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Java Classes and Objects


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:

MyClass.java

Create a class named "MyClass" with a variable x:

public class MyClass {
  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. We have already created the class named MyClass, so now we can use this to create objects.

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

Example

Create an object called "myObj" and print the value of x:

public class MyClass {
  int x = 5;

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

Run example »



Multiple Objects

You can create multiple objects of one class:

Example

Create two objects of MyClass:

public class MyClass {
  int x = 5;

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

Run example »


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:

  • MyClass.java
  • OtherClass.java

MyClass.java

public class MyClass {
  int x = 5;
}

OtherClass.java

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

When both files have been compiled:

C:\Users\Your Name>javac MyClass.java
C:\Users\Your Name>javac OtherClass.java

Run the OtherClass.java file:

C:\Users\Your Name>java OtherClass

And the output will be:

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