1. Intro to OOP with Java, C. Thomas Wu
INHERITANCE
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2. Defining Classes with Inheritance
Intro to OOP with Java, C. Thomas Wu
Defining Classes with Inheritance
Case Study:
Suppose we want implement a class roster that contains both undergraduate and graduate students.
Each student’s record will contain his or her name, three test scores, and the final course grade.
The formula for determining the course grade is different for graduate students than for undergraduate students.
Undergrads: pass if avg test score >= 70
Grads: pass if avg test score >= 80
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3. Modeling Two Types of Students
Intro to OOP with Java, C. Thomas Wu
Modeling Two Types of Students
There are two ways to design the classes to model undergraduate and graduate students.
We can define two unrelated classes, one for undergraduates and one for graduates.
We can model the two kinds of students by using classes that are related in an inheritance hierarchy.
Two classes are unrelated if they are not connected in an inheritance relationship.
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4. Classes for the Class Roster
Intro to OOP with Java, C. Thomas Wu
Classes for the Class Roster
For the Class Roster sample, we design three classes:
Student
UndergraduateStudent
GraduateStudent
The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects.
The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects.
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5. Inheritance Hierarchy
Intro to OOP with Java, C. Thomas Wu
Inheritance Hierarchy
+ computeCourseGrade() : int
+ UndergraduateStudent() : void
+ GraduateStudent() : void
+ computeCourseGrade() : int
+ computeCourseGrade() : int
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6. Definition of GraduateStudent & UndergraduateStudent classes
Intro to OOP with Java, C. Thomas Wu
Definition of GraduateStudent & UndergraduateStudent classes
class GraduateStudent extends Student { //constructor not shown
public void computeCourseGrade() { int total = 0; total = test1 + test2 + test3; if (total / 3 >= 80) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } } }
class UndergraduateStudent extends Student { //Constructor not shown
public void computeCourseGrade() { int total = 0; total = test1 + test2 + test3; if (total / 3 >= 70) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } } }
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7. Intro to OOP with Java, C. Thomas Wu
Declaring a Subclass
A subclass inherits data and methods from the superclass. In the subclass, you can also:
Add new data
Add new methods
Override the methods of the superclass
Modify existing behaviour of parent
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8. Intro to OOP with Java, C. Thomas Wu
Inheritance Rules
The private members of the superclass are private to the superclass
The subclass can access the members of the superclass according to the accessibility rules
The subclass can include additional data and/or method members
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9. Inheritance Rules (continued)
Intro to OOP with Java, C. Thomas Wu
Inheritance Rules (continued)
4. The subclass can override, that is, redefine the methods of the superclass
The overriding method in subclass must have similar
Name
Parameter list
Return type
5. All members of the superclass are also members of the subclass
Similarly, the methods of the superclass (unless overridden) are also the methods of the subclass
Remember Rule 1 & 2 when accessing a member of the superclass in the subclass
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10. Intro to OOP with Java, C. Thomas Wu
Inheritance Rules (continued)
6. (Using the Keyword super)
The keyword super refers to the direct superclass of a subclass . This keyword can be used in two ways:
To call a superclass constructor
super(); //must be the first statement in subclass’s constructor
To call a superclass method
super.methodname();
this is only used if the subclass overrides the superclass method
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11. (Accessibility Modifier)
Intro to OOP with Java, C. Thomas Wu
INHERITANCE
(Accessibility Modifier)
Sometimes , it is called visibility modifier
Not all properties can be accessed by sub class.
Super class can control a data accessing from subclass by giving the type of accessing to the members and methods.
A class can declare the data members or method as a public, private or protected.
If it is not declared, the data or method will be set to default type.
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12. Intro to OOP with Java, C. Thomas Wu
INHERITANCE
Data Accessibility
Super class
int a
public int b
protected int c
private int d
Package A
Package B
Sub class B
public int b
protected int c
Sub class A
int a
public int b
protected int c
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13. Intro to OOP with Java, C. Thomas Wu
Refer to the previous slide
Super class has 2 subclasses : Subclass A and Subclass B.
Subclass A is defined in same package with superclass, subclass B is defined outside the package.
There are 4 accessibility data types: public, protected, private and default.
Subclass A can access all properties of superclass except private.
But, subclass B can only access the properties outside the package which are public and protected.
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14. Intro to OOP with Java, C. Thomas Wu
class ClassX {
private int m;
public String toString()
return new String("(" + m + ")"); }
public class ClassY extends ClassX
private int n;
return new String("(" + m + " , " + n + ")");
class TestAccesibility
public static void main(String [] args)
ClassX x = new ClassX;
ClassY y = new ClassY;
System.out.println("x = " + x);
System.out.println("y = " + y);
What’s wrong with the code?
How to fix it?
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15. Inheritance and Constructors
Intro to OOP with Java, C. Thomas Wu
Inheritance and Constructors
Unlike members of a superclass, constructors of a superclass are not inherited by its subclasses.
You must define a constructor for a class or use the default constructor added by the compiler.
The statement
super();
calls the superclass’s constructor.
super(); must be the first statement in the subclass contructor.
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16. Intro to OOP with Java, C. Thomas Wu
A call to the constructor of the superclass must be in the first statement in the child constructor.
public Box(double l, double w, double h) {
super(l,w);
height = h; }
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17. Intro to OOP with Java, C. Thomas Wu
Rectangle myRectangle = new Rectangle(5, 3);
Box myBox = new Box(6, 5, 4);
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18. Superclass’s Constructor Is Always Invoked
Intro to OOP with Java, C. Thomas Wu
Superclass’s Constructor Is Always Invoked
A subclass constructor may invoke its superclass’s constructor. If none is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example, the constructor of class A:
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19. Example on the Impact of a Superclass without no-arg Constructor
Intro to OOP with Java, C. Thomas Wu
Example on the Impact of a Superclass without no-arg Constructor
Find out the error in the program:
class Fruit {
public Fruit(String name) {
System.out.println("Fruit constructor is invoked"); }
public class Apple extends Fruit {
public Apple(String name) {
System.out.println(“Apple constructor is invoked");
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