Java Classes (Part 1)

What is OOP?

OOP stands for Object-Oriented Programming.

Procedural programming is about writing procedures or methods that perform operations on the data, while object-oriented programming is about creating objects that contain both data and methods.

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Object-oriented programming has several advantages over procedural programming:

• OOP is faster and easier to execute
• OOP provides a clear structure for the programs
• OOP helps to keep the Java code DRY “Don’t Repeat Yourself”, and makes the code easier to maintain, modify and debug
• OOP makes it possible to create full reusable applications with less code and shorter development time

Create a Class:

To create a class, use the keyword class.

public class Main {
  int x = 5;
}

Remember from the Java Syntax chapter that a class should always start with an uppercase first letter, and that the name of the java file should match the class name.

Create an Object:

To create an object of Main, specify the class name, followed by the object name, and use the keyword new.

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x); // Output 5
  }
}

You can access attributes by creating an object of the class, and by using the dot syntax (.):

Multiple Objects:

You can create multiple objects of one class.

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj1 = new Main();  // Object 1
    Main myObj2 = new Main();  // Object 2
    System.out.println(myObj1.x); // Output 5
    System.out.println(myObj2.x); // Output 5
  }
}

Multiple Classes:

You can also create an object of a class and access it in another class. This is often used for better organization of classes.

public class Main {
  int x = 5;
}

class Second {
  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x); // Output 5
  }
}

Modify Attributes:

You can also modify attribute values.

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x); // Output 5
    myObj.x = 40;
    System.out.println(myObj.x); // Output 40
  }
}

If you don’t want the ability to override existing values, declare the attribute as final.

public class Main {
  final int x = 10;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}