Defining classes in java with examples

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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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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Declaring Classes

This is a class declaration. The class body (the area between the braces) contains all the code that provides for the life cycle of the objects created from the class: constructors for initializing new objects, declarations for the fields that provide the state of the class and its objects, and methods to implement the behavior of the class and its objects.

The preceding class declaration is a minimal one. It contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,

class MyClass extends MySuperClass implements YourInterface < // field, constructor, and // method declarations >

means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.

You can also add modifiers like public or private at the very beginning—so you can see that the opening line of a class declaration can become quite complicated. The modifiers public and private, which determine what other classes can access MyClass , are discussed later in this lesson. The lesson on interfaces and inheritance will explain how and why you would use the extends and implements keywords in a class declaration. For the moment you do not need to worry about these extra complications.

In general, class declarations can include these components, in order:

1. Modifiers such as public, private, and a number of others that you will encounter later. (However, note that the private modifier can only be applied to Nested Classes.)
2. The class name, with the initial letter capitalized by convention.
3. The name of the class’s parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
4. A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
5. The class body, surrounded by braces, <>.

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