Flash cards
Review the key moves
What is the main idea behind Java Classes and Objects?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
public ___ Main {Put the learning moves in the order that makes the concept easiest to apply.
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:
myObjMultiple Objects
You can create multiple objects of one class:
MainUsing 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.javaRun the Second.java file:
C:\Users\
Your Name
>java SecondAnd the output will be
5You will learn much more about classes and objects in the next chapters.