Chapter 9 Oriented Programming Inheritance

Chapter 9: Oriented Programming: Inheritance • Inheritance – – It’s a form of Software reuse in which a new class is created from an existing class by: • Absorbing existing class’s data and behaviors • Enhanced with new capabilities A Subclass extends a superclass • • Subclass – More specialized group of objects – Has behaviors inherited from the superclass – Has additional behaviors Class hierarchy – Direct superclass : Inherited explicitly (one level up hierarchy) – Indirect superclass : Inherited two or more levels up hierarchy – Single inheritance • – Inherits from one superclass Multiple inheritance • Inherits from multiple superclasses – • Java does not support multiple inheritance Superclasses and subclasses – Object of one class “is an” object of another class • – Example: A Car is a Vehicle. – Class Car inherits from class Vehicle – Vehicle: superclass – Car: subclass Superclass typically represents larger set of objects than subclasses 97 • protected access • Intermediate level of protection between public and private • A superclass’s protected members can be accessed by • • the superclass members • its subclass members • Class members in the same package Subclass access to superclass members • Keyword super and a dot (.) • Methods of a subclass cannot directly access private members of their superclass. A subclass can change the state of private superclass instance variables only through non-private methods provided in the superclass and inherited by the subclass. • Superclass and subclass relationship • Example: CommissionEmployee/BasePlusCommissionEmployee inheritance hierarchy • CommissionEmployee – • • First name, last name, SSN, commission rate, gross sale amount BasePlusCommissionEmployee – First name, last name, SSN, commission rate, gross sale amount – Base salary Class CommissionEmployee • Extends class Object • Every class in Java extends an existing class, except Object class • Every class inherits Object’s methods • New class implicitly extends Object – If it does not extend another class 98 • The Java compiler sets the superclass of a class to Object when the class declaration does not explicitly extend a superclass. • Copying and pasting code from one class to another can spread errors across multiple source code files. To avoid duplicating code (and possibly errors), use inheritance, rather than the “copy-and-paste” approach, in situations where you want one class to “absorb” the instance variables and methods of another class. • With inheritance, the common instance variables and methods of all the classes in the hierarchy are declared in a superclass. When changes are required for these common features, software developers need only to make the changes in the superclass—subclasses then inherit the changes. Without inheritance, changes would need to be made to all the source code files that contain a copy of the code in question. • Class BasePlusCommissionEmployee • Extends class CommissionEmployee • Has instance variable baseSalary • Inherits public and protected members from class CommissionEmployee • The Constructor is not inherited • A compilation error occurs if a subclass constructor calls one of its superclass constructors with arguments that do not match exactly the number and types of parameters specified in one of the superclass constructor declarations. • When a superclass method is overridden in a subclass, the subclass version often calls the superclass version to do a portion of the work ( prefix the superclass method name with the keyword super and a dot (.) separator). • When a program creates a subclass object, the subclass constructor immediately calls the superclass constructor (explicitly, via super, or implicitly). The superclass constructor’s body executes to initialize the superclass’s instance variables that are part of the subclass object, then the subclass constructor’s body executes to initialize the subclass-only instance variables. • Java ensures that even if a constructor does not assign a value to an instance variable, the variable is still initialized to its default value (e.g., 0 for primitive numeric types, false for booleans, null for references). 99  Example: public class CommissionEmployee { private String name; private double grossSales; private double commissionRate; //extends Object public CommissionEmployee( String n, double sales, double rate ) { //super(); name = n; setGrossSales( sales ); setCommissionRate( rate ); System.out.println("CommissionEmployee Constructor"); } public String getName() { return name; } public void setName(String n){ name = n; } public void setGrossSales( double sales ) { grossSales = ( sales < 0.0 ) ? 0.0 : sales; } public double getGrossSales() { return grossSales; } public void setCommissionRate( double rate ) { commissionRate = ( rate > 0.0 && rate < 1.0 ) ? rate : 0.0; } public double getCommissionRate(){ return commissionRate; } public double earnings() { return commissionRate * grossSales; } public String toString() { return String.format( "commission employee: %s\ngross sales: %.2f\ncommission rate: %.2f", name, grossSales, commissionRate ); } } 100 public class BasePlusCommissionEmployee extends CommissionEmployee { private double baseSalary; public BasePlusCommissionEmployee( String name1, double sales, double rate, double salary ){ super( name1, sales, rate ); setBaseSalary( salary ); System.out.printf( "BasePlusCommissionEmployee constructor:\n%s\n", this ); } public void setBaseSalary( double salary ) { baseSalary = ( salary < 0.0 ) ? 0.0 : salary; } public double getBaseSalary() { return baseSalary; } public double earnings() { return getBaseSalary() + getCommissionRate()*getGrossSales(); //return getBaseSalary() + super.getCommissionRate()* super.getGrossSales(); // return getBaseSalary() + super.earnings(); // not allowed to write: return getBaseSalary() + commissionRate*grossSales; } public String toString() { return String.format( "%s\n%s: %.2f", super.toString(), "base salary", getBaseSalary() ); }} public class BasePlusCommissionEmployeeTest { public static void main( String args[] ) { CommissionEmployee employee1 = new CommissionEmployee( "Jone", 2000, .02 ); System.out.println( "\nEmployee1 information obtained by get methods: " ); System.out.printf( "Employee name is: %s\n", employee1.getName() ); System.out.printf( "Earnings is %.2f\n\n", employee1.earnings() ); // Error to write //System.out.printf( "Base Salary is %.2f\n", employee1.getBaseSalary() ); BasePlusCommissionEmployee employee = new BasePlusCommissionEmployee( "Bob", 5000, .04, 300 ); System.out.println( "Employee information obtained by get methods: \n" ); System.out.printf( "Employee name is: %s\n", employee.getName() ); System.out.printf( "Base Salary is %.2f\n", employee.getBaseSalary() ); System.out.printf( "Earnings is %.2f\n", employee.earnings() ); employee.setBaseSalary( 1000 ); // set base salary System.out.printf( "\nUpdated employee information obtained by toString:\n\n%s\n", employee.toString() ); }} 101 public class Box{ private double width; private double height; private double depth; public Box(double w, double h, double d){ width = w; height = h; depth = d; System.out.println("In Box constructor"); } public double getW(){return width;} public double getH(){return height;} public double getD(){return depth;} public double getVolume(){ return width*height*depth; } public String toString(){ return String.format("width is: "+width+" Height is: "+height+" Depth is: "+depth); }} public class MatchBox extends Box{ double weight; public MatchBox(double w, double h, double d, double m) { super(w,h,d); weight = m; System.out.println("MatchBox Constructor"); } public double getWeight(){ return weight; } public String toString(){ return String.format(super.toString()+" weight = "+weight); } public static void main(String args[]) { MatchBox mb1 = new MatchBox(4,6,8,12); Box b1 = new Box(3,5,10); System.out.println("call toString in MatchBox\n"+mb1); System.out.println("Volume is : "+mb1.getVolume()); System.out.println("width of MatchBox is " + mb1.getW()); System.out.println("width of Box is " + b1.getW()); System.out.println("weight of MatchBox is " + mb1.weight); //mb1.getWeight(); // b1.getWeight(); error System.out.println("call to String in Box\n"+b1); }} 102