Getting Input from the User

In the first two chapters of Book VI, you find out how to use two basic Swing user-interface components-labels and buttons-and how to handle events generated when the user clicks one of those buttons. If all you ever want to write are programs that display text when the user clicks a button, you can put the book down now. But if you want to write programs that actually do something worthwhile, you need to use other Swing components.

In this chapter, you find out how to use components that get information from the user. First, I cover two components that get text input from the user: text fields, which get a line of text, and text areas, which get multiple lines. Then I move on to two components that get either/or information from the user: radio buttons and check boxes.

Along the way, I tell you about some features that let you decorate these controls to make them more functional. Specifically I look at scroll bars (which are commonly used with text areas) and borders (which are used with radio buttons and check boxes).

Using Text Fields

A text field is a box that the user can type text in. You create text fields by using the JTextField class. Table 3-1 shows some of the more interesting and useful constructors and methods of this class.

Table 3-1: Handy JTextField Constructors and Methods

Open table as spreadsheet

Open table as spreadsheet

When you create a text field by calling a constructor of the JTextField class, you can specify the width of the text field and an initial text value, as in these examples:

JTextField text1 = new JTextField(15); JTextField text2 = new JTextField("Initial Value", 20);

The width is specified in columns, which is a vague and imprecise measurement that's roughly equal to the width of one character in the font that the text field uses. You have to experiment a bit to get the text fields the right size.

The usual way to work with text fields is to create them in the frame constructor, and then retrieve text entered by the user in the actionPerformed method of an action listener attached to one of the frame's buttons using code like this:

String lastName = textLastName.getText();

Here the value entered by the user into the textLastName text field is assigned to the String variable lastName.

Looking at a sample program

Figure 3-1 shows the operation of a simple program that uses a text field to ask for the user's name. If the user enters a name, the program uses JOptionPane to say good morning to the user by displaying the middle message box shown in Figure 3-1. But if the user clicks the button without entering anything, the program displays the second JOptionPane message shown at the bottom.

Figure 3-1: The Namer application in action.

The code for this program is shown in Listing 3-1.

Listing 3-1: Saying Good Morning with a Text Field

import javax.swing.*; import java.awt.event.*; public class Namer extends JFrame → 4 { public static void main(String [] args) { new Namer(); } private JButton buttonOK; private JTextField textName; → 12 public Namer() { this.setSize(325,100); this.setTitle("Who Are You?"); this.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); ButtonListener bl = new ButtonListener(); JPanel panel1 = new JPanel(); panel1.add(new JLabel("Enter your name: ")); → 25 textName = new JTextField(15); → 27 panel1.add(textName); buttonOK = new JButton("OK"); buttonOK.addActionListener(bl); panel1.add(buttonOK); this.add(panel1); this.setVisible(true); } private class ButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { String name = textName.getText(); → 46 if (name.length() == 0) → 47 { JOptionPane.showMessageDialog( Namer.this, "You didn't "Moron", JOptionPane.INFORMATION_MESSAGE); } else { JOptionPane.showMessageDialog( Namer.this, "Good morning " + name, "Salutations", JOptionPane.INFORMATION_MESSAGE); } textName.requestFocus(); → 63 } } } }

This program isn't very complicated, so the following paragraphs just hit the highlights:

Open table as spreadsheet

Using text fields for numeric entry

You need to take special care if you're using a text field to get numeric data from the user. The getText method returns a string value. You can pass this value to one of the parse methods of the wrapper classes for the primitive numeric types. For example, to convert the value entered into a text box to an int, you use the parseInt method:

int count = Integer.parseInt(textCount.getText());

Here the result of the getText method is used as the parameter to the parseInt method.

Table 3-2 lists the parse methods for the various wrapper classes. Note that each of these methods throws NumberFormatException if the string can't be converted. As a result, you need to call the parseInt method in a try/catch block to catch this exception.

Then you can call this method whenever you need to check to see if a text field has a valid integer. For example, here's the actionPerformed method for a program that gets the value entered in a textCount text field and displays it in a JOptionPane message box if the value entered is a valid integer:

public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { if (isInt(textCount, "You must enter an integer.")) { JOptionPane.showMessageDialog(Number.this, "You entered " + Integer.parseInt(textCount.getText()), "Your Number", JOptionPane.INFORMATION_MESSAGE); } textCount.requestFocus(); } }

Here the isInt method is called to make sure the text entered by the user can be converted to an int. If so, the text is converted to an int and displayed in a message box. (In this example, the name of the outer class is Number, which is why the first parameter of the showMessageDialog method specifies Number.this.)

