Defining a class

How to define a class that you can use to construct Circle objects: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             /** Constructor 1 for a circle object */

    public Circle(double newRadius) /** Constructor 2 for a circle object */
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

Using the Circle class to construct some circles

We can use the Circle class to create 2 Circle objects as follows: 

    public static void main()
    {
        Circle circle1 = new Circle();  
                         // Invokes Circle() to make this circle

        Circle circle2 = new Circle(2); 
                         // Invokes Circle(double) to make this circle
    }

Demo in BlueJ - show the objects in main().      Step into the constructors

UML: a standardized modeling representation description

The Unified Modeling Language representation of classes and objects: 

    public static void main()
    {
        Circle circle1 = new Circle();  
                         // Invokes Circle() to make this circle

        Circle circle2 = new Circle(2); 
                         // Invokes Circle(double) to make this circle
    }

Demo in BlueJ - show the objects in main().      Step into the constructors

How a class definition represents objects

A Java class uses variables to define data fields (properties) of objects: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             /** Constructor 1 for a circle object */

    public Circle(double newRadius) /** Constructor 2 for a circle object */
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

How a class definition represents objects

A Java class uses methods to define the actions of objects: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             /** Constructor 1 for a circle object */

    public Circle(double newRadius) /** Constructor 2 for a circle object */
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

How a class definition represents objects

Important note:     methods to define the actions of objects do not have the static qualifier 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             /** Constructor 1 for a circle object */

    public Circle(double newRadius) /** Constructor 2 for a circle object */
    {
        radius = newRadius;
    }

    public double getArea()      /** Do NOT use static qualifier ! */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Do NOT use static qualifier ! */
    {
       radius = newRadius;
    }
}

Constructors

A class provides special methods called constructors which are invoked (only) to create a new object: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             /** Constructor 1 for a circle object */

    public Circle(double newRadius) /** Constructor 2 for a circle object */
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

Constructors are designed to perform initializing actions, such as initializing the data fields of objects

The main( ) method

A class that represents real world objects usually does not need a main( ) method: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             // called when you use: new Circle()

    public Circle(double newRadius) // called when you use: new Circle(number)
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

Without a main( ) method, such class cannot be run as a Java program

The main( ) method

You may put a main( ) method in the Circle class to test the methods: 

public class Circle 
{
    public double radius = 1;       /** The radius of this circle */

    public Circle() { }             // called when you use: new Circle()

    public static void main(String[] args) 
    {                                      
        Circle circle1 = new Circle();     
        Circle circle2 = new Circle(2);    
    }                                      

    public Circle(double newRadius) // called when you use: new Circle(number)
    {
        radius = newRadius;
    }

    public double getArea()      /** Return the area of this circle */
    {
        return 3.14159 * radius * radius;
    }

    public void setRadius(double newRadius) /** Set new radius for this circle */
    {
       radius = newRadius;
    }
}

I prefer to write a separate class to do the testing...

How to use the methods on an object

Example invoking methods on objects: 

    public static void main()
    {
        Circle circle1 = new Circle();  // Create a Circle object circle1
        Circle circle2 = new Circle(2); // Create a Circle object circle2

	double area1 = circle1.getArea(); // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);

	double area2 = circle2.getArea(); // Tell circle2 to run getArea()
	System.out.println("Area of circle2 = " + area2);


	circle1.setRadius(5);          // Tell circle1 to run setRadius()

	area1 = circle1.getArea();     // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);
    }

Preventing undesirable behavior in objects

The Circle class implementation allows a user to access the object variables directly: 

    public static void main()
    {
        Circle circle1 = new Circle();  // Create a Circle object circle1
        Circle circle2 = new Circle(2); // Create a Circle object circle2

	double area1 = circle1.getArea(); // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);

	circle1.radius = 10;   // Update the radius variable directly

	area1 = circle1.getArea();     // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);
    }

Because: 

public class Circle 
{
    public double radius = 1;  // The radius variable is not private  
    ....

Preventing undesirable behavior in objects

We can prevent direct access to variables in a class by using the private qualifier: 

    public static void main()
    {
        Circle circle1 = new Circle();  // Create a Circle object circle1
        Circle circle2 = new Circle(2); // Create a Circle object circle1

	double area1 = circle1.getArea(); // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);

	circle1.radius = 10;   // Compile error !!! 

	area1 = circle1.getArea();     // Tell circle1 to run getArea()
	System.out.println("Area of circle1 = " + area1);
    }

Because: 

public class Circle 
{
    private double radius = 1;  // Disallows direct access to radius  
    ....