CSE 1030 Lecture 7/Jan/2007

Writing Classes


The class is the basic unit of Java programming. Everything in Java is an object, where each object is of a class type.

A class is a data type definition. The object is a variable of the class type. e.g. String (class).

String s = "This is a string";

Another way is that the class is a category of objects. e.g. Car (class) can have objects My Toyota and Your Ford. These objects are of type Car.

Class Definition Syntax

We have already used:

public class My Program
{

public static void main(String[] args)
{
system.out.println ("Hello World");
}
}


We now use:

public class ClassName
{
//Attributes, variables contained either by the class itself or by objects of the class type.

//Constructors: special methods called when an object is created.

//Other methods: manipulate the attributes.

}


What is public?

This is an access specifier, it determines what other classes can access this resource. Another access specifier is private.

public: anyone can use this resource.
private: only methods within the class itself can access the resource.

There are another two: protected and (blank), i.e. no access specifier.

The ability to make resources private, or specify their access is fundamental to object-oriented programming.

It obscures details, abstracting away complexity, prevents other programs from messing with the internals of the class, and forces other programmers to interact with the class in a manner you specify. This is, in effect, the Application Programming Interface (API).

What is "static"?

A static feature of a class is a feature associated with the class is a feature associated with the class type itself, not with any particular object.

Example, the Car class: all are fueled by Gasoline (static attribute). My Toyota has a fuel level of a quarter tank and Your Ford has a fuel level of a half tank. This "fuel level" is a non-static attribute.

Example, the Jet class: all are fueled by JetA (static attribute). Air Canada Airbus has a fuel level of a full tank and Bill Gates' Private Jet has a fuel level of three-quarters of a tank. This "fuel level" is a non-static attribute.

Declaration of Access Specifiers

(access specifier) static (type of attribute) (attribute name) = initialized value;

Examples: public static String fuel = "Gasoline";
public static int x = 0;
public static double y = 1.2;


In the fuel example, the type of fuel will never change. However fuel is a public attribute that can be changed by other classes. We therefore use the final keyword to ensure this attribute is not writable , and the initial value becomes mandatory.

public static final String fuel = "Gasoline";


Static constructor?

Makes no sense, there's no such thing.


Static method?

Just fine. Write static after the access specifier as in public static void main(...)

They cannot access non-static attributes of an object.


Utility class

A class where every feature is static.



Here is a temporary link to a test video by Louis.