A Class for Circles

Background. Now we will develop some rather rudimentary classes to illustrate some of the principles and issues we have worked with so far. Our first example relates to circles. We keep it simple and don't really do any drawing. But we will get a radius from a user and compute relevant things such as the area and the circumference.

Whether you are aware of it or not, the radius is the most salient measure related to a circle. If you know it you should be able to find out just about anything else you want about the circle. You may remember that one classic definition is it is the set of points in a plane that are equidistant from one center point.

So, ultimately, our circles will be represented by a single number in the computer. They are not represented by a drawn circle.

Remember, it took three numbers to represent the time. When we set the time we used computer syntax

setTime(hh,mm,ss)

It required some extra methods to cast this into the more familiar

hh:mm:ss

form.

You should call the following file Circle.java and save it in its own directory. Whether or not you want to create a package is up to you. You will notice we sometimes make use of the this reference rather than referring to an instance of an object by its name.

import java.text.DecimalFormat;

public class Circle extends Object
{

private double radius;
final double PI = 3.14159;

public Circle( )
{

setCircle(0);

}

public Circle(double rad)
{

setCircle(rad);

}

public void setCircle(double rad)
{

radius = rad;

}

public void setRadius(double rad)
{

radius = rad;

}

public double getRadius( )
{

return radius;

}

public double computeArea( Circle input )
{

return PI * this.getRadius( ) * this.getRadius( );

}

public double computeCircumference( Circle input )
{

return 2 * PI * this.getRadius( );

}

public String toPrecisionFourString(double someNumber)
{

DecimalFormat precisionFour = new DecimalFormat("0.0000");
return precisionFour.format(someNumber);

}

}

You should compile this, likely with the more typical

javac Circle.java

We start with a discussion of the code and methods.

import java.text.DecimalFormat

the overall inclusive class is called Circle that extends Object

declare an instance variable double radius

declare a constant instance variable final double PI = 3.14159

define a constructor Circle( )

receives no arguments

uses a later method setRadius to initialize the radius to zero

define a constructor Circle( )

receives a single double precision argument rad

uses a later method setRadius to initialize the radius to rad

a setRadius( ) method is declared and developed

it receives a single double precision argument

rad for the radius

it assigns the instance variable radius equal to the rad that is passed

a getRadius( ) method is declared and developed

it receives no arguments

it returns the radius of the object

a computeArea( ) method is declared and developed

it receives one argument

an object of type Circle

it returns a double precision number representing the area of the circle

the area is computed using the constant PI times the radius squared

the radius is obtained using this.getRadius( )

a computeCircumference( ) method is declared and developed

it receives one argument

an object of type Circle

it returns a double precision number representing the circumference of the circle

the area is computed using 2 times the constant PI times the radius

the radius is obtained using this.getRadius( )

a toPrecisionFourString( ) method is declared and developed

it receives one double precision argument

it returns a String

it makes use of the DecimalFormat class in the text package to create a format that always has exactly four places after the decimal

it returns whatever double precision number was passed to it in this format as a string

Now you need the following CircleTestGUI.java to construct/instantiate a Circle object and work with it.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class CircleTestGUI extends JApplet implements ActionListener
{

// declare GUI components
JLabel lblRadiusInput;
JTextField txtRadiusInput;
JLabel lblEchoInput;
JTextField txtEchoInput;
JLabel lblAreaOutput;
JTextField txtAreaOutput;
JLabel lblCircumferenceOutput;
JTextField txtCircumferenceOutput;
JLabel lblClickToCompute;
JButton compute;
JPanel panInterface;
JPanel panGUI;
// declare two global instance variables related to inputs
boolean validInput;
double radiusInput = 0.0;

public void init( )
{

JPanel panInterface = new JPanel( );
panInterface.setLayout( new GridLayout(1, 1) );

panInterface = createGUI( );

setContentPane(panInterface);

}

public void actionPerformed(ActionEvent userClick)
{

// call method to obtain and validate input
// perform computations only if method returns true
if (validateInput( ))
{

if (userClick.getSource( ) == compute)
{

// instantiate an object of type Circle
// with the valid radius inputted by the user
Circle One = new Circle(radiusInput);
// echo back the input to the user
txtEchoInput.setText(Double.toString(One.getRadius( )));
// call the method to compute the area
// convert the numeric answer to a string with four places after the decimal point
// set the text in the appropriate JTextField
txtAreaOutput.setText(One.toPrecisionFourString(One.computeArea(One)));
// call the method to compute the circumference
// convert the numeric answer to a string with four places after the decimal point
// set the text in the appropriate JTextField
txtCircumferenceOutput.setText(One.toPrecisionFourString(One.computeCircumference(One)));
// a finalizer to free up memory allocation on Circle One
One = null;

}

}

}

public JPanel createGUI( )
{

JPanel panGUI = new JPanel( );
panGUI.setLayout( new GridLayout(5, 2, 5, 5) );

