Intro to Classes and Objects

Lesson 8 - OOP

There are four main OOP (object oriented programming) concepts in Java.

• Encapsulation (setters and getters, we will discuss this today)

• Abstraction

• Inheritance

• Polymorphism

Today OOP topics

• non static methods

• this

• constructors and copy constructors

• objects are passed to functions as references

• toString

OOP - Encapsulation

הסתרת מידע

• public and private

• getters and setters

What are Classes and objects?

Objects and Classes in Java

What is Class and Object in Java OOPS? Learn with Example

• Class - Dogs

• Data members or objects- size, age, color, breed, etc.

• Methods- sleep(), toString()

Attempt 1 🐶

// Class Declaration
public class Dog {
    // Instance Variables
    String breed;
    String name;
    String size;
    int age;
    String color;


    // non static method
    public void sleep() {
        System.out.println(name + " is sleeping");
    }

    //static method
    public static void whoIsSleeping(Dog dog){
        System.out.println(dog.name + " is sleeping");
    }

    public String toString() {
        return ("Name : "+name+ "\nBreed is: "+breed+"\nSize is: "+size+"\nAge is: "+age+"\nColor is: "+color+ "\n");
    }


    public static void main(String[] args) {
        Dog maltese = new Dog();
        maltese.breed="Maltese";
        maltese.size="Small";
        maltese.age=2;
        maltese.color="white";

        maltese.sleep();

        System.out.println();
        System.out.println(maltese);

        maltese.name="fluffy";
        Dog.whoIsSleeping(maltese);
    }
}

null is sleeping

Breed is: Maltese
Size is: Small
Age is: 2
Color is: white

fluffy is sleeping

• we have a static method (doesnt belong to class), whoIsSleeping()

• we have a non static method (belongs to class), sleep()

We got null is slepping , we forgot to initialize the name we should use a constructor

Attempt 2 - Adding constructors

here we will add 2 constructors

• a copy constructor

• a constructor that receives arguments

Main.java

public class Main {
    public static void main(String[] args) {
        Dog fluffy = new Dog("Fluffy","Maltese",
                "Small",2,"white");

        fluffy.sleep();

        System.out.println();
        System.out.println(fluffy);

        Dog snowball = new Dog(fluffy); //call copy constructor

        System.out.println();
        snowball.name = "Snowball";
        System.out.println(snowball.name);
    }
}

Dog.java

// Class Declaration
public class Dog {
    // Instance Variables
    String name;
    String breed;
    String size;
    int age;
    String color;

    //constructor - ADDED THIS
    Dog(String my_name, String breed, String size, int age, String my_color) {
        this.breed=breed;
        name=my_name;
        this.age = age;
        this.size=size;
        color= my_color;
    }

    //copy constructor -  ADDED THIS
    Dog(Dog other) {
        breed = other.breed;
        name = other.name;
        age = other.age;
        size = other.size;
        color = other.color;
    }

    // non static method
    public void sleep() {
        System.out.println(name + " is sleeping");
    }

    //static method
    public static void whoIsSleeping(Dog dog){
        System.out.println(dog.name + " is sleeping");
    }

    public String toString() {
        return ("Name : "+name+ "\nBreed is: "+breed+"\nSize is: "+size+
                "\nAge is: "+age+"\nColor is: "+color+ "\n");
    }
}

OneFile.java

// Class Declaration
public class Dog {
    // Instance Variables
    String name;
    String breed;
    String size;
    int age;
    String color;

    //constructor - ADDED THIS
    Dog(String my_name, String breed, String size, int age, String my_color) {
        this.breed=breed;
        name=my_name;
        this.age = age;
        this.size=size;
        color= my_color;
    }

    //copy constructor -  ADDED THIS
    Dog(Dog other) {
        breed = other.breed;
        name = other.name;
        age = other.age;
        size = other.size;
        color = other.color;
    }

     // non static method
    public void sleep() {
        System.out.println(name + " is sleeping");
    }

    //static method
    public static void whoIsSleeping(Dog dog){
        System.out.println(dog.name + " is sleeping");
    }

   public String toString() {
        return ("Name : "+name+ "\nBreed is: "+breed+"\nSize is: "+size+"\nAge is: "+age+"\nColor is: "+color+ "\n");
    }


    public static void main(String[] args) {
        Dog fluffy = new Dog("Fluffy","Maltese",
                "Small",2,"white");

        fluffy.sleep();

        System.out.println();
        System.out.println(fluffy);

        Dog snowball = new Dog(fluffy); //call copy constructor

        System.out.println();
        snowball.name = "Snowball";
        System.out.println(snowball.name);
    }
}

Fluffy is sleeping

Breed is: Maltese
Size is: Small
Age is: 2
Color is: white

Snowball

Attempt 3 - Adding encapsulation

we should not be able to directly change and get properties of the object for example here

public static void main(String[] args) {
        Dog fluffy = new Dog( "Fluffy", "Maltese", "Small", 2, "white");
        fluffy.name = "Snowball";
    }

• but rather use setters and getters

• the fields should also be private (can't be accessed outside the class)

so now we have

Main.java

public class Main {
    public static void main(String[] args) {
        Dog fluffy = new Dog("Fluffy","Maltese","Small",2,"white");

        Dog snowball = new Dog(fluffy); //call copy constructor

        System.out.println();
        snowball.setName("Snowball");
        System.out.println(snowball.getName());
    }
}

Dog.java

// Class Declaration
public class Dog {
    // Instance Variables
    private String name;
    private String breed;
    private String size;
    private int age;
    private String color;

    //constructor
    Dog(String my_name, String breed, String size, int age, String my_color) {
        this.breed=breed;
        name=my_name;
        this.age = age;
        this.size=size;
        color= my_color;
    }

    //copy constructor
    Dog(Dog other) {
        breed = other.breed;
        name = other.name;
        age = other.age;
        size = other.size;
        color = other.color;
    }

    // non static method
    public void sleep() {
        System.out.println(name + " is sleeping");
    }

    //static method
    public static void whoIsSleeping(Dog dog){
        System.out.println(dog.name + " is sleeping");
    }

    //getters and setters
    // === ADDED THIS ===
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getBreed() {
        return breed;
    }

    public void setBreed(String breed) {
        this.breed = breed;
    }

    public String getSize() {
        return size;
    }

    public void setSize(String size) {
        this.size = size;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }


    public String toString() {
        return ("Name : "+name+ "\nBreed is: "+breed+"\nSize is: "+size+"\nAge is: "+age+"\nColor is: "+color+ "\n");
    }
}

Snowball

An array of Dogs 🐶🐩🐕

public class Main {
    public static void main(String[] args) {
        String[] names = {"fluffy", "snowball", "shugi", "cookie"};
        Dog[] dogs = new Dog[4];

        for (int i = 0; i < dogs.length; i++)
            dogs[i] = new Dog(names[i],"Maltese","Small",2,"white");

        for (int i = 0; i < dogs.length; i++)
            System.out.println(dogs[i]);

    }
}

Name : fluffy
Breed is: Maltese
Size is: Small
Age is: 2
Color is: white

Name : snowball
Breed is: Maltese
Size is: Small
Age is: 2
Color is: white

...