Creating a validation class

Of course, a method can return only one value. The only way to coax a method into returning two values is to return an object that contains both of the values. And to do that, you have to create a class that defines the object. Here's an example of a class you could use as the return value of a method that validates integers:

public class IntValidationResult { public boolean isValid; public int value; }

And here's a class that provides a static method named isInt that validates integer data and returns an IntValidationResult object:

public class Validation { public static IntValidationResult isInt( JTextField f, String msg) { IntValidationResult result = new IntValidationResult(); try { result.value = Integer.parseInt(f.getText()); result.isValid = true; return result; } catch (NumberFormatException e) { JOptionPane.showMessageDialog(f, "Entry Error", msg, JOptionPane.ERROR_MESSAGE); f.requestFocus(); result.isValid = false; result.value = 0; return result; } } }

Here's an actionPerformed method that uses the isInt method of this class to validate the textCount field:

public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { IntValidationResult ir; ir = Validation.isInt(textCount, "You must enter an integer."); if (ir.isValid) { JOptionPane.showMessageDialog(Number2.this, "You entered " + ir.value, "Your Number", JOptionPane.INFORMATION_MESSAGE); } textCount.requestFocus(); } }

Using Text Areas

A text area is similar to a text field, but lets the user enter more than one line of text. If the user enters more text into the text area than can be displayed at once, the text area can use a scroll bar to let the user scroll to see the entire text. Figure 3-2 shows a text area in action.

Figure 3-2: A frame that uses a text area.

To create a text area like the one shown in Figure 3-2, you must actually use two classes. First, you use the JTextArea class to create the text area. But unfortunately, text areas by themselves don't have scroll bars. So you have to add the text area to a second component called a scroll pane, created by the JScrollPane class. Then you add the scroll pane-not the text area-to a panel so it can be displayed.

Creating a text area isn't as hard as it sounds. Here's the code I used to create the text area shown in Figure 3-2, which I then added to a panel:

textNovel = new JTextArea(10, 20); JScrollPane scroll = new JScrollPane(textNovel, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER); panel1.add(scroll);

Here the first statement creates a text area, giving it an initial size of 10 rows and 20 columns. Then the second statement creates a scroll pane. Notice that the text area object is passed as a parameter to the constructor for the JScrollPane, along with constants that indicate whether the scroll pane should include vertical or horizontal scroll bars (or both). Finally, the third statement adds the scroll pane to the panel named panel1.

The following sections describe the constructors and methods of the JTextArea and JScrollPane classes in more detail.

The JTextArea class

Table 3-3 lists the most popular constructors and methods of the JTextArea class, which you use to create text areas. In most cases, you use the second constructor, which lets you set the number of rows and columns to display. The rows parameter governs the height of the text area, while the cols parameter sets the width.

Table 3-3: Clever JTextArea Constructors and Methods

Open table as spreadsheet

Open table as spreadsheet

To retrieve the text that the user enters into a text area, you use the getText method. For example, here's an actionPerformed method from an action listener that retrieves text from a text area:

public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { String text = textNovel.getText(); if (text.contains("All work and no play")) JOptionPane.showMessageDialog(textNovel, "Can't you see I'm working?", "Going Crazy", JOptionPane.ERROR_MESSAGE); } }

Here a message box is displayed if the text contains the string All work and no play.

Notice that in addition to the getText method, the JTextArea class has methods that let you add text to the end of the text area's current value (append), insert text in the middle of the value (insert), and replace text (replace). You use these methods to edit the value of the text area.

The JScrollPane class

Text areas aren't very useful without scroll bars. To create a text area with a scroll bar, you use the JScrollPane class, whose constructors and fields are listed in Table 3-4. Note that this table doesn't show any methods for the JScrollPane class. The JScrollPane class does have methods (plenty of them, in fact). But none of them are particularly useful for ordinary programming, so I didn't include any of them in the table.

Table 3-4: Essential JScrollPane Constructors and Fields

Open table as spreadsheet

Open table as spreadsheet

The usual way to create a scroll pane is to use the second constructor. You use the first parameter of this constructor to specify the component you want to add scroll bars to. For example, to add scroll bars to a textNovel text area, you specify textNovel as the first parameter.

