Unit 01 1

Inheritance

• Recall What Inheritance is About

• The extends Keyword

• The Object Class

• Overriding versus Overloading

• What is Actually Inherited?

• Single vs. Multiple Inheritance

• Up-Casting

Unit 01 2

Recall What Inheritance is About

• Inheritance allows a software developer to derive a new class from an existing one. –Reusability

• The existing class is called the parent class, base class or super class.

• The new class is called the child class, derived class or subclass.

• As the name implies, the child inherits characteristics of the parent.

• That is, the child class inherits the methods and data defined in the parent class.

• Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class.

• Inheritance creates an is-a relationship, meaning the child is a more specific version of the parent.

Student

GraduateStudent

defendThesis()

Unit 01 3

The extends Keyword• In Java, the keyword extends is used to establish an inheritance.

1. public class Student{2. private String name;3. private int id;4. private double gpa;5.6. public Student(int id, String name, double gpa){...}7. public Student(int id, double gpa){...}8. public String getName(){...}9. public int getId(){...}10. public double getGPA(){...}11. public String toString(){...}12. public void setGPA(double newGPA){...}13. }

1. public class GraduateStudent extends Student{2. private String thesisTitle;3. public GraduateStudent(...){...}4. public String toString(){...} // overridden5. public void defendThesis(){...} // added6. }

Unit 01 4

The Object Class

• A class called Object is defined in the java.lang package of the Java

standard class library.

• If a class is not explicitly defined to be the child of an existing class, it is

assumed to be the child of the Object class.

• Thus, the Object class is the ultimate parent of all classes.

• The Object class contains a few useful methods, toString, equals, etc.,

which are inherited by all classes.

• The toString method in the Object class is defined to return the name of the

object's class and its location in the memory, while the equals method

determines if two references are aliases.

• Although all objects are guaranteed to have a toString and equals methods,

it is often desirable to override these methods when we write our classes.

Unit 01 5

Overriding Methods• A child class can override a method it inherited from its super class.• The new method must have the same signature as the parent's method.

1. public class Student{2. private String name;3. private int id;4. private double gpa;5. //...6. public String toString(){7. return "ID: "+id+“ Name: "+name+“ GPA: "+gpa;8. }9. //...10. }

1. public class GraduateStudent extends Student{2. private String thesisTitle;3. //...4. public String toString() { // overridden5. return super.toString()+“ Thesis Title: "+thesisTitle;6. }7. //...8. }

Unit 01 6

Overloading vs Overriding

• Comparison between overloading and overriding

Overriding deals with two methods, one in a parent class and one in a child class with the same signature

Overriding lets you define a similar operation in different ways for different object types

Overloading deals with multiple methods in the same class with the same name but different signatures.

Overloading lets you define a similar operation in different ways for different data

Unit 01 7

Constructors are not Inherited

• Constructors are not inherited, even though they have public visibility.• However, the super reference can be used within the child's constructor to

call the parent's constructor.• In that case, the call to parent's constructor must be the first statement.

1. public class GraduateStudent extends Student{2. private String thesisTitle;3. //...4. public GraduateStudent(int id, String name, double gpa, String title){5. super(id, name, gpa);6. thesisTitle = title;7. }8. //...9. }

• If super is not used to call the parent's constructor, the compiler inserts a call to the parent's default constructor. An error results if no such default constructor exists.

Unit 01 8

What then is Inherited• Fields and Methods are inherited if their access modifiers allow.

• public variables and methods are inherited, but private ones are not.

• When an instance of a subclass is created, so is that of its super class.

GraduateStudent s = new GraduateStudent(202020, “Usman”, 3.7, “JavaMania”);

Unit 01 9

The protected Visibility Modifier

• We saw that fields that are declared private cannot be inherited.• We also know that it is a very bad practice to declare fields as public. So is there a compromise? Yes, protected modifier.

1. public class Student{2. protected String name;3. protected int id;4. protected double gpa;5. //...6. }

1. public class GraduateStudent extends Student{2. private String thesisTitle;3. //...4. public String toString(){5. return "ID: "+id+"Name: "+name+"GPA: "+gpa+"ThesisTitle: "+thesisTitle;6. }7. //...8. }

Unit 01 10

Single vs. Multiple Inheritance

• Multiple inheritance allows a class to be derived directly from two or more classes, inheriting the members of all parents.

• Collisions, such as the same variable name in two parents, have to be resolved - leading to complex code.

• To avoid this complexity, Java supports only single inheritance, meaning that a derived class can have only one direct parent class.

Student

GraduateStudent

defendThesis()

Object

Unit 01 11

Class Hierarchy

• A child class of one parent can be the parent of another child, forming class hierarchies.

• Two children of the same parent are called siblings - Example, Faculty and Staff.

• Good class design puts all common features as high in the hierarchy as is reasonable

Unit 01 12

Up-Casting

• An object reference can refer to an object of its class, or to an object of any class related to it by inheritance

• For example, if the Employee class is used to derive a class, Faculty, then an Employee reference can be used to point to a Faculty object.

• This assigning is called up-casting, because we are going up the inheritance tree.

• Up-casting is considered to be a widening conversion since the subclass object has more methods than its superclass.

Unit 01 13

Why Up-Casting

• The advantage of up-casting is that some methods apply to a superclass and all its subclasses.

• Such a method can be designed to accept a superclass reference so that it can be called with a superclass object or subclass object.

• For example, a method that generates pay-slip for all employees may have the following heading:

• Now since the method is expecting Employee as argument, we can call it with an object of MonthlyEmployee or HourlyEmployee.

1. MonthlyEmployee secretary = new MonthlyEmployee("Al-Amir", 8000);

2. HourlyEmployee consultant = new HourlyEmployee("Umar", 500);

3. //...

4. generatePaySlip(secretary);

5. generatePaySlip(consultant);

Unit 01 14

Exercises

Write each of the following classes:

1. A class, Person, with an instance variable, name. Write appropriate accessor and mutator methods for the class. Also write a toString method for the class.

2. Employee, that extends Person, and has: two additional instance variables, ID and payRate; and an additional method, pay, that returns the payRate. Override the toString method accordingly.

3. Faculty, that extends Employee, and has two additional variables; officeHours of type String and teachingCourses, which is an array of Courses. Each course has course code and course title. Write appropriate accessor, mutator and toString methods for the class.

4. TestPersons, that has two methods: void printPerson(Person p) that prints its argument. And a main method that creates an instance of each of the above classes and prints it using printPerson method.

Unit 01 15

Exampleclass A {

String a = "A String";

String method1(){

return "method1() from class A.";

}

static String method2(){

return "method2() from class A.";

}

}

class ShadowingVsOverriding extends A {

double a = 7;

String method1(){

return "method1() from class ShadowingVsOverriding.";

}

static String method2(){

return "method2() from class ShadowingVsOverriding.";

}

Unit 01 16

Example – Cont’d

public static void main(String args[]){

//A obj = new ShadowingVsOverriding();

ShadowingVsOverriding obj = new ShadowingVsOverriding();

System.out.println(obj.a);

System.out.println(obj.method1());

System.out.println(obj.method2());

}

7.0

method1() from class ShadowingVsOverriding.

method2() from class ShadowingVsOverriding.

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