Java how to use classes

Introduction to Java
The complete History of Java Programming Language
C++ vs Java vs Python
How to Download and Install Java for 64 bit machine?
Setting up the environment in Java
How to Download and Install Eclipse on Windows?
JDK in Java
How JVM Works – JVM Architecture?
Differences between JDK, JRE and JVM
Just In Time Compiler
Difference between JIT and JVM in Java
Difference between Byte Code and Machine Code
How is Java platform independent?

Decision Making in Java (if, if-else, switch, break, continue, jump)
Java if statement with Examples
Java if-else
Java if-else-if ladder with Examples
Loops in Java
For Loop in Java
Java while loop with Examples
Java do-while loop with Examples
For-each loop in Java
Continue Statement in Java
Break statement in Java
Usage of Break keyword in Java
return keyword in Java

Object Oriented Programming (OOPs) Concept in Java
Why Java is not a purely Object-Oriented Language?
Classes and Objects in Java
Naming Conventions in Java
Java Methods
Access Modifiers in Java
Java Constructors
Four Main Object Oriented Programming Concepts of Java
Inheritance in Java
Abstraction in Java
Encapsulation in Java
Polymorphism in Java
Interfaces in Java
‘this’ reference in Java

Classes

The introduction to object-oriented concepts in the lesson titled Object-oriented Programming Concepts used a bicycle class as an example, with racing bikes, mountain bikes, and tandem bikes as subclasses. Here is sample code for a possible implementation of a Bicycle class, to give you an overview of a class declaration. Subsequent sections of this lesson will back up and explain class declarations step by step. For the moment, don’t concern yourself with the details.

public class Bicycle < // the Bicycle class has // three fields public int cadence; public int gear; public int speed; // the Bicycle class has // one constructor public Bicycle(int startCadence, int startSpeed, int startGear) < gear = startGear; cadence = startCadence; speed = startSpeed; >// the Bicycle class has // four methods public void setCadence(int newValue) < cadence = newValue; >public void setGear(int newValue) < gear = newValue; >public void applyBrake(int decrement) < speed -= decrement; >public void speedUp(int increment) < speed += increment; >>

A class declaration for a MountainBike class that is a subclass of Bicycle might look like this:

public class MountainBike extends Bicycle < // the MountainBike subclass has // one field public int seatHeight; // the MountainBike subclass has // one constructor public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) < super(startCadence, startSpeed, startGear); seatHeight = startHeight; >// the MountainBike subclass has // one method public void setHeight(int newValue) < seatHeight = newValue; >>

MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it (mountain bikes have seats that can be moved up and down as the terrain demands).

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Java Classes and Objects

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 :

Main.java

Create a class named » Main » with a variable x:

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 Main , so now we can use this to create objects.

To create an object of Main , 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:

Multiple Objects

You can create multiple objects of one class:

Example

Create two objects of 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

When both files have been compiled:

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

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