The second parameter tells the scroll pane whether or not to create a vertical scroll bar. The value you specify for this parameter should be one of the first three fields listed in Table 3-4:

• VERTICAL_SCROLLBAR_ALWAYS: Choose this if you always want the scroll pane to show a vertical scroll bar.
• VERTICAL_SCROLLBAR_AS_NEEDED: Specify this if you want to see the vertical scroll bar only when the text area contains more lines that can be displayed at once; the vertical scroll bar is shown only when it's needed.
• VERTICAL_SCROLLBAR_NEVER: Choose this if you never want to see a vertical scroll bar on-screen.

The third parameter uses the three HORIZONTAL_SCROLLBAR constants to indicate whether the scroll pane includes a horizontal scroll bar always, never, or only when necessary.

Thus the following code adds scroll bars to a text area. The vertical scroll bar is always shown, but the horizontal scroll bar is shown only when needed:

JScrollPane scroll = new JScrollPane(textNovel, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);

Using Check Boxes

A check box is a control that the user can click to either check or uncheck. Check boxes are usually used to let the user specify Yes or No to an option. Figure 3-3 shows a frame with three check boxes.

Figure 3-3: A frame with three check boxes.

To create a check box, you use the JCheckBox class. Its favorite constructors and methods are shown in Table 3-5.

Table 3-5: Notable JCheckBox Constructors and Methods

Open table as spreadsheet

Open table as spreadsheet

As with any Swing component, if you want to refer to the component in both the frame class constructor and a listener, you have to declare class variables to refer to the check box components, like this:

JCheckBox pepperoni, mushrooms, anchovies;

Then you can use statements like these in the frame constructor to create the check boxes and add them to a panel (in this case, panel1):

pepperoni = new JCheckBox("Pepperoni"); panel1.add(pepperoni); mushrooms = new JCheckBox("Mushrooms"); panel1.add(mushrooms); anchovies = new JCheckBox("Anchovies"); panel1.add(anchovies);

Notice that I didn't specify the initial state of these check boxes in the constructor. As a result, they're initially unchecked. If you want to create a check box that is initially checked, call the constructor like this:

Pepperoni = new JCheckBox("Pepperoni", true);

In an event listener, you can test the state of a check box by using the isSelected method, and you can set the state of a check box by calling its setSelected method. For example, here's an actionPerformed method that displays a message box and unchecks all three check boxes when the user clicks the OK button:

public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { String msg = ""; if (pepperoni.isSelected()) msg += "Pepperoni "; if (mushrooms.isSelected()) msg += "Mushrooms "; if (anchovies.isSelected()) msg += "Anchovies "; if (msg.equals("")) msg = "You didn't order any toppings."; else msg = "You ordered these toppings: " + msg; JOptionPane.showMessageDialog(buttonOK, msg, "Your Order", JOptionPane.INFORMATION_MESSAGE); pepperoni.setSelected(false); mushrooms.setSelected(false); anchovies.setSelected(false); } }

Here the name of each topping selected by the user is added to a text string. For example, if you select Pepperoni and Anchovies, the following message is displayed:

You ordered these toppings: Pepperoni Anchovies

Suppose your restaurant has anchovies on the menu, but you refuse to actually make pizzas with anchovies on them. Here's an actionPerformed method from an action listener that displays a message if the user tries to check the Anchovies check box and unchecks the box:

public void actionPerformed(ActionEvent e) { if (e.getSource() == anchovies) { JOptionPane.showMessageDialog(anchovies, "We don't do anchovies here.", "Yuck!", JOptionPane.WARNING_MESSAGE); anchovies.setSelected(false); } }

Using Radio Buttons

Radio buttons are similar to check boxes, but with a crucial difference: Radio buttons travel in groups, and a user can select only one radio button in each group at a time. When you click a radio button to select it, whatever radio button was previously selected is automatically deselected. Figure 3-4 shows a frame with three radio buttons.

To work with radio buttons, you use two classes. First, you create the radio buttons themselves with the JRadioButton class, whose constructors and methods are shown in Table 3-6. Then you create a group for the buttons with the ButtonGroup class. You must add the radio buttons themselves to a panel (so they are displayed) and to a button group (so they're properly grouped with other buttons).

Figure 3-4: A frame with three radio buttons.

