Overloading Constructors

Background. Notice in the TimeBasicsTest.java that when we instantiate/construct a TimeBasics object we set all the values to zero and then set the time based on the user's inputs in the next line of code. We repeat the code here for ease of reference.

// instantiate an object of type Circle
// with the valid time inputted by the user
TimeBasics thisTime = new TimeBasics( );
thisTime.setTime(hoursInput, minutesInput, secondsInput);

This code is contained in the actionPerformed( ) method that gets executed if the user clicks on the button and has given valid inputs.

But we can improve on this somewhat by creating another constructor with the same name, but with a different number of arguments. Doing this is called constructor overloading. Remember, the constructor must have the same name as the overall inclusive class. How the compiler differentiates between which constructor you want to use depends on how many arguments you pass when you invoke it.

With this in mind we have slightly modified the TimeBasics.java class to contain another constructor. You should still call this TimeBasics.java and compile it using the

javac -d C:\j2sdk1.4.0\jre\classes TimeBasics.java

command. The only changes in the code are highlighted in blue.

package javaPackages.TimeClasses;
import java.text.DecimalFormat;

// This class maintains the time in a 24 hour format
// and has a method to convert this to an AM/PM scale

public class TimeBasics extends Object
{

private int hour; // from 0 to 23
private int minute; // from 0 to 59
private int second; // from 0 to 59

// The TimeBasics constructor initializes each instance variable (instantiation of an object) to zero.
// It also ensures that each TimeBasics object starts in a state consistent with its specifications
public TimeBasics( )
{

setTime(0,0,0);

}
// A constructor that takes three argunments
// it is differentiated from the constructor
// with no arguments by the compiler fue to
// the number of arguments that are passed
public TimeBasics(int h, int m, int s)
{

setTime(h,m,s);

}

// Set a new time using military/universal representation.
// Perform validity checks on the data.
// Set invalid values to zero to ensure consistency.
public void setTime( int h, int m, int s)
{

// if hour number is outside the acceptable range set it to 0
if (h < 0 || h >= 24) hour = 0;
else hour = h;
// if minutes number is outside the acceptable range set it to 0
if (m < 0 || m >= 60) minute = 0;
else minute = m;
// if seconds number is outside the acceptable range set it to 0
if (s < 0 || s >= 60) second = 0;
else second = s;

}

// This method converts a time to a string in military/universal format
public String toUniversalString()
{

DecimalFormat twoDigits = new DecimalFormat("00");

return twoDigits.format(hour) + ":" + twoDigits.format(minute) + ":" + twoDigits.format(second);

}

// Convert string into AM/PM time format
public String toAMPMString( )
{

String AMPM;
int tempHour;
// setting up a decimal format to make certain numbers
// have two digits with no decimal places
DecimalFormat twoDigits = new DecimalFormat("00");
// checking for noon and midnight
if (hour == 12 || hour == 0)

tempHour = 12;

// if not noon or midnight then
// cast hour into correct number of hours
// since noon or midnight
else

tempHour = hour % 12;

// determine whether it is AM or PM by hour
if (hour < 12)

AMPM = "AM";

else

AMPM = "PM";

return tempHour + ":" + twoDigits.format(minute) + ":" + twoDigits.format(second) + AMPM;

}

public String echoInput( )
{

DecimalFormat twoDigits = new DecimalFormat("00");

return twoDigits.format(hour) + ":" + twoDigits.format(minute) + ":" + twoDigits.format(second);

}

}

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javaPackages.TimeClasses.TimeBasics;

public class TimeBasicsTest extends JApplet implements ActionListener
{

// declare GUI components
JLabel lblHoursInput;
JTextField txtHoursInput;
JLabel lblMinutesInput;
JTextField txtMinutesInput;
JLabel lblSecondsInput;
JTextField txtSecondsInput;
JLabel lblEchoInput;
JTextField txtEchoInput;
JLabel lblUniversalTime;
JTextField txtUniversalTime;
JLabel lblAMPMTime;
JTextField txtAMPMTime;
JLabel lblClickToCompute;
JButton compute;
JPanel panInterface;
// declare four global variables related to inputs
boolean validInput;
int hoursInput = 0;
int minutesInput = 0;
int secondsInput = 0;

public void init( )
{

JPanel panInterface = new JPanel();
panInterface = createGUI();

setContentPane(panInterface);

