Download SEF_COJ_5_Inheritance_Ver2

Software Engineering
Module: Core of Java
Topic: Inheritance
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Inheritance
By the end of this session, you will be able to:
• Define inheritance
• Explain the need for Inheritance
• Write Java code to create classes and subclasses in
an inheritance hierarchy
• Explain constructor calling chain
• Use “super” keyword
• Create hierarchy of classes and use them
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2
Inheritance
Write a java program for the following
specifications
• There will be three shapes - Square, Triangle
and Circle.
• When the user selects a shape the shape rotates
clockwise 3600 if it is png.
EXERCISE
Change In Specification:
• Add one more Shape – amoeba
• When user selects amoeba it rotates only if it is a jpg image
One more change:
The amoeba rotation point should be at the corner not centre.
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Inheritance
OO Thinking
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Inheritance
Inheritance is the concept of a child class (sub
class) automatically inheriting the variables and
methods defined in its parent class (super class).
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Inheritance
Deriving a Subclass
To derive a child class, we use the extends keyword.
Example: Suppose we have a parent class called Person.
Then a subclass Student can be created as . . .
public class Person {
protected String name;
protected String address;
/**
* Default constructor
*/
public Person(){
System.out.println(“Inside
Person:Constructor”);
name = "";
address = "";
}
. . . .
}
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public class Student extends Person
{
public Student(){
System.out.println(“Inside
Student:Constructor”);
}
. . . .
}
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Inheritance
Create classes for Shapes activity specifications as
follows and complete the program.
• Superclass Shape with methods rotate() and
playSound().
• Subclasses Square, Circle, Triangle, Amoeba.
• Then create a main class and then use the above
class to complete the previous Activity.
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EXERCISE
7
Inheritance
What one can do in a Sub-class regarding Attributes
The inherited attributes can be used directly, just like
any other attributes.
You can declare new attributes in the subclass that are
not in the super class.
You can declare an attribute in the subclass with the
same name as the one in the super class, thus hiding it
(not recommended).
A subclass does not inherit the private members of its
parent class. However, if the super class has public or
protected methods for accessing its private fields, these
can also be applied by the subclass
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Inheritance
What one can do in a Sub-class regarding Methods
The inherited methods can be used directly as they are.
You can declare new methods in the subclass that are
not in the super class.
You can write a new instance method in the subclass
that has the same signature as the one in the super
class, thus overriding it.
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Inheritance
Object Class
Object class is mother of all classes in java .
Following are some of the important methods of the Object class:
getClass()
equals()
toString()
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Inheritance
Need for “super”
Let us consider the following code snippet
class A {
int a1,a2,a3;
public A(int m1, int m2,
int m3){
a1 = m1;
a2 = m2;
a3 = m3;
}
}
class B extends A {
int a4,a5;
public B(int x1, int x2,
int x3, int x4, int x5) {
a1=x1;
a2=x2;
a3=x3;
a4=x4;
a5=x5;
}
}
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class B extends A{
int a4,a5;
public B(intx1,intx2,intx3,
intx4,intx5) {
super(x1,x2,x3);
a4=x4;
a5=x5;
}
}
Calls the super
class
constructor
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Inheritance
What is “super” keyword?
A subclass can explicitly call a constructor of its immediate
super class using the super constructor call e.g.: super().
A super constructor call in the constructor of a subclass
will result in the execution of relevant constructor from the
super class, based on the arguments passed.
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Inheritance
Few things to remember when using the super constructor call:
The super() call must occur as the first statement in a
constructor.
The super() call can only be used in a constructor (not in
ordinary methods).
Another use of super is to refer to members of the super class
(just like the this reference ).
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Inheritance
How Constructor Method of a Super Class gets Called
A subclass constructor invokes the constructor of the superclass
implicitly.
When an object of the Student class (subclass) is instantiated,
the default constructor of its super class i.e. Person, is invoked
implicitly before the statements in the constructor method of
the subclass are executed.
Used explicitly when passing parameters to the constructor of
the super class.
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Inheritance
Constructor Calling Chain
Carnivores
Mammal
Animal
class Carnivores
extends Mammal
class Mammal
extends Animal
class Animal
If an object of Carnivores is created as
Carnivores carniv = new Carnivores();
Constructor of
Carnivores
Constructor of
Mammal
Constructor of
Animal
{
System.out.println
(“Carnivores
executed last");
{
System.out.printl
n(“Mammal
executed
second");
{
System.out.println
(" Animal
executed first.");
}
}
}
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Inheritance
Constructor Calling Chain
public class Animal {
public Animal()
{
System.out.println
(“Animal executed
first.");
}
}
public class Mammal
extends Animal {
public Mammal()
{
System.out.println
(“Mammal executed
second");
}
}
public class Carnivore
extends Mammal{
public Carnivore()
{
System.out.println
("Carnivores
executed last");
}
}
Output :
Animal executed first.
Mammal executed second
Carnivores executed last
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Inheritance
Java compiler and “super” keyword
The Java compiler automatically inserts the necessary constructor
calls in the process of constructor chaining, or we can do it
explicitly.
The Java compiler inserts a call to the parent constructor (super)
if we don't have a constructor call as the first statement of our
constructor.
Example:
public Point(int x, int y) {
super(); // Automatically done if you don't call constructor here.
m_x = x;
m_y = y;
}
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Inheritance
Default Constructor
class A{
int x1;
public A(int x){
x1=x;
}
}
class Test{
public static void
main(String args[]){
A oa= new A();
}
}
class Test{
public static void
main(String args[]){
A oa= new A(10);
}
}
Note : The java compiler does not insert default constructor into
class A when there is already a user defined constructor.
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Inheritance
Execute the following code
class SuperClass{
public SuperClass(){
System.out.println(“In SuperClass”);
}
}
class SubClass extends SuperClass{
public SubClass( int att1){
System.out.println("In SubClass");
}
}
class SubSubClass extends SubClass{
public SubSubClass(){
System.out.println("In SubSubClass");
}
}
class MainClass{
public static void main(String args[]){
new SubSubClass();
}
}
class SuperClass{
public SuperClass(){
System.out.println(“In SuperClass”);
}
}
class SubClass extends SuperClass{
public SubClass( int att1){
System.out.println("In SubClass");
}
}
class SubSubClass extends SubClass{
public SubSubClass(){
super(10);
System.out.println("In SubSubClass");
}
}
class MainClass{
public static void main(String args[]){
new SubSubClass();
}
}
When the super class constructor is overloaded, it must be called explicitly.
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Inheritance
Create a class Employee and the sub classes
Manager and Clerk,
Employee:
Instance variables: name, empId, salary.
Methods: set and get methods for name,
empId, getSalary,
setSalary --- Method
Manager:
Instance variables: type
Methods: setSalary()
Clerk:
Instance variables : int speed , int accuacy
Methods: setSalary()
EXERCISE
Provide proper constructors for all classes.
Create a general class “MyClass”. In this clas,s create objects of
Manager, Clerk and Employee class and then set the name, empId
and salary attributes for each object and display them.
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Inheritance
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