Table 3-6: Various JRadioButton Constructors and Methods

Open table as spreadsheet

Open table as spreadsheet

The usual way to create a radio button is to declare a variable to refer to the button as a class variable so it can be accessed anywhere in the class. For example

JRadioButton small, medium, large;

Then, in the frame constructor, you call the JRadioButton constructor to create the radio button:

small = new JRadioButton("Small");

You can then add the radio button to a panel in the usual way.

To create a button group to group radio buttons that work together, just call the ButtonGroup class constructor:

ButtonGroup group1 = new ButtonGroup();

Then call the add method of the ButtonGroup to add each radio button to the group:

group1.add(small); group1.add(medium); group1.add(large);

Where button groups really come in handy is when you have more than one set of radio buttons on a form. For example, suppose that in addition to choosing the size of the pizza, the user can also choose the style of crust-thin or thick. In that case, you use a total of five radio buttons and two button groups. The constructor code that creates the radio buttons might look something like this:

ButtonGroup size = new ButtonGroup(); ButtonGroup crust = new ButtonGroup(); small = new JRadioButton("Small"); medium = new JRadioButton("Medium"); large = new JRadioButton("Large"); size.add(small); size.add(medium); size.add(large); thin = new JRadioButton("Thin Crust"); thick = new JRadioButton("Thick Crust"); crust.add(thin); crust.add(thick);

(To keep this example simple, I omitted the statements that add the radio buttons to the panel.)

Using Borders

A border is a decorative element that visually groups components by drawing a line around them. Figure 3-5 shows a frame that shows some radio buttons and check boxes inside borders.

Figure 3-5: A frame with borders.

You can apply a border to any object that inherits JComponent, but the usual technique is to apply the border to a panel and add any components you want to appear within the border to the panel. To create a border, you call one of the static methods listed in Table 3-7. Each of these methods creates a border with a slightly different visual style. You then apply the Border object to a panel by calling the panel's setBorder method.

The BorderFactory class is in the javax.swing package, but the Border interface that defines the resulting border objects is in javax.swing.border. Thus you need to include this import statement at the beginning of the class-in addition to importing javax.swing.*-if you plan on using borders:

import javax.swing.border.*;

For example, here's a snippet of code that creates a panel, creates a titled border, and applies the border to the panel:

JPanel sizePanel = new JPanel(); Border b1 = BorderFactory.createTitledBorder("Size"); sizePanel.setBorder(b1);

Then any components you add to sizePanel appear within this border.

The last method listed in Table 3-7 needs a little explanation. It simply adds a title to a border created by any of the other created methods of the BorderFactory class. For example, you can create a raised bevel border with the title Options like this:

Border b = BorderFactory.createRaisedBevelBorder(); b = BorderFactory.createTitledBorder(b, "Options");

Designing a Pizza Ordering Program

To give you an idea of how borders work together with radio buttons and check boxes, Listing 3-2 presents the complete code for the program that created the frame that was shown in Figure 3-5. When the user clicks the OK button, this program displays a message box summarizing the user's order. For example, if the user orders a medium pizza with pepperoni and mushrooms, the following message is displayed:

You ordered a medium pizza with the following toppings: Pepperoni Mushrooms

If you order a pizza with no toppings, the message you get looks something like this:

You ordered a medium pizza with no toppings.

