03 Java Language And OOP Part III

Java Language and OOP
Part III
By Hari Christian

Agenda
• 01 Class - Definition
• 02 Create and Declare a Class
• 03 Object - Definition
• 04 Create and Declare an Object
• 05 Constructor - Basic
• 06 Constructor - Overload

Agenda
• 07 Method - Basic
• 08 Method - Passing by Value
• 09 Method - Passing by Reference
• 10 Method - Overload
• 11 Return Statement
• 12 Package
• 13 Import
• 14 Organizing Statement
• 15 Access Modifier

Class - Definition
• When you write code in Java, you are writing
classes or interfaces
• Within those classes, as you know, are
variables and methods
• How you declare your classes, methods, and
variables dramatically affects your code's
behavior, for example, a public method can be
accessed from code running anywhere in your
application

Class - Definition
• Source File Declaration Rules:
– There can be only one public class per source code file
– Comments can appear at the beginning or end of any line in the
source code file
– If there is a public class in a file, the name of the file must match
the name of the public class. For example, a class declared as
public class Dog { } must be in a source code file named
Dog.java
– If the class is part of a package, the package statement must be
the first line in the source code file, before any import statements
that may be present

Class - Definition
• Source File Declaration Rules:
– If there are import statements, they must go between the
package statement and the class declaration
– If there isn't a package statement, then the import statement(s)
must be the first line(s) in the source code file
– If there are no package or import statements, the class
declaration must be the first line in the source code file
– Import and package statements apply to all classes within a
source code file. In other words, there's no way to declare
multiple classes in a file and have them in different packages, or
use different imports
– A file can have more than one nonpublic class
– Files with no public classes can have a name that does not
match any of the classes in the file

Create and Declare a Class
• Car.java
public class Car { // Only one public class
// Variable
int gear;
String color;
// Methods
public void accelerate();
public void brake();
}

Object - Definition
• An object is an instance of a class
• An object belongs to a class
• An object belongs to a reference type

Create and Declare an Object
• CarTest.java
public class CarTest {
public static void main(String[] args) {
Car honda = new Car();
honda.setGear(5);
honda.setColor(“white”);
Car Ferrari = new Car();
Car Pagani = new Car();
}
}

Constructor
• Constructor is called when create a new object
• Constructor allocates memory for a new
instance of an object
• Constructor typically initialize the fields of the
new object

Constructor
• Constructor has the same name as the class
• Constructor is written without return type
• All classes always have at least one constructor
• A constructor in the object's parent class is
always called

Constructor
• There is Instance Initializer similar to constructor
{
System.out.println("I am an instance initializer!");
}
• No Destructors

Constructor
• Example:
public class Parent {
public Parent() { } // default constructor
Parent p = new Parent();
}
}

Constructor
• Example:
public Parent() {
System.out.println(“PARENT”);
}
Parent p = new Parent();
}
}

Constructor
• Example:
public Parent() {
}
Parent p = new Parent(“TEST”); // Error or not?
}
}

Constructor
• Example:
public Parent() {
}
Parent p = new Parent(); // Error or not?
}
}

Constructor
• Example:
public Parent(String name) {
System.out.println(“PARENT = ” + name);
}
Parent p = new Parent(“Hari”);
}
}

Constructor
• Example:
public Parent(String name) {
System.out.println(“PARENT = ” + name);
}
}
}

Constructor
• Example:
public Parent() {
System.out.println(“PARENT ”);
}
}
public class Child extends Parent {
public Child() {
System.out.println(“CHILD”);
}
}

Constructor
• Example:
public class TestConstructor {
Child c = new Child(); // Error or not?
}
}

Constructor
public class Employee {
private String nik;
private String name;
public Employee() {
}
public Employee(String nik, String name) {
this.nik = nik;
this.name = name;
}
// Setter Getter
}

Constructor - Overload
• Example:
public class TestEmployee {
Employee h = new Employee(“1”, “Hari”);
Employee r = new Employee();
r.setNik(“2”);
r.setNik(“Rifki”);
}
}

Methods - Basic
• Methods are the OOP name for functions
• A method is always declared inside a class
• Format:
[access] [modifiers] returnType methodName([param]) [throw] {
Statement;
}

Methods - Basic
• Example:
public int addition(int num1, int num2) {
return num1 + num2;
}

Methods – Passing by Value
• "Passing by value" means that the argument's
value is copied and is passed to the method.
Inside the method you can modify the copy at
will, and the modifications don't affect the
original argument.

