Thread: Need help on objects

Originally Posted by alex067

...
My question is:

1) the public class method is the actual circle object?

No, the public class named Circle is a class. All of that code is (presumably) in a file named Circle.java
A class is not an object. It is kind of a "blueprint" that tells the Java compiler how to create an object and how to access the object's characteristics.

Originally Posted by alex067

2) how do i define a circle object?
3) how do i instantiate the circle object?

Somewhere in your code (maybe in the main() function of the class that you are executing with the "java" command) you declare an object of that class and invoke its constructor.

Since your Circle class constructor takes a double precision argument that will be the radius of the circle, then it can go like this:

public class Whatever {
    public static void main(String [] args) {
        // A local variable within this main() function that will be
        // used to create the Circle object
        double rad;
 
        // Do "something" here to get a value for rad so that you can use it in the Circle constructor
        // Maybe define a static function named getRadiusFromUser in this class (not the Circle class).
        //
        // For preliminary testing you can just assign a value:
        rad = 1.23456;// Or whatever...
 
       // Then...
 
        // Create a Circle object with this radius:
        Circle circle = new Circle(rad);
        // Now you have a Circle with the value of radius that you gave it.

Then, to retrieve the value of the radius, you can do something like:

 
        // Use the circle object method to retrieve the value of
        // its radius for the output summary.  (You could
        // use the local variable here, but, in general,
        // it might be better to make sure that the object
        // is what you think it should be.)
        //
        // Use the "dot" notation to access an instance method:
        double radi = circle.getRadius();

To get the instance method to calculate the circumference:

        // Use the circle method to calculate the circumference
        // from its radius
        double circ = circle.calcCircumference();

Similarly for the area.

Finally, print the values of the variables:

        // Now print the summary of radius, circumference and area...
        System.out.printf("Radius        = %f\n", radi);
        System.out.printf("Circumference = %f\n", circ);
        System.out.printf("Area          = %f\n", area);

Originally Posted by alex067

4) and what are instance methods? How do I output results using an instance method?

They are functions defined in the class that do something with an object of that class.

You can do it as I showed it above, using the object "dot" notation to store the value in a local variable, or you can just do something like

        System.out.printf("Area          = %f\n", circle.calcArea());

Output:

You entered 1.23456
Radius        = 1.234560
Circumference = 7.756969
Area          = 4.788222

Here's how it goes: With the Circle class an application can create (instantiate) as many Circle objects as you please. Each object has its own radius value (determined by the argument you feed to the constructor). For each object you can use that object's instance methods to calculate that particular object's area and circumference.


Cheers!

Z

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