Java Classes and Objects

Java is an object-oriented language. You describe things with classes and create real copies of them called objects.

What are Classes and Objects?

A class is a plan or template. An object is a real thing made from that plan.

Class	Objects
Student	Ali, Ayesha, Bilal
Car	A white Mehran, a red Civic
Product	Rice bag, cooking oil, tea pack

A class has fields (data, also called attributes) and methods (actions).

Creating a Class and an Object

Example

java

class Student {
  String name;
  int marks;

  void introduce() {
    System.out.println("I am " + name + " and I got " + marks + " marks");
  }
}

public class Main {
  public static void main(String[] args) {
    Student s1 = new Student();
    s1.name = "Ali";
    s1.marks = 82;

    Student s2 = new Student();
    s2.name = "Ayesha";
    s2.marks = 95;

    s1.introduce();
    s2.introduce();
  }
}

Output

I am Ali and I got 82 marks
I am Ayesha and I got 95 marks

The new keyword creates an object. Each object has its own copy of the fields.

Constructors

A constructor is a special method that runs when an object is created. It has the same name as the class and no return type. Use it to set starting values:

Example

java

class Product {
  String name;
  int price;

  Product(String name, int price) {
    this.name = name;
    this.price = price;
  }

  void show() {
    System.out.println(name + ": Rs " + price);
  }
}

public class Main {
  public static void main(String[] args) {
    Product tea = new Product("Tea pack", 450);
    Product rice = new Product("Rice 5kg", 1800);
    tea.show();
    rice.show();
  }
}

Output

Tea pack: Rs 450
Rice 5kg: Rs 1800

this.name means "the field of this object", so Java can tell it apart from the parameter name.

Encapsulation: private Fields

It is good practice to make fields private so other code cannot change them in a wrong way. Give access through getter and setter methods:

Example

java

class BankAccount {
  private int balance = 0;

  public void deposit(int amount) {
    if (amount > 0) {
      balance += amount;
    }
  }

  public int getBalance() {
    return balance;
  }
}

public class Main {
  public static void main(String[] args) {
    BankAccount acc = new BankAccount();
    acc.deposit(5000);
    acc.deposit(-100);   // ignored
    System.out.println(acc.getBalance());
  }
}

Output

Access Modifiers

Modifier	Who can use it
`public`	Any class
`private`	Only the same class
`protected`	The same package and child classes
(none)	Only classes in the same package

Inheritance in One Look

A class can extend another class and reuse its fields and methods:

Example

java

class Animal {
  void eat() { System.out.println("Eating..."); }
}

class Cat extends Animal {
  void meow() { System.out.println("Meow"); }
}
// new Cat() can call both eat() and meow()

The four big ideas of OOP are encapsulation, inheritance, polymorphism and abstraction. Start with classes, objects and constructors; the rest builds on them.

Exercise

Which keyword creates a new object from a class?

classthisnewcreate