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Java Foundations: Objects and Classes

  1. Java Foundations Objects and Classes
  2. Your Course Instructors Svetlin Nakov George Georgiev
  3. The Judge System Sending your Solutions for Automated Evaluation
  4. Testing Your Code in the Judge System  Test your code online in the SoftUni Judge system:
  5. Objects and Classes Using Objects and Classes Defining Simple Classes
  6. Table of Contents 1. Objects: Set of Data Fields 2. Classes: Define the Object Structure 3. Built-in Classes 4. Defining Simple Classes  Fields  Constructors  Methods class DateTime Day: int Month: int Year: int AddDays(…) Subtract(…) object peterBirthday Day = 27 Month = 11 Year = 1996
  7. Objects and Classes
  8. Objects  An object holds a set of named values  E.g. birthday object holds day, month and year  Creating a birthday object: 9 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
  9. 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 10
  10. 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 11 LocalDate date1 = LocalDate.of(2018, 5, 5); LocalDate date2 = LocalDate.of(2016, 3, 5); LocalDate date3 = LocalDate.of(2013, 3, 2);
  11. Classes vs. Objects  Classes provide structure for creating objects  An object is a single instance of a class 12 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
  12. Using the Built-In API Classes Math, Random, BigInteger ...
  13.  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 14 LocalDateTime today =; double cosine = Math.cos(Math.PI); Random rnd = new Random(); int randomNumber = rnd.nextInt(99);
  14.  You are given a list of words  Randomize their order and print each word on a separate line Problem: Randomize Words 15 Note: the output is a sample. It should always be different! a b b a PHP Java C# Java PHP C#
  15. Solution: Randomize Words 16 Scanner sc = new Scanner(; String[] words = sc.nextLine().split(" "); Random rnd = new Random(); 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));
  16.  Calculate n! (n factorial) for very big n (e. g. 1000) Problem: Big Factorial 17 50 3041409320171337804361260816606476884437764156 8960512000000000000 5 120 10 3628800 12 479001600 88 1854826422573984391147968456455462843802209689 4939934668442158098688956218402819931910014124 4804501828416633516851200000000000000000000
  17. Solution: Big Factorial 18 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!
  18. Defining Classes Creating Custom Classes
  19.  Specification of a given type of objects from the real-world  Classes provide structure for describing and creating objects Defining Simple Classes 20 class Dice { … } Class name Class body Keyword
  20. Naming Classes  Use PascalCase naming  Use descriptive nouns  Avoid abbreviations (except widely known, e.g. URL, HTTP, etc.) 21 class Dice { … } class BankAccount { … } class IntegerCalculator { … } class TPMF { … } class bankaccount { … } class intcalc { … }
  21.  Class is made up of state and behavior  Fields store values  Methods describe behaviour Class Members 22 class Dice { private int sides; public void roll() { … } } Field Method
  22.  Store executable code (algorithm) Methods 23 class Dice { public int sides; public int roll() { Random rnd = new Random(); int sides = rnd.nextInt(this.sides + 1); return sides; } }
  23. Getters and Setters 24 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
  24.  A class can have many instances (objects) Creating an Object 25 class Program { public static void main(String[] args) { Dice diceD6 = new Dice(); Dice diceD8 = new Dice(); } } Use the new keyword Variable stores a reference
  25.  Special methods, executed during object creation Constructors 26 class Dice { public int sides; public Dice() { this.sides = 6; } } Overloading default constructor Constructor name is the same as the name of the class
  26.  You can have multiple constructors in the same class Constructors (2) 27 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); } }
  27.  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 28
  28. Solution: Students (1) 29 public Student(String firstName, String lastName, int age, String city) { this.firstName = firstName; this.lastName = lastName; this.age = age; = city; // TODO: Implement Getters and Setters }
  29. Solution: Students (2) 30 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(); }
  30. Solution: Students (3) 31 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; }
  31.  …  …  … Summary  Classes define templates for objects  Fields  Constructors  Methods  Objects  Hold a set of named values  Instances of a classes
  32.  …  …  … Next Steps  Join the SoftUni "Learn To Code" Community  Access the Free Coding Lessons  Get Help from the Mentors  Meet the Other Learners

Notes de l'éditeur

  1. Hello, I am Svetlin Nakov from SoftUni (the Software University). We are here again with my colleague George Georgiev, to continue teaching this free Java Foundations course, which (as you already know) covers important concepts from Java programming, such as arrays, lists, maps, methods, strings, classes, objects and exceptions, and prepares you for the "Java Foundations" official exam from Oracle. In this lesson your instructor George will explain the concept of objects and classes and will give you real world examples with live Java coding. In programming objects hold a set of data fields. For example, the “Peter’s birthday” object could hold a day (27), plus a month (11, which is November), plus a year (1996). In Java, developers can define classes to describe a particular structure of an object. For example, a DateTime class could contain a day, plus a month, plus a year, all of which are integers. Classes in Java typically hold fields (to keep object’s data) + constructors (to create and initialize objects) + methods (to implement functionality, related to object’s data). Let's learn how to use objects and simple classes in Java through live coding examples, and later solve some hands-on exercises to gain practical experience. Let's start!
  2. Before the start, I would like to introduce your course instructors: Svetlin Nakov and George Georgiev, who are experienced Java developers, senior software engineers and inspirational tech trainers. They have spent thousands of hours teaching programming and software technologies and are top trainers from SoftUni. I am sure you will like how they teach programming.
  3. Most of this course will be taught by George Georgiev, who is a senior software engineer with many years of experience with Java, JavaScript and C++. George enjoys teaching programming very much and is one of the top trainers at the Software University, having delivered over 300 technical training sessions on the topics of data structures and algorithms, Java essentials, Java fundamentals, C++ programming, C# development and many others. I have no doubt you will benefit greatly from his lessons, as he always does his best to explain the most challenging concepts in a simple and fun way.
  4. Before we dive into the course, I want to show you the SoftUni judge system, where you can get instant feedback for your exercise solutions. SoftUni Judge is an automated system for code evaluation. You just send your code for a certain coding problem and the system will tell you whether your solution is correct or not and what exactly is missing or wrong. I am sure you will love the judge system, once you start using it!
  5. // Solution to problem "01. Student Information". import java.util.Scanner; public class Main { public static void main(String[] args) { Scanner sc = new Scanner(; String name = sc.nextLine(); int age = Integer.parseInt(sc.nextLine()); double grade = Double.parseDouble(sc.nextLine()); System.out.printf("Name: %s, Age: %d, Grade: %.2f", name, age, grade); } }
  6. Did you like this lesson? Do you want more? Join the learners' community at Subscribe to my YouTube channel to get more free video tutorials on coding and software development. Get free access to the practical exercises and the automated judge system for this coding lesson. Get free help from mentors and meet other learners. Join now! It's free. SOFTUNI.ORG