Java Classes and Objects — Java interactive lesson

Java Classes/Objects

Java is an object-oriented programming language.

Everything in Java is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes , such as weight and color, and methods , such as drive and brake.

A Class is like an object constructor, or a "blueprint" for creating objects.

Create a Class

To create a class, use the keyword class .

In this example, we create a class named " Main " with a variable x :

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

In Java, an object is created from a class. After defining a class, you can create objects from it using the new keyword:

myObj

Multiple Objects

You can create multiple objects of one class:

Main

Using 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 (one class has all the attributes and methods, while the other class holds the main() method (code to be executed)).

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory/folder:

Main.java
Second.java

public class Main {
 int x = 5;
}

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

When both files have been compiled

C:\Users\
Your Name
>javac Main.java
C:\Users\
Your Name
>javac Second.java

Run the Second.java file:

C:\Users\
Your Name
>java Second

And the output will be

You will learn much more about classes and objects in the next chapters.