Listing 3-2: The Pizza Order Program

import javax.swing.*; import java.awt.event.*; import javax.swing.border.*; public class Pizza extends JFrame { public static void main(String [] args) { new Pizza(); } private JButton buttonOK; → 12 private JRadioButton small, medium, large; private JCheckBox pepperoni, mushrooms, anchovies; public Pizza() { this.setSize(320,200); this.setTitle("Order Your Pizza"); this.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); ButtonListener bl = new ButtonListener(); JPanel mainPanel = new JPanel(); → 24 JPanel sizePanel = new JPanel(); → 26 Border b1 = → 27 BorderFactory.createTitledBorder("Size"); sizePanel.setBorder(b1); → 29 ButtonGroup sizeGroup = new ButtonGroup(); → 31 small = new JRadioButton("Small"); → 33 small.setSelected(true); sizePanel.add(small); sizeGroup.add(small); medium = new JRadioButton("Medium"); → 38 sizePanel.add(medium); sizeGroup.add(medium); large = new JRadioButton("Large"); → 42 sizePanel.add(large); sizeGroup.add(large); mainPanel.add(sizePanel); → 46 JPanel topPanel = new JPanel(); → 48 Border b2 = BorderFactory.createTitledBorder( "Toppings"); topPanel.setBorder(b2); pepperoni = new JCheckBox("Pepperoni"); → 53 topPanel.add(pepperoni); mushrooms = new JCheckBox("Mushrooms"); topPanel.add(mushrooms); anchovies = new JCheckBox("Anchovies"); topPanel.add(anchovies); mainPanel.add(topPanel); → 62 buttonOK = new JButton("OK"); → 64 buttonOK.addActionListener(bl); mainPanel.add(buttonOK); this.add(mainPanel); → 68 this.setVisible(true); } private class ButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { String tops = ""; → 79 if (pepperoni.isSelected()) tops += "Pepperoni "; if (mushrooms.isSelected()) tops += "Mushrooms "; if (anchovies.isSelected()) tops += "Anchovies "; String msg = "You ordered a "; → 87 if (small.isSelected()) msg += "small pizza with "; if (medium.isSelected()) msg += "medium pizza with "; if (large.isSelected()) msg += "large pizza with "; if (tops.equals("")) → 95 msg += "no toppings."; else msg += "the following toppings: " + tops; JOptionPane.showMessageDialog( → 100 buttonOK, msg, "Your Order", JOptionPane.INFORMATION_MESSAGE); pepperoni.setSelected(false); → 104 mushrooms.setSelected(false); anchovies.setSelected(false); small.setSelected(true); } } } }

I cover everything in this program in this chapter (or in previous chapters), so I just hit the highlights here:

Open table as spreadsheet

Using Sliders

As Figure 3-6 shows, a slider is a component that lets a user pick a value from a set range (say, from 0 to 50) by moving a knob. Sliders are a convenient way to get numeric input from the user when the input falls within a set range of values.

Figure 3-6: A frame with a slider.

To create a slider control, you use the JSlider class. Table 3-8 shows its constructors and methods.

Table 3-8: Selected JSlider Constructors and Methods

Open table as spreadsheet

Open table as spreadsheet

To create a barebones slider, just call the JSlider constructor. You can create a slider that ranges from 0 to 100 like this:

slider = new JSlider();

Here the slider variable is declared as a class variable of type JSlider.

If you want to specify the minimum and maximum values, use this constructor:

slider = new JSlider(0, 50);

Here the slider lets the user choose a value from 0 to 50. The initial position of the knob is 25, midway between the minimum and maximum values.

To set a different initial value, use this constructor:

slider = new JSlider(0, 0, 50);

Here the slider ranges from 0 to 50, and the initial value is 0.

You usually want to add at least some adornments to the slider to make it more usable. The slider shown in Figure 3-6 has minimum and maximum tick-mark values with labels visible. Here's the code used to create it:

slider = new JSlider(0, 50, 0); slider.setMajorTickSpacing(10); slider.setMinorTickSpacing(1); slider.setPaintTicks(true); slider.setPaintLabels(true); panel1.add(slider);

To get the value of the slider, you use the getValue method. For example, here's the actionPerformed method for the action listener attached to the OK button in Figure 3-6:

public void actionPerformed(ActionEvent e) { if (e.getSource() == buttonOK) { int level = slider.getValue(); JOptionPane.showMessageDialog(slider, "Remember, this is for posterity. " + "Tell me...how do you feel?", "Level " + level, JOptionPane.INFORMATION_MESSAGE); } }

Here a message box is displayed when the user clicks the OK button. The current setting of the slider component is retrieved and stored in an int variable named level, which is then used to create the title for the message box.

You can also add an event listener that reacts whenever the user changes the value of the slider. To do that, you use the addChangeListener method. The parameter must be an object that implements the ChangeListener interface, which defines a single method named stateChanged. Here's an example of a class that can be used to react to slider changes:

private class SliderListener implements ChangeListener { public void stateChanged(ChangeEvent e) { if (slider.getValue() == 50) { JOptionPane.showMessageDialog(slider, "No! Not 50!", "The Machine", JOptionPane.WARNING_MESSAGE); } } }

To wire an instance of this class to the slider, use this method:

slider.addChangeListener(new SliderListener());

Then the stateChanged method is called whenever the user moves the knob to another position. It checks the value of the slider and displays a message box if the user has advanced the slider all the way to 50.