Performed (Action. Event e) • A listener that responds to button clicks in an applet or in a JFrame must implement the Action. Listener interface. Chapter 7 111

Programming Example: Smiley Face as a JFrame • Class JFrame and every class derived from class JFrame has a paint method. • The paint method can be redefined to draw a figure. Chapter 7 112

Programming Example: Smiley Face as a Jframe, cont. • class Happy. Face Chapter 7 113

Programming Example: Smiley Face as a Jframe, cont. • class Show. Happy. Face Chapter 7 114

Programming Example: Smiley Face as a Jframe, cont. Chapter 7 115