Methods - Passing by Value
• Example:
public class Method {
int number = 5;
hitung(number);
System.out.println(“[main] number = ” + number);
}
public static void hitung(int number) {
number = 7;
System.out.println(“[hitung] number = ” + number);
}
}

Methods - Passing by Reference
• "Passing by reference" means that a reference
to (i.e., the address of) the argument is passed
to the method. Using the reference, the method
is actually directly accessing the argument, not a
copy of it. Any changes the method makes to the
parameter are made to the actual object used as
the argument. After you return from the method,
that object will retain any new values set in the
method.

Methods - Passing by Reference
• Example:
Student s = new Student();
input(s);
System.out.println(“[main] nim = ” + s.getNim());
}
public static void input(Student s) {
s.setNim(“007”);
System.out.println(“[input] nim = ” + s.getNim());
}
}

Methods - Overload
• We can have several methods with the same
name in a class
• This same methods is called Overload Methods
• This same methods must have DIFFERENT
parameter

Methods - Overload
• Example:
public static void hitung(int number) {
}
public static void hitung(double number) {
}
}

Methods - Overload
• Example:
public static void addition(int num1, int num2) {
}
public static void addition(double num1, double num2) {
}
}

Methods - Overload
• Example:
public static void calcAge(Date birthDate) {
}
public static void calcAge(Date date1, Date date2) {
}
}

Methods - Chaining
• Example:
String x = "abc";
String y = x.concat("def").toUpperCase().replace('C','x');
System.out.println("y = " + y); // result is "y = ABxDEF“
• Tips: read it from left, use the result to invoking
2nd methods and so on.

Transfer of Control Statement - Return
• Format:
return;
return Expression;
• Explanation:
 return, is only used in method with no return value
 return Expression, is always used in every method
that actually does return a value

• Example:
void deleteCustomer(Customer c) {
if (c == null) return;
// Delete Code here
}

• Example:
int addition(int num1, int num2) {
return num1 + num2;
}
OR
int addition(int num1, int num2) {
int result = num1 + num2;
return result;
}

Package
• Must be unique
• Package = Directory
• The identifier rules applied
• There are thus three purposes to packages:
– To organize namespaces in your program so your
classes don't collide with each other
– To organize namespaces for all organizations, so all
program names don't collide with each other
– To control visibility of classes

Package
• Package a.b.c.D =
– a
• b
– c
» D.java
• Put in the very beginning of Java Code. Ex:
package id.ac.istb.javatraining
public class Test {}

Import
• You do import when you calling a class from
another package
• Only one class with the same name can be
imported
• There 3 ways to use class from another package
– Import whole class in package
– import the spesific class
– Using full name

Import Whole Package
• Import whole class in package example:
import java.util.*; // Use * to import whole class
public class TestImport {
// Scanner is belong to java.util package
Scanner scan = new Scanner(System.in);
}

Import The Spesific Class
• Import the spesific class example:
import java.util.Scanner;
Scanner scan = new Scanner(System.in);
}

Import Full Name
• Using full name example:
java.util.Scanner scan = new java.util.Scanner(System.in);
}

Import Static
• Static import example (new since JDK 5):
• Without static import:
System.out.println(Integer.MAX_VALUE);
}
• With static import:
import static java.lang.Integer.*;
System.out.println(MAX_VALUE);
}

Organizing Statement - Local
• Local variable
Local variable live only inside the block of if, loop
or method
• Example:
boolean valid = true
if (valid) {
int number = 5; // local variable
}

Organizing Statement - Instance
• Instance variable / class variable
Instance variable live only inside the class
• Example:
public class Test {
static int number = 5; // instance variable
System.out.println(number);
}
}

Organizing Statement - This
• Using this keyword
• Example:
public class Test {
static int number = 5; // instance variable
int number = 10; // local variable
System.out.println(number);
System.out.println(this.number);
}
}

Empty Statement
• Empty statement (semi colon) does nothing
• Example:
; // empty variable
if (true) {
; // empty variable
} else {
System.out.println(“Do Something Here”);
}
}

Access Modifier
• There are four access modifier in Java:
default
• Visible only in the same package
private
• Visible only in the same class
protected
• Visible only in the same class or subclass
public
• Visible everywhere

Access Modifier - class
 default
package a.b.c;
class D { /*some code */ }
Class D will be visible only to classes in the same
package a.b.c, (typically this means the same directory)
and nowhere else
 public
package a.b.c;
public class D { /*some code */ }
Class a.b.c.D will be visible to all packages everywhere

Access Modifier – field/method
 default
int getValue();
Visible only to classes in the same package
 public
public int getValue();
Visible to all packages everywhere
 private
private int getValue();
Visible only inside class
 protected
protected int getValue();
Visible only to classes in the same package and subclass

03 Java Language And OOP Part III

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