11. Java Objects and classes

Using Objects and Classes
Defining Simple Classes
Objects and Classes
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Table of Contents
1. Objects
2. Classes
3. Built in Classes
4. Defining Simple Classes
 Fields
 Constructors
 Methods

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#fund-java
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Objects
 An object holds a set of named values
 E.g. birthday object holds day, month and year
 Creating a birthday object:
5
Birthday
day = 27
month = 11
year = 1996
LocalDate birthday =
LocalDate.of(2018, 5, 5);
System.out.println(birthday);
Create a new object of
type LocalDate
Object
fields
Object
name

Classes
 In programming classes provide the structure for objects
 Act as a blueprint for objects of the same type
 Classes define:
 Fields (private variables), e.g. day, month, year
 Getters/Setters, e.g. getDay, setMonth, getYear
 Actions (behavior), e.g. plusDays(count),
subtract(date)
 Typically a class has multiple instances (objects)
 Sample class: LocalDate
 Sample objects: birthdayPeter, birthdayMaria
6

Objects - Instances of Classes
 Creating the object of a defined class is
called instantiation
 The instance is the object itself, which is
created runtime
 All instances have common behaviour
7
LocalDate date1 = LocalDate.of(2018, 5, 5);

Classes vs. Objects
 Classes provide structure for
creating objects
 An object is a single
instance of a class
8
class
LocalDate
day: int
month: int
year: int
plusDays(…)
minusDays(…)
Class actions
(methods)
Class name
Class fields
object
birthdayPeter
day = 27
month = 11
year = 1996
Object
name
Object
data

Using the Built-In API Classes
Math, Random, BigInteger ...

 Java provides ready-to-use classes:
 Organized inside Packages like:
java.util.Scanner, java.utils.List, etc.
 Using static class members:
 Using non-static Java classes:
Built-In API Classes in Java
10
LocalDateTime today = LocalDateTime.now();
double cosine = Math.cos(Math.PI);
Random rnd = new Random();
int randomNumber = rnd.nextInt(99);

 You are given a list of words
 Randomize their order and print each word on a separate line
Problem: Randomize Words
11Check your solution here: https://judge.softuni.bg/Contests/1319/
Note: the output is a sample.
It should always be different!
a b
b
a
PHP Java C#
Java
PHP
C#

Solution: Randomize Words
12
Scanner sc = new Scanner(System.in);
String[] words = sc.nextLine().split(" ");
for (int pos1 = 0; pos1 < words.length; pos1++) {
int pos2 = rnd.nextInt(words.length);
//TODO: Swap words[pos1] with words[pos2]
}
System.out.println(String.join(
System.lineSeparator(), words));
Check your solution here: https://judge.softuni.bg/Contests/1319/

 Calculate n! (n factorial) for very big n (e.g. 1000)
Problem: Big Factorial
13
50
3041409320171337804361260816606476884437764156
8960512000000000000
5 120 10 3628800 12 479001600
88
1854826422573984391147968456455462843802209689
4939934668442158098688956218402819931910014124
4804501828416633516851200000000000000000000

Solution: Big Factorial
14
import java.math.BigInteger;
...
int n = Integer.parseInt(sc.nextLine());
BigInteger f = new BigInteger(String.valueOf(1));
for (int i = 1; i <= n; i++) {
f = f.multiply(BigInteger
.valueOf(Integer.parseInt(String.valueOf(i))));
}
System.out.println(f);
Use the
java.math.BigInteger
N!

Defining Classes
Creating Custom Classes

 Specification of a given type of objects
from the real-world
 Classes provide structure for describing
and creating objects
Defining Simple Classes
16
class Dice {
…
}
Class name
Class body
Keyword

Naming Classes
 Use PascalCase naming
 Use descriptive nouns
 Avoid abbreviations (except widely known, e.g. URL,
HTTP, etc.)
17
class Dice { … }
class BankAccount { … }
class IntegerCalculator { … }
class TPMF { … }
class bankaccount { … }
class intcalc { … }

 Class is made up of state and behavior
 Fields store values
 Methods describe behaviour
Class Members
18
class Dice {
private int sides;
public void roll() { … }
}
Field
Method

 Store executable code (algorithm)
Methods
19
class Dice {
public int sides;
public int roll() {
int sides = rnd.nextInt(this.sides + 1);
return sides;
}
}

Getters and Setters
20
class Dice {
. . .
public int getSides() { return this.sides; }
public void setSides(int sides) {
this.sides = sides;
}
public String getType() { return this.type; }
public void setType(String type) {
this.type = type;
}
}
Getters & Setters

 A class can have many instances (objects)
Creating an Object
21
class Program {
public static void main(String[] args) {
Dice diceD6 = new Dice();
Dice diceD8 = new Dice();
}
}
Use the new
keyword
Variable stores a
reference

 Special methods, executed during object creation
Constructors
22
class Dice {
public int sides;
public Dice() {
this.sides = 6;
}
}
Overloading default
constructor
Constructor name is
the same as the name
of the class

 You can have multiple constructors in the same class
Constructors (2)
23
class Dice {
public int sides;
public Dice() { }
public Dice(int sides) {
this.sides = sides;
}
}
class StartUp {
public static void main(String[] args) {
Dice dice1 = new Dice();
Dice dice2 = new Dice(7);
}
}

 Read students until you receive "end" in the following format:
 "{firstName} {lastName} {age} {hometown}"
 Define a class Student, which holds the needed information
 If you receive a student which already exists (matching
firstName and lastName), overwrite the information
 After the end command, you will receive a city name
 Print students which are from the given city in the format:
"{firstName} {lastName} is {age} years old."
Problem: Students
24

Solution: Students (1)
25
public Student(String firstName, String lastName,
int age, String city){
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.city = city;
// TODO: Implement Getters and Setters
}

26
List<Student> students = new ArrayList<>();
String line;
while (!line.equals("end")) {
// TODO: Extract firstName, lastName, age, city from the input
Student existingStudent = getStudent(students, firstName, lastName);
if(existingStudent != null) {
existingStudent.setAge(age);
existingStudent.setCity(city);
} else {
Student student = new Student(firstName, lastName, age, city);
students.add(student);
}
line = sc.nextLine();
}

27
static Student getStudent(List<Student> students, String firstName,
String lastName) {
for (Student student : students){
if(student.getFirstName().equals(firstName)
&& student.getLastName().equals(lastName))
return student;
}
return null;
}

 …
 …
 …
Summary
 Classes define templates for object
 Fields
 Constructors
 Methods
 Objects
 Hold a set of named values
 Instance of a class

 https://softuni.bg/courses/programming-fundamentals

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11. Java Objects and classes

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