Java Notes

Example - Generic Calc

This program converts miles into kilometers. It provides a basic plan for making a program that, when a button is pressed, takes input from a field, computes a new value based on that, and puts the result in another field. Extending this program is the basis for many other examples. To customize it:

Change the documentation.
Change the names of the components (window, fields, button, etc). Remember to save the file in a name that matches the new class name.
Add additional labels, fields, or buttons that are needed.
The button listener may have to be changed to convert the input to the correct type to match the parameter(s) of the method that does the calculations.

// GenericCalc.java - Input field, button, output field generic program.
// Fred Swartz, 2004-10-25

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

////////////////////////////////////////////////////// class GenericCalc
class GenericCalc {
    //====================================================== method main
    public static void main(String[] args) {
        JFrame window = new GenericCalcGUI();
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true);
    }
}


/////////////////////////////////////////////////// class GenericCalcGUI
class GenericCalcGUI extends JFrame {
    //=============================================== instance variables
    private JTextField m_milesTf      = new JTextField(10);
    private JTextField m_kilometersTf = new JTextField(10);
    private JButton    m_convertBtn   = new JButton("Convert");


    //====================================================== constructor
    public GenericCalcGUI() {
        //... Add a listener to the button
        m_convertBtn.addActionListener(new ConvertListener());

        //... Create content panel, set layout, add components
        JPanel content = new JPanel();
        content.setLayout(new FlowLayout());   // use FlowLayout
        content.add(new JLabel("Miles"));      // create and add label
        content.add(m_milesTf);                // add input field
        content.add(m_convertBtn);             // add button
        content.add(new JLabel("Kilometers")); // create and add label
        content.add(m_kilometersTf);           // add output field
        
        //... Set window characteristics
        this.setContentPane(content);
        this.pack();
        this.setTitle("Miles to Kilometers");
        this.setResizable(false);
    }//end constructor
    
    
    ///////////////////////////////////////// inner class ConvertListener
    class ConvertListener implements ActionListener {
        //============================================= actionPerformed 
        public void actionPerformed(ActionEvent e) {
            // This only copies input field to output field.
            String in = m_milesTf.getText(); // get input
            double mi = Double.parseDouble(miles);
            double km = convertMilesToKilometers(mi);
            m_kilometersTf.setText("" + km);   // set output
        }
    }//end class ConvertListener
    
    
    //=========================================== convertMilesToKilometers
    // Performing the pure logic of the program (often called the model)
    //    is typically done in a separate class.  
    //    Here we only put it into a separate method.
    // You might think this is too trivial to put into a method.
    //    Perhaps in this case, but it is a very good habit to
    //    acquire because in "real" programs there will be 
    //    a big payoff in clarity.
    public double convertMilesToKilometers(double miles) {
        return mi * 1.62;
    }
    
}//endclass GenericCalcGUI