1. 1 Inheritance Chapter 9

2. 2 Module Outcomes To develop a subclass from a superclass through inheritance To invoke the superclass ’ s constructors and methods using the super keyword To override methods in the subclass To explore the useful methods (equals(Object), hashCode(), toString(), finalize(), clone(), and getClass()) in the Object class To comprehend polymorphism, dynamic binding, and generic programming

3. 3 Module Outcomes To describe casting and explain why explicit downcasting is necessary To understand the effect of hiding data fields and static methods To restrict access to data and methods using the protected visibility modifier To declare constants, unmodifiable methods, and nonextendable class using the final modifier

4. 4 Superclasses and Subclasses UML Diagram

5. 5 // Cylinder.java: Class definition for describing Cylinder public class Cylinder extends Circle { private double length = 1; /** Return length */ public double getLength() { return length; } /** Set length */ public void setLength(double length) { this.length = length; } /** Return the volume of this cylinder */ public double findVolume() { return findArea() * length; } supertype subtype

6. 6 Cylinder cylinder = new Cylinder(); System.out.println("The length is " + cylinder.getLength()); System.out.println("The radius is " + cylinder.getRadius()); System.out.println("The volume of the cylinder is " + cylinder.findVolume()); System.out.println("The area of the circle is " + cylinder.findArea()); The length is 1.0 The radius is 1.0 The volume of the cylinder is 3.14159 The area of the circle is 3.14159 The output is

7. 7 Using the Keyword super To call a superclass constructor To call a superclass method The keyword super refers to the superclass of the class in which super appears. This keyword can be used in two ways:

8. 8 CAUTION: to call superclass constructor from subclass Must use super Cannot call a superclass constructor’s name in a subclass – syntax error statement that uses the keyword super must appear first in the constructor.

9. 9 NOTE Properties and methods of superclass are inherited in subclass Superclass's constructors are NOT inherited in the subclass Superclass's constructors can only be invoked from the subclasses' constructors, using super. If the keyword super is not explicitly used, the superclass's no-arg constructor is automatically invoked.

10. 10 Superclass’s Constructor Is Always Invoked A constructor may invoke an overloaded constructor or its superclass’s constructor. If none of them is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example, Constructor Chaining

11. 11 Run project CircleCylinder Study the code and design various tests

12. 12 How is inheritance made possible? Constructor chaining

13. 13 Constructing an instance of a class invokes all the superclasses’ constructors along the inheritance chain. This is called constructor chaining. Example: Constructor Chaining

14. 14 Constructor Chaining public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { // no overloaded constructor nor super(), so call super(); System.out.println("Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { // calls overloaded constructor, no super() added this("Invoke Employee’s overloaded constructor"); System.out.println("Employee's no-arg constructor is invoked"); } public Employee(String s) { // statement only, call super(); System.out.println(s); } class Person { public Person() { // statement only. Insert super(); which does nothing System.out.println("Person's no-arg constructor is invoked"); }

15. 15 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 1. Start from the main method animation

16. 16 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 2. Invoke Faculty constructor animation

17. 17 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 3. Invoke Employee’s no- arg constructor animation

18. 18 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 4. Invoke Employee(String) constructor animation

19. 19 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 5. Invoke Person() constructor animation

20. 20 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 6. Execute println animation

21. 21 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 7. Execute println animation

22. 22 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 8. Execute println animation

23. 23 Trace Execution public class Faculty extends Employee { public static void main(String[] args) { new Faculty(); } public Faculty() { System.out.println("(4) Faculty's no-arg constructor is invoked"); } class Employee extends Person { public Employee() { this("(2) Invoke Employee’s overloaded constructor"); System.out.println("(3) Employee's no-arg constructor is invoked"); } public Employee(String s) { System.out.println(s); } class Person { public Person() { System.out.println("(1) Person's no-arg constructor is invoked"); } 9. Execute println animation

24. 24 Order of invoking constructors See BJ_Faculty_0

25. 25 Example on the Impact of a Superclass without no-arg Constructor public class Apple extends Fruit { } class Fruit { public Fruit(String name) { System.out.println("Fruit's constructor is invoked"); } Find out the errors in the program: Will the Apple class compile? Run: BJ_Apple_Fruit

26. 26 Check projects BJ_Apple_Fruit: Not compiled –Fruit: 1-arg constructor –Apple: no constructor BJ_Apple_Fruit2: Not compiled –Fruit: 1-arg constructor –Apple: 0-arg constructor BJ_Apple_Fruit3: compiled –Fruit: 0-arg constructor, 1-arg constructor –Apple: 0-arg constructor BJ_Apple_Fruit4: compiled –Fruit: 1-arg constructor –Apple: super(1-arg)

27. 27 Declaring a Subclass A subclass extends properties and methods from the superclass. You can also: F Add new properties F Add new methods F Override the methods of the superclass Question: Can we treat a subclass as a subset of its parent class?

28. 28 Overriding Methods in the Superclass A subclass inherits methods from a superclass. Sometimes it is necessary for the subclass to modify the implementation of a method defined in the superclass. This is referred to as method overriding. // Cylinder.java: New cylinder class that overrides the findArea() // method defined in the circle class. public class Cylinder extends Circle { /** Return the surface area of this cylinder. The formula is * 2 * circle area + cylinder body area */ public double findArea() { return 2 * super.findArea() + 2 * getRadius() * Math.PI * length; } // Other methods are omitted }

29. 29 NOTE An instance method can be overridden only if it is accessible. private method m() in superclass private method m() in subclass cannot override the m() in superclass The two m() are unrelated

30. 30 Examples Class A and subclass B both contain method show() –Method_Overriding project Show() in B overrides show() in A How to call the show() method of A from B? To_Bypass_Overriding project Show() in B calls show() in A by calling super.show()

31. 31 NOTE How about static methods in superclass? a static method can be inherited a static method cannot be overridden If you redefine a static method in a subclass, the method defined in the superclass is hidden. See staticMethodOverride.java

32. 32 Check: BJ_Method_Overriding BJ_Method_Overriding1 BJ_Method_Overriding2 Overriding_vs_Overloading

33. 33 The Object Class All Java classes descend from the java.lang.Object class. If no inheritance is specified when a class is defined, the superclass of the class is Object.

34. 34 The toString() method in Object The toString() method returns a string representation of the object. Default implementation returns: a string with class name of the object is an instance, @, and a number representing this object. Ex: Cylinder myCylinder = new Cylinder(5.0, 2.0); System.out.println(myCylinder.toString()); displays something like Cylinder@15037e5. - not very helpful or informative. - should override the toString method to a more meaning string