}

public void actionPerformed(ActionEvent userClick)
{

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

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

// instantiate an object of type Circle
// with the valid times inputted by the user
TimeBasics thisTime = new TimeBasics(hoursInput, minutesInput, secondsInput);
// echo back the input to the user
txtEchoInput.setText(thisTime.echoInput( ));
// call the method to convert the inputs
// to universal time
// set the text in the appropriate JTextField
txtUniversalTime.setText(thisTime.toUniversalString( ));
// call the method to compute the circumference
// convert the numeric answer to a string
// set the text in the appropriate JTextField
txtAMPMTime.setText(thisTime.toAMPMString( ));
// a finalizer to free up memory allocation on TimeBasics thisTime
thisTime = null;

}

}

}

public JPanel createGUI( )
{

JPanel panGUI = new JPanel();
panGUI.setLayout( new GridLayout(7, 2, 5, 5) );

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

txtHoursInput = new JTextField(3);
panGUI.add(txtHoursInput);

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

txtMinutesInput = new JTextField(3);
panGUI.add(txtMinutesInput);

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

txtSecondsInput = new JTextField(3);
panGUI.add(txtSecondsInput);

// 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(10);
txtEchoInput.setEditable(false);
panGUI.add(txtEchoInput);

// developing output row for universal time
lblUniversalTime = new JLabel("In universal time: ", SwingConstants.RIGHT);
panGUI.add(lblUniversalTime);

txtUniversalTime = new JTextField(10);
txtUniversalTime.setEditable(false);
panGUI.add(txtUniversalTime);

// developing output row for AM/PM time
lblAMPMTime = new JLabel("In AM/PM time: ", SwingConstants.RIGHT);
panGUI.add(lblAMPMTime);

txtAMPMTime = new JTextField(10);
txtAMPMTime.setEditable(false);
panGUI.add(txtAMPMTime);

return panGUI;

}

public boolean validateInput( )
{

String errorMessage = "";
validInput = true;

try
{

hoursInput = Integer.parseInt(txtHoursInput.getText());

}

catch (NumberFormatException nfeInteger)
{

// give error message when input can't be parsed to integer
errorMessage = errorMessage + "You need to enter an integer for the hour\n";
// set boolean variable to false when
// received an invalid input
validInput = false;
// blanking out inputs and any previous outputs
// when input is invalid
txtHoursInput.setText("");

}

try
{

minutesInput = Integer.parseInt(txtMinutesInput.getText());

}

catch (NumberFormatException nfeInteger)
{

// give error message when input can't be parsed to integer
errorMessage = errorMessage + "You need to enter an integer for the minutes\n";
// set boolean variable to false when
// received an invalid input
validInput = false;
// blanking out inputs and any previous outputs
// when input is invalid
txtMinutesInput.setText("");

}

try
{

secondsInput = Integer.parseInt(txtSecondsInput.getText());

}

catch (NumberFormatException nfeInteger)
{

// give error message when input can't be parsed to integer
errorMessage = errorMessage + "You need to enter an integer for the seconds\n";
// set boolean variable to false when
// received an invalid input
validInput = false;
// blanking out inputs and any previous outputs
// when input is invalid
txtSecondsInput.setText("");

}

if (!validInput)
{

JOptionPane.showMessageDialog(null, errorMessage, "Input Error", JOptionPane.ERROR_MESSAGE);
txtEchoInput.setText("");
txtUniversalTime.setText("");
txtAMPMTime.setText("");

}

return validInput;

}

}