Sonu wiziq

OOP in Java

IT training and classes
Trainer: Sonu

OOP in Java
 Java is fundamentally Object-Oriented
 Every line of code you write in Java must be inside a Class
(not counting import directives)
 Clear use of
 Variables
 Methods
 Re-use through “packages”
 Modularity, Encapsulation, Inheritance, Polymorphism

etc

OOP Vocabulary Review
 Classes



Definition or a blueprint of a
userdefined datatype
Prototypes for objects

 Objects


Nouns, things in the world

 Constructor


Given a Class, the way to create an
Object (that is, an Instance of the
Class) and initialize it

 Attributes


Properties an object has

 Methods


Actions that an object can do

Object
Anything we can put a
thumb on

The Structure of Classes
class name {
declarations

instance variables
and symbolic constants

constructor definitions

how to create and
initialize objects

method definitions
}
These parts of a class can
actually be in any order

how to manipulate those
objects (may or may not
include its own “driver”,
i.e., main( ))

Defining a Class
Comparison with C++
 Java gives you the ability to write classes or user-defined data types

similar to the way C++ does, with a few differences

 Points to consider when defining a class


There are no global variables or functions. Everything resides
inside a class. Remember we wrote our main method inside a
class



Specify access modifiers (public, private or protected ) for each member
method or data members at every line.



No semicolon (;) at the end of class



All methods (functions) are written inline. There are no separate
header and implementation files.

The Point Class
class Point {
private int x;
private int y;
public Point (……) {……}
public void Display (……) {
……….

}

}

instance variables
and symbolic constants
how to create and
initialize objects
how to manipulate those
objects (may or may not
include its own “driver”,
i.e., main( ))

Defining a Class
Comparison with C++ (cont)
 Points to consider when defining a class (cont)


Automatic initialization of class level data members if you do
not initialize them


Primitive types
 Numeric (int, float etc) with zero
 Char with null
 Boolean with false



Object References
 With null



Remember, the same rule is not applied to local variables.
Using a local variable without initialization is a compile time
error.
public void someMethod () {
int x;
//local variable
System.out.println(x); //compile time error
}

Defining a Class
Comparison with C++ (cont)
 Points to consider when defining a class (cont)


Access Modifiers








Constructor







public
: Accessible anywhere by anyone
Private
: Only accessible within this class
Protected : Accessible only to the class itself and to it’s subclasses or
other classes in the same “package”
Package : Default access if no access modifier is provided.
Accessible to all classes in the same package

Same name as class name
Does not have a return type
No initialization list
JVM provides a zero-argument constructor only if a class
doesn’t define it’s own constructor

Destructor


Destructors are not required in a java class

Task - Defining a Class
 Create a class for Student


should be able to store the following
characteristics of student



Roll No
Name



Provide default, parameterized and
copy constructors



Provide standard getters/setters for
instance variables




Make sure, roll no has never assigned a
negative value i.e. ensuring the correct
state of the object

Provide print method capable of
printing student object on console

Student
Attributes:
Roll NO
Name
Methods:
constructors
getters/setters
print

Student Implementation Code
// Student.java
/*
Demonstrates the most basic features of a class. A student is defined
by their name and rollNo. There are standard get/set accessors for
name and rollNo.
NOTE A well documented class should include an introductory
comment like this. Don't get into all the details – just introduce the
landscape.
*/
public class Student {
private String name;
private int rollNo;

Student Implementation Code cont.
// Standard Setters
public void setName (String name) {
this.name = name;
}
// Note the masking of class level variable rollNo
public void setRollNo (int rollNo) {
if (rollNo > 0) {
this.rollNo = rollNo;
}else {
this.rollNo = 100;
}
}

// Standard Getters
public String getName ( ) {
return name;
}

public int getRollNo ( ) {
return rollNo;
}

// Constructor that uses a default value instead of taking an argument.
public Student() {
name = “not set”;
rollNo = 100;
}
// parameterized Constructor for a new student
public Student(String name, int rollNo) {
setName(name);
//call to setter of name
setRollNo(rollNo);
//call to setter of rollNo
}
// Copy Constructor for a new student
public Student(Student s) {
name = s.name;
rollNo = s.rollNo;
}

// method used to display method on console

public void print () {
System.out.println("Student name:" +name+ ", roll no:" +rollNo);
}

} // end of class

Using a Class
Comparison with C++


Objects of a class are always created on heap using the “new”
operator followed by constructor


Student s = new Student () // no pointer operator “*” between
// Student and s



Only String constant is an exception
 String greet = “Hello” ;



// No new operator

However you can use
 String greet2 = new String(“Hello”);



Members of a class ( member variables and methods also known
as instance variables/methods ) are accessed using “.” operator.
There is no “” operator in java



s.setName(“Ali”);
SsetName(“Ali”) is incorrect and will not compile in java

Using a class
Comparison with C++
 Differences from C++ (cont)


Objects are always passed by reference whereas
primitive data types are passed by value.



