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 called "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.