// developing input row
lblRadiusInput = new JLabel("Please input the radius: ", SwingConstants.RIGHT);
panGUI.add(lblRadiusInput);

txtRadiusInput = new JTextField(6);
panGUI.add(txtRadiusInput);

// developing command button row for program execution
lblClickToCompute = new JLabel("Click button to solve: ", SwingConstants.RIGHT);
panGUI.add(lblClickToCompute);

compute = new JButton("Compute");
compute.addActionListener(this);
panGUI.add(compute);

// developing output row for echo of the input
lblEchoInput = new JLabel("Echoing your input: ", SwingConstants.RIGHT);
panGUI.add(lblEchoInput);

txtEchoInput = new JTextField(5);
txtEchoInput.setEditable(false);
panGUI.add(txtEchoInput);

// developing output row for area
lblAreaOutput = new JLabel("The area of the circle: ", SwingConstants.RIGHT);
panGUI.add(lblAreaOutput);

txtAreaOutput = new JTextField(15);
txtAreaOutput.setEditable(false);
panGUI.add(txtAreaOutput);

// developing output row for circumference
lblCircumferenceOutput = new JLabel("The circumference of the circle: ", SwingConstants.RIGHT);
panGUI.add(lblCircumferenceOutput);

txtCircumferenceOutput = new JTextField(15);
txtCircumferenceOutput.setEditable(false);
panGUI.add(txtCircumferenceOutput);

return panGUI;

}

public boolean validateInput( )
{

validInput = true;

try
{

radiusInput = Double.parseDouble(txtRadiusInput.getText( ));

}

catch (NumberFormatException nfeDouble)
{

// give error message when input can't be parsed to double
JOptionPane.showMessageDialog(null, "You need to enter a number", "Input Error", JOptionPane.ERROR_MESSAGE);
// set boolean variable to false when
// received an invalid input
validInput = false;
// blanking out inputs and any previous outputs
// when input is invalid
txtRadiusInput.setText("");
txtEchoInput.setText("");
txtAreaOutput.setText("");
txtCircumferenceOutput.setText("");

}

return validInput;

}

}

Notice the four decimal places on the area and the circumference.

Now we will discuss the code.

we need to import the javax.swing.* package

import java.awt.*

import java.awt.event

the overall inclusive class is called CircleTestGUI which extends JApplet and implements ActionListener

declare the GUI components

a label for the radius input = lblRadiusInput

a text field for the radius input = txtRadiusInput

a label for echoing the input = lblEchoInput

a text field for echoing the input = txtEchoInput

a label for outputting the area = lblAreaOutput

a text field for outputting the area = txtAreaOutput

a label for outputting the circumference = lblCircumferenceOutput

a text field for outputting the circumference = txtCircumferenceOutput

a label to tell the user to click the compute button = lblClickToCompute

a button to cause the program to compute = compute

a panel to contain the GUI called panInterface

a panel for developing the GUI called panGUI

declare the variables associated with user inputs

a boolean validInput to indicate whether the user's inputs pass the program's validity tests

initialize and declare a variable for the user's radius input = radiusInput

the init( ) method

instantiates a panel to be the panel that contains all the other panels called panInterface

sets its layout to be a grid with one row and one column

calls the methods to create the GUI called createGUI( )

sets the content pane for the applet to be panInterface

the actionPerformed( ) method

calls the validateInput( ) method to retrieve and validate the user's input

if the input is acceptable

determines the source of the user's click

if the user clicks the compute button

creates a new instance of a Circle object called One set by the user's inputs

echoes the input in the txtEchoInput text field

gets the radius using getRadius( )

sets the text using the built in setText( ) method

updates the display in the txtAreaOutput text field

calls the computeArea( ) method in the Circle class

uses the class method toPrecisionFourString( ) to format it for display

sets the text using the built in setText( ) method

updates the display in the txtCircumferenceOutput text field

calls the computeCircumference( ) method in the Circle class

uses the class method toPrecisionFourString( ) to format it for display

sets the text using the built in setText( ) method

destroys the Circle object called One

createGUI( )

creates the panel to contain the input components

sets the layout to a GridLayout with five rows and two columns

instantiates the appropriate JLabels and JTextFields

instantiates a JButton

sets editability properties appropriately

returns this panel to the calling location

the validateInput( ) method

initializes validInput to true

uses a try block to

get the text from txtRadiusInput

parse it as a double

uses a catch block if it can't be parsed as a double to

the errorMessage is displayed using the showMessageDialog( ) method of the JOptionPane class

set validInput to false

set the txtRadiusInput to blank

set the text of txtEchoInput to blank

set the text of txtAreaOutput to blank

set the text of txtCircumferenceOutput to blank

return the boolean validInput

We will do one more of these "simpler" classes before working some more sophisticated problems.