All methods use the run-time, not compile-time, types
(i.e. all Java methods are like C++ virtual functions)



The types of all objects are known at run-time



All objects are allocated on the heap (always safe to
return objects from methods)

Task - Using Student Class
 Create objects of

Student class by
calling default,
parameterized and
copy constructors.

Student
Attributes:
Roll NO
Name
Methods:
constructors
getters/setters
print

class

ali

 Call Students class

various methods on
objects

Attributes:
Roll NO: 89
Name: ali raza
Methods:
getters/setters
print

object

Student Client Code
public class Test{
public static void main (String args[]){
// Make two students
Student s1 = new Student("ali", 15);
Student s2 = new Student();
//call to default costructor
s1.print();
s2.print();
s2.setName("usman");
s2.setRollNo(20);
System.out.print("Student name:" + s2.getName());
System.out.println(" rollNo:" + s2.getRollNo());
//continue….

Student Client Code
System.out.println("calling copy constructor");
Student s3 = new Student(s2); //call to copy constructor
s2.print();
s3.print();
s3.setRollNo(-10); //Roll No would be set to 100
s3.print();
/*NOTE: public vs. private
A statement like "b.rollNo = 10;" will not compile in a client
of the Student class when rollNo is declared protected or private */
} //end of main
} //end of class

Static
 A class can have static



Variables
Methods

 Static variables and methods



Are associated with the class itself!!
Not associated with the object

 Therefore Statics can be accessed without instantiating an object!
 Generally accessed by class name
 Cannot refer to a non-static instance variable in a static method


No this reference

Static Variable & Methods
 Occurs as a single copy in the class
 For example;

System.out is a static variable
 JOptionPane.showInputDialog(String)


Static Fun
Object: ali
Type: Student
Name: ali raza
Roll No: 5
Methods: getName, setName
getRollNo, setRollNo
toString
Class: Student
countStudents: 2
Method: getCountStudents()

Object: usman
Type: Student
Name: usman shahid
Roll No: 5
Methods: getName, setName
getRollNo, setRollNo
toString

Garbage collection and finalize
 Java performs garbage collection and eliminates the need to

free objects explicitly.
 When an object has no references to it anywhere, except in

other objects that are also unreferenced, its space can be
reclaimed.
 Before the object is destroyed, it might be necessary for the

object to perform some actions.
 For example closing an open file. In such a case define a
finalize() method with the actions to be performed
before the object is destroyed.

finalize
 When a finalize method is defined in a class, Java run time calls

finalize() whenever it is about to recycle an object of that
class.

protected void finalize()
{
// code
}
 A garbage collector reclaims objects in any order or never

reclaim them.

 System.gc()



Request the JVM to run the garbage collector
Not necessary it will run

Memory Mangement
public class Test{
public static void main|(String args[]){
Student s1 = new Student(“ali”);
Student s2 = new Student(“raza”);
s1= s2;
}
}
No Memory leakage in Java, Automatic
Garbage Collection will take care of such
scenarios

Stack

Heap

s1

0F59

0F59

s2
03D2

name

ali

03D2
name

raza

Modify Student Class
public class Student {
…..
private static int countStudents = 0;
public static int getCountStudents() {
return countStudents;
}
…….

// Constructor that uses a default value instead of taking an argument.
public Student() {
name = “not set”;
rollNo = 100;
countStudents += 1;
}
// parameterized Constructor for a new student
public Student(String name, int rollNo) {
setName(name);
//call to setter of name
setRollNo(rollNo);
//call to setter of rollNo
countStudents += 1;
}
// Copy Constructor for a new student
public Student(Student s) {
name = s.name;
rollNo = s.rollNo;
countStudents += 1;
}

// Overridden methods
// Overriding toString method of class java.lang.Object
public String toString () {
return ("name: "+name + "RollNo: " + rollNo);
}
//Overriding finalize method of Object class
protected void finalize () {
countStudents -= 1;
}
} // end of class

Student Client Code
public class Test{
public static void main (String args[]){
int numObjs;
numObjs = Student.getCountStudents();
System.out.println("Students Objects:"+numObjs);
Student s1 = new Student(“Sonu", 15);
System.out.println("Student:" + s1.toString() );

Student Client Code
Student s2 = new Student(“Gaurav", 49);
System.out.println("Student:" +s2);

//implicit call to toString()

s1 = null;
System.gc(); // request the JVM to run the garbage collector But
// there is no gaurantee that garbage collector will run
} //end of main
} //end of class

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