INHERITANCE
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
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.
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.
Inheritance Hierarchy
+ computeCourseGrade() : int
+ UndergraduateStudent() : void
+ GraduateStudent() : void
+ computeCourseGrade() : int
+ computeCourseGrade() : int
Definition of GraduateStudent &
UndergraduateStudent classes
class GraduateStudent extends Student {
class UndergraduateStudent extends Student {
//constructor not shown
public void computeCourseGrade() {
int total = 0;
//Constructor not shown
public void computeCourseGrade() {
int total = 0;
total = test1 + test2 + test3;
total = test1 + test2 + test3;
if (total / 3 >= 70) {
courseGrade = "Pass";
} else {
courseGrade = "No Pass";
}
if (total / 3 >= 80) {
courseGrade = "Pass";
} else {
courseGrade = "No Pass";
}
}
}
}
}
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
Inheritance Rules
1. The private members of the superclass are
private to the superclass
2. The subclass can access the members of the
superclass according to the accessibility rules
3. The subclass can include additional data and/or
method members
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
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
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.
INHERITANCE
Data Accessibility
Super class
Package B
int a
public int b
protected int c
private int d
Sub class B
Sub class A
public int b
protected int c
int a
public int b
protected int c
Package A
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.
What’s
wrong with
the code?
How to fix
it?
class ClassX
{
private int m;
public String toString()
{
return new String("(" + m + ")");
}
}
public class ClassY extends ClassX
{
private int n;
public String toString()
{
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);
}
}
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.
• 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;
}
Rectangle myRectangle = new Rectangle(5, 3);
Box myBox = new Box(6, 5, 4);
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:
public A() {
}
public A(double d) {
// some statements
}
public A() {
super();
}
is equivalent to
is equivalent to
public A(double d) {
super();
// some statements
}
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");
}
}