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?

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

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);
    }
}

// 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");
    }
}

// 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

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());
    }
}

// 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

...

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