java - Add values when JCheckBox clicked

Question

My program contains a label with the caption “Choose a coffee” and four check boxes – Americano, Espresso, Double Espresso and Latte. Note: A JCheckBox array is recommended here)

I need to add event handling code to allow the user to purchase one or more items. The bill amount is displayed in a label after the user has made their selections. The prices are Americano €3.75, Espresso €4.00, Double Espresso €4.50 and Latte €3.50. An array is suitable here also. As the user makes a choice a label is displayed showing the bill.

I cant figure out how to add the cost when the check box is selected and remove the cost when it is de selected using the arrays. Any help appreciated.

This is my code so far:

package Lab4EventHandling;

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

class Frame3 extends JFrame implements ActionListener {

    private Container cPane;
    private JLabel tMsg, bMsg;

    private JCheckBox americano, espresso, doubleEspresso, latte;
    JCheckBox[] boxes = new JCheckBox[]{americano, espresso, doubleEspresso, latte};

    private JPanel checkPanel = new JPanel(new GridLayout(0,1));
    private Color cl;

    private double cost = 0;

    private final int WINDOW_WIDTH = 200;
    private final int WINDOW_HEIGHT = 200;
    private final int x = 550;
    private final int y = 400;


    public Frame3()
    {
        cPane = getContentPane();

        cl = new Color(150, 150, 250);
        cPane.setBackground(cl);
        this.setLayout(new BorderLayout(0,1));
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLocation(x,y);

        this.add(checkPanel, BorderLayout.CENTER);

        tMsg = new JLabel("Choose a coffee:" ,SwingConstants.CENTER);
        tMsg.setBorder(BorderFactory.createEmptyBorder(4,4,4,4));

        this.add(tMsg, BorderLayout.PAGE_START);

        americano = new JCheckBox("Americano", false);
        checkPanel.add(americano);
        americano.addActionListener(this);

        espresso = new JCheckBox("Espresso", false);
        checkPanel.add(espresso);
        espresso.addActionListener(this);

        doubleEspresso = new JCheckBox("Double Espresso", false);
        checkPanel.add(doubleEspresso);
        doubleEspresso.addActionListener(this);

        latte = new JCheckBox("Latte", false);
        checkPanel.add(latte);
        latte.addActionListener(this);

        bMsg = new JLabel("Bill is ");
        bMsg.setHorizontalAlignment(SwingConstants.CENTER);
        bMsg.setBorder(BorderFactory.createEmptyBorder(4,4,4,4));
        this.add(bMsg, BorderLayout.SOUTH);


        this.setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
        this.setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {

        Double[] array = {3.75, 4.00, 4.50, 3.50}; 

        for (JCheckBox box : boxes) {

        if(americano.isSelected())
        {
            cost += 3.75;
            String r = String.valueOf(cost);
            bMsg.setText(r);
        }

        if(espresso.isSelected())
        {
            cost += 4.00;
            String r = String.valueOf(cost);
            bMsg.setText(r);
        }

        else if(doubleEspresso.isSelected())
        {
            cost = 4.50;
            String r = String.valueOf(cost);
            bMsg.setText(r);
        }

        else  if(latte.isSelected())
        {
            cost = 3.50;
            String r = String.valueOf(cost);
            bMsg.setText(r);
        }
    }

}

public class Frame3Test{

    public static void main(String [] args)
    {
        Frame3 f = new Frame3();
        f.setVisible(true);

    }
}

Answer 1

Your first problem (which you may or may not have noticed) is your array of JCheckBoxes only contains null elements:

// check boxes are null here
private JCheckBox americano, espresso, doubleEspresso, latte;

// array created with only null elements
JCheckBox[] boxes = new JCheckBox[]{americano, espresso, doubleEspresso, latte};

So you need to create the array after instantiating the actual check boxes.

...

americano = new JCheckBox("Americano", false);
checkPanel.add(americano);
americano.addActionListener(this);

espresso = new JCheckBox("Espresso", false);
checkPanel.add(espresso);
espresso.addActionListener(this);

doubleEspresso = new JCheckBox("Double Espresso", false);
checkPanel.add(doubleEspresso);
doubleEspresso.addActionListener(this);

latte = new JCheckBox("Latte", false);
checkPanel.add(latte);

boxes = new JCheckBox[] {
    americano, espresso, doubleEspresso, latte
};

Then the assignment is suggesting to use arrays because you can create another parallel array of prices (which you did). But since you need these prices in parallel you cannot use a for each loop. You need the index. Then you need to recalculate the entire cost every time anything is selected or deselected.

final double[] prices = {
    3.75, 4.00, 4.50, 3.50
};

...

double total = 0.0;

for(int i = 0; i < boxes.length; i++) {
    if(boxes[i].isSelected()) {
        total += prices[i];
    }
}

There's two other notes that seem to be outside the scope of the assignment:

• You should always make a class for this kind of association.
• You should never use double for money. Use BigDecimal or something like it.

Using a class makes logic simpler and not using double makes the calculation not incur error for this decimal addition.

class PricePair {
    JCheckBox jCheckBox;
    BigDecimal price;
}

BigDecimal total = new BigDecimal("0.00");

for(PricePair option : options) {
    if(option.jCheckBox.isSelected()) {
        total = total.add(option.price);
    }
}