Object Oriented Programming - 5. Class & Object

Object Oriented
Programming
Andi Nurkholis, S.Kom., M.Kom.
Study Program of Informatics
Faculty of Engineering and Computer Science
SY. 2019-2020
March 16, 2020

OOP
Everything in OOP is associated with classes
and objects, along with its attributes and
methods.
3

Class
A Class is like an object constructor, or a "blueprint"
for creating objects.
A class describes the characteristics of objects in
general.
4

5
Create a Class
To create a class, use the keyword class:
public class MyClass {
int x = 5;
}
MyClass.java
Create a class named "MyClass" with a variable x:
Keyword class ClassName()

Create an Object
In Java, an object is created from a class. We have already created the
class named MyClass, so now we can use this to create objects.
To create an object of MyClass, specify the class name, followed by the
object name, and use the keyword new:
6
Class ObjectName = new Class();

Example
7
int x = 5;
public static void main(String[] args) {
MyClass myObj = new MyClass();
System.out.println(myObj.x);
}
}
Create an object called "myObj" and print the value of x:

Multiple Objects
8
int x = 5;
MyClass myObj1 = new MyClass(); // Object 1
System.out.println(myObj1.x);
System.out.println(myObj2.x);
}
}
Create two objects of MyClass:

Using Multiple Classes
You can also create an object of a class and access it in another class.
This is often used for better organization of classes (one class has all the
attributes and methods, while the other class holds the main() method
(code to be executed)).
Remember that the name of the java file should match the class name. In
this example, we have created two files in the same directory/folder:
• MyClass.java
• OtherClass.java
9

Example
10
class OtherClass {
}
}
int x = 5;
}

11
Java Class Attributes
In the previous slide, we used the term "variable" for x in the example (as
shown below). It is actually an attribute of the class. Or you could say that
class attributes are variables within a class:
Create a class called "MyClass" with two attributes: x and y:
int x = 5, y = 3;
}

Accessing Attributes
12
int x = 5;
}
}
You can access attributes by creating an object of the class, and by
using the dot syntax (.):

Modify Attributes
You can also modify attribute values:
13
int x;
myObj.x = 40;
}
}

Modify Attributes (cont.)
14
int x = 10;
myObj.x = 25; // x is now 25
}
}
Or override existing values:

15
Final Attributes
If you don't want the ability to override existing values, declare the
attribute as final:
The final keyword is useful when you want a variable to always store the
same value, like PI (3.14159...).
The final keyword is called a "modifier".

Example
16
final int x = 10;
myObj.x = 25; // will generate an error:
cannot assign a value to a final variable
}
}

Multiple Objects
If you create multiple objects of one class, you can change the attribute
values in one object, without affecting the attribute values in the other:
17

Example
18
int x = 5;
myObj2.x = 25;
System.out.println(myObj1.x); // Outputs 5
System.out.println(myObj2.x); // Outputs 25
}
}

19
Multiple Attributes
You can specify as many attributes as you want:
public class Person {
String fname = "John“, lname = "Doe"; int age = 24;
Person myObj = new Person();
System.out.println("Name: " + myObj.fname + " " +
myObj.lname);
System.out.println("Age: " + myObj.age);
}
}

Java Class Methods
20
A method is a block of code which only runs when it is called.
You can pass data, known as parameters, into a method.
Methods are used to perform certain actions, and they are also known
as functions.
Why use methods? To reuse code: define the code once, and use it
many times.

Example
Create a method named myMethod() in MyClass:
21
static void myMethod() {
System.out.println("Hello World!");
}
myMethod();
}
}
// Outputs "Hello World!"

Static vs. Non-Static
22
You will often see Java programs that have either static or public
attributes and methods.
In the example above, we created a static method, which means that it
can be accessed without creating an object of the class, unlike public,
which can only be accessed by objects:

23
Example
// Static method
static void myStaticMethod() {
System.out.println("Static methods can be called
without creating objects");
}
// Public method
public void myPublicMethod() {
System.out.println("Public methods must be called
by creating objects");
}

Example (cont.)
24
// Main method
myStaticMethod(); // Call the static method
// myPublicMethod(); This would compile an error
MyClass myObj = new MyClass(); // Create an
object of MyClass
myObj.myPublicMethod(); // Call the public
method on the object
}
}

Access Methods With an Object
Create a Car object named myCar. Call the fullThrottle() and speed()
methods on the myCar object:
25
// Create a Car class
public class Car {
// Create a fullThrottle() method
public void fullThrottle() {
System.out.println("The car is going as fast
as it can!");
}

Example (cont.)
26
// Create a speed() method and add a parameter
public void speed(int maxSpeed) {
System.out.println("Max speed is: " +
maxSpeed);
}

27
Example (cont.)
// Inside main, call the methods on the myCar object
Car myCar = new Car(); // Create a myCar object
myCar.fullThrottle(); // Call the fullThrottle() method
myCar.speed(200); // Call the speed() method
}
}
// The car is going as fast as it can!
// Max speed is: 200

Using Multiple Classes
28
Like we specified in previous slide, it is a good practice to create an
object of a class and access it in another class.
Remember that the name of the java file should match the class name.
In this example, we have created two files in the same directory:
• Car.java
• OtherClass.java

Example
29
public class Car {
public void fullThrottle() {
System.out.println("The car is going as fast as
it can!");
}
public void speed(int maxSpeed) {
System.out.println("Max speed is: " + maxSpeed);
}
}

Example (cont.)
30
class OtherClass {
Car myCar = new Car(); // Create a myCar object
myCar.fullThrottle(); // Call the fullThrottle() method
myCar.speed(200); // Call the speed() method
}
}

Thank You, Next …
Relation between Classes
Study Program of Informatics
Faculty of Engineering and Computer Science
SY. 2019-2020
Andi Nurkholis, S.Kom., M.Kom.
March 16, 2020

Object Oriented Programming - 5. Class & Object

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