Here are the key points about object equality in Java: - The == operator checks if the objects are the same instance - The equals() method checks if the objects have the same value - By default, equals() only returns true if comparing the same instance - Classes should override equals() to check for value equality instead of reference equality So in this case: - jack1 == jack2 will print false as they are different instances - jack1.equals(jack2) will print false by default, as equals() isn't overridden

1. Object
Orientated
Programming
with
Java
Jussi
Pohjolainen
Tampere
University
of
Applied
Sciences

2. Object
Orientated
Concepts
• Class
• Object
• Inheritance
• Constructors
• Abstract
class
• Interface
• Polymorphism

3. Class
class Student {
private String name;
public Student(String name) {
this.name = name;
}
public void setName(String name) {
this.name = name
}
public void getName() {
return name;
}
}

4. CreaAng
objects
Student jack = new Student(“Jack”);
Student bill = new Student(“Bill”);
System.out.println( jack.getName() );
System.out.println( bill.getName() );

5. Reference?
Student jack = new Student(“Jack”);
Student bill = jack;
jack.setName(“Lisa”);
// What is the output?
System.out.println(bill.getName());

6. Reference
• In
Java,
every
object
is
passed
by
reference
• If
you
want
to
clone
a
object,
you
use
special
techniques
(later
on
the
material)

7. Inheritance
class Person {
private String name;
...
}
class Student extends Person {
private int id;
...
}

8. Constructors
class Person {
private String name;
public Person() {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
public Student() {
System.out.println(“Student”);
}
}
// What is the output?
Student s = new Student();

9. Default
Constructor
• If
programmer
does
not
define
a
constructor,
Java
creates
a
default
constructor:
class Person {
}
=>
class Person {
public Person() {
super();
}
}

10. Default
Constructor
class Person {
private String name;
public Person() {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
}
// What is the output?
Student s = new Student();

11. Default
Constructor
Problem
class Person {
private String name;
public Person(String name) {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
}
// What is the output?
Student s = new Student();

12. Abstract
Class
• You
cannot
create
a
object
from
abstract
class
• Abstract
class
may
contain
abstract
method
• Abstract
method
is
a
method
declaraAon
which
must
be
implemented
in
inherited
classes

13. Abstract
Class
abstract class Graphic {
abstract double calculateSurfaceArea();
}
class Circle extends Graphic {
private int radius;
double calculateSurfaceArea() {
...
}
}

14. Abstract
Class
abstract class A {
abstract void m();
}
abstract class B extends A {
// What is the implementation of the class B?
}

15. Interface
• Interface
is
a
abstract
class
that
contain
only
abstract
methods
• Interface
can
contain
also
public
staAc
final
variables
• Class
can
inherit
only
one
class,
but
it
can
implement
many
interfaces

16. Interface
interface class A {
public void m();
}
class B implements A {
public void m() {
...
}
}

17. Polymorphism
• Declaring
a
object:
– Graphic c;!
• IniAalizing
the
object:
– c = new Graphic();!
• This
is
also
possible:
– c = new Circle();!
– c = new Rect();!
• If
Circle and
Rect are
inherited
from
Graphic!

18. Polymorphism
class Polymorphism {
public static void main(String [] args) {
// What are the possible objects to be passed?
method(??)
}
public static void method(Graphic c) {
...
}
}

19. Polymorphism
interface R {
...
}
class Polymorphism {
public static void main(String [] args) {
// What are the possible objects to be passed?
method(??)
}
public static void method(R r) {
...
}
}

20. Cloning
class Person implements Cloneable {
private name;
public Person(String name) {
this.name = name;
}
public Object clone() {
return new Person(this.name);
}
}
Person a = new Person(“jack”);
Person b = a.clone();

21. Equals
// What happens here?
Student jack1 = new Student(“Jack”);
Student jack2 = new Student(“Jack”);
System.out.println( jack1 == jack2 ); // true or false?
System.out.println( jack1.equals(jack2) ); // ?