GUI Components

More Low Level GUI Components

Review Questions for GUIs:
1. Write Java code that creates a panel and makes the layout for the panel BorderLayout.
2. Write Java code that creates a button. The text on the button should initially read "off", but when it is clicked it should alternate between "on" and "off".
3. Add the button you created in #2 to the south region of the panel you created in #1.
4. Assume that you have a JButton button already created, and that this button's text is the number of times it has been clicked (currently 0). Write Java code that handles clicks on this button.

Text Fields

Allows user to enter a small amount of text
When the user hits return to indicate that the text field is complete, an action event is signaled
Class: JTextField. The constructor takes the number of columns in the field

Example: JTextField textField = new JTextField(25); // a text field with 25 columns

Use an ActionListener object to handle the event when text is entered in the text field. You must implement the actionPerformed() method.

Example:
JTextField textfield = new JTextField(25);
textfield.addActionListener(this); // our class will implement the ActionListener
// interface
...
frame.getContentPane().add(textfield);

public void actionPerformed(ActionEvent e)
{
String text = textField.getText();
}

Example: Write a program that processes strings entered in a text field. Update a button's text to contain the most recently entered string.

Labels

Class: JLabel
Use a JLabel to display text and images
Useful methods:

void setText(String s)
String getText()

Constructors:

JLabel() // creates a label with no image and with an empty string for the title
JLabel(Icon image) // creates a label instance with the specified image
JLabel(String text) // creates a label with the specified text

Example: Write an application which contains a button and a label. The button's text should contain an integer equal to the number of button clicks, and the label should contain a label of the form "Number of button clicks: 3".

Check Boxes

Class: JCheckBox
Used for yes or no input from the user
User checks the box by clicking it and unchecks it by clicking it again
A check box generates an item event. Use the addItemListener() method to add a listener to a check box.
Implement this method in the ItemListener interface:

public void itemStateChanged(ItemEvent e)

The itemStateChanged() method is invoked when an item is selected or deselected by user.
If you have multiple check boxes, you may want to have your class implement the ItemListener interface - you can determine which check box triggered the item event using the getItemSelectable() method
Use the ItemEvent method getStateChange() to determine if the check box is SELECTED or DESELECTED.

javax.swing.JCheckBox

JCheckBox(String label) // the parameter is the label on the checkbox

JCheckBox(String label, boolean selected) // selected is the initial state of the checkbox. true = selected, false = not selected

boolean isSelected() // returns state of the checkbox (true = selected)

void setSelected(boolean state) // sets checkbox to a new state

java.awt.event.ItemEvent

getItemSelectable() // returns the originator of the event

getStateChange() // returns the type of state change (selected or deselected)

Example:
class GUIEx implements ItemListener
{
    ...
    pepperoniButton.addItemListener(this);
    ...
    public void itemStateChanged(ItemEvent e)
    {
       Object src = e.getItemSelectable();

       if(src == pepperoniButton) { ... // put pepperoni in the list of included
         ingredients}
       else if(src == cheeseButton) ...
    }

    if(e.getStateChange() == ItemEvent.DESELECTED) {// remove item from the
     ingredient list}

Example: Write a program with 3 checkboxes. The 3 checkboxes should contain the text "Box 1", "Box 2", and "Box 3". Include a label for each box which contains "Box n is selected" or "Box n is not selected", depending on the state of each box.

Exercise: Write a program that a person might use while ordering a pizza. Include 5 checkboxes for 5 different ingredients: pepperoni, cheese, peppers, pineapple, and onion. Use a label to indicate which toppings will be used for the pizza.

Radio Buttons

Checkboxes: If you have 3 boxes, any or all of them can be checked.
Radio Buttons: Only one button in a group of buttons can be selected at any time.
Constructor: JRadioButton(String label, boolean state) // label = label on check box, state indicates whether button is initially checked or not
When one button in a group is selected/checked, the previously selected box/button is automatically unchecked.
Ex: Use radio buttons for user to select pizza size - it only makes sense to have one option checked.

To create associated radio buttons, e.g. small, medium and large buttons for pizza size:
1. Construct one object of type ButtonGroup for each group of buttons.

2. Add objects of type JRadioButton to the button group.

3. The button group object deselects the previously selected button when a new button is clicked.

4. The ButtonGroup object forms a relationship between the states of the buttons which are added to it. It does NOT replace the need to put the radio buttons on a panel, content pane, etc. A ButtonGroup object is not a GUI component, so do not try to add it to a container.

5. A radio button generates an action event when clicked, so implement actionPerformed() in the ActionListener interface to handle this event.

Example:
JRadioButton smallButton = new JRadioButton("small", false); // false --> not initially selected
JRadioButton medButton = new JRadioButton("medium", true); // text for button is "medium"
JRadioButton largeButton = new JRadioButton("large", false);

ButtonGroup group = new ButtonGroup(); // this is how we tie the 3 buttons together, so only one option can be chosen
group.add(smallButton);
group.add(medButton);
group.add(largeButton);

java.util.EventObject (ancestor of ActionEvent class)

Object getSource() // returns the object on which the event initially occurred.

Example: Write a program that uses radio buttons to allow the user to choose a pizza size. Indicate the current size choice on a label.

Borders

Separate groups of radio buttons from each other using borders
Place a border on a panel, and then add a group of radio buttons (or other components) to the panel
A border can be added to any component that inherits from JComponent
Call a static method of BorderFactory to create a border
Border styles:

Lowered bevel
Raised bevel
Etched
Line

Example:
JPanel buttonPanel = new JPanel();
buttonPanel.add(smallButton);
buttonPanel.add(mediumButton);
buttonPanel.add(largeButton);

Border etched = BorderFactory.createRaisedBevelBorder();
buttonPanel.setBorder(etched);

Methods from javax.swing.BorderFactory

static Border createEtchedBorder() // creates a line border with a 3D effect
static Border createLoweredBevelBorder() // creates a border that gives the effect of a lowered surface
static Border createRaisedBevelBorder() // creates a border that gives the effect of a raised surface
static Border createLineBorder(Color color) // creates a simple line border

Example:
Border border = BorderFactory.createLineBorder(Color.blue);
panel.setBorder(border);

Exercise: Add a border to the panel containing the buttons that represent the pizza's size in our last example.