Swing

1. Swing

2. Creating an Applet with a Graphical User Interface There are three steps that a programmer must complete to create an applet with a graphical user interface: Step 1. Declare and define the components that will be placed on the applet. Step 2. Choose a Layout Manager and use this manager to direct the placement of the components on the applet. Step 3. Add appropriate event listeners to the components you have placed in the applet, and provide the appropriate (actionPerformed) instructions for each event listener In this lecture we focus on Step 1 – declaring and defining components.

3. Swing Differences between Swing and AWT Naming Conventions All Swing components begin with a capital J -- JPanel, JButton, JScrollBar, JApplet, etc.. Most Swing Components are “Lightweight” • Use Java code rather than native code to draw in the underlying window. • The Exceptions are JFrame, JApplet, JWindow, andJDialog Use Content Pane for adding Components to “Heavyweight” Containers To add Components to a JApplet (in method init( )) use Container cp = new getContentPane( ); cp.add(new JButton(“Start”);

4. Part of the AWT Class Hierarchy Object Component Button Checkbox Choice List Label Canvas TextComponent Container Scrollbar TextArea TextField Panel Window ScrollPane Applet Dialog Frame

5. Container Panel Window ScrollPane Applet Heavyweight Swing components Dialog Frame The “Swing Set” All Swing classes are derived from the AWT abstract class Container AWT classes Swing classes JComponent JApplet JDialog JFrame JWindow

6. Object Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects implement the methods of this class. Component The Component class is the abstract superclass of many of the AWT classes. It represents something that has a position, a size, can be painted on the screen, and can receive input events. Some Methods Defined in Class Component: setForeground(Color) set the component’s foreground color setBackground(Color) set the component’s background color setFont(Font) set the component’s font paint(Graphics) repaint the component repaint( ) schedule the component for repainting addxxxListener(xxxListener) add an xxx listener for the component (where xxx is Component, Focus, Key, Mouse, or MouseMotion) An xxxListener is an object that waits for an event to occur, then executes certain actions in response to the event.

7. JLabel JPanel JTextComponent JComboBox JScrollBar AbstractButton JList JScrollPane others JMenuItem JButton JToggleButton JTextField JTextArea JEditorPane Heavyweight Swing components JCheckBox JRadioButton Swing Components Swing classes JComponent JApplet JDialog JFrame JWindow

8. An Applet Displaying the Components to be Discussed: ComponentIntro1 The Essence of the Structure of the Code of the Nine Applets to be Displayed Next: import java.awt.*; import javax.swing.*; public class Xxx extends JApplet { <declarations of Components> //in a few cases other stuff public void init() { <Component registration> //using method add(...) and } //in a few cases other stuff } For Component JPanel, an “inner” subclass of class JPanel has been employed that implements method paint(Graphics).

9. Container The Container class is the abstract superclass representing all components that can hold other components. JComponent is derived from this class, Some Methods Defined in Class Container (and JComponent): setLayout(LayoutManager) associate a particular layout with the container add(Component) add a component to the container The following components are placed within Container objects: JLabel Labels A Label object consists of non-editable text. In addition, input events can be associated with the following components: JButton Buttons This class creates a labeled button. JCheckBox (ButtonGroup) A check box has an "on" (true) and "off" (false) state. Clicking on the check box toggles its state between "on" and "off ". A group of radio buttons or check boxes in which only 1 can be in its "on" state at any time. CheckBoxes

10. JComboBox A combo box presents the user with a pop-up menu of choices. The current choice is displayed as the title of the menu. ChoiceLists TextComponent A text component is the superclass of any component that allows the editing of some text. JTextArea A text area is a multi-line area for displaying text. It can be set to allow editing or to be read-only. TextAreas JTextField A text field presents the user with a single editable line of text. TextFields JList A list presents the user with a scrolling list of text items that can be set up either so that the user can pick only 1 item or pick multiple items. ScrollingLists JScrollBar A scroll bar provides a convenient means of allowing a user to select from a range of values. ScrollBars JPanel A JPanel component represents a blank rectangular area of the screen onto which can appear a drawing. Panels

11. The Form of the HTML File Used in the Previous 9 Applets: <HTML> <HEAD> <TITLE>Xxx</TITLE> </HEAD> <BODY> <H2>Component: Yyy</H2> <applet code=“Xxx.class" width=www height=hhh> </applet> <P> <A HREF=“Xxx.java"> Java Source </A> </BODY> </HTML> where Xxx is the public class name, Yyy is the component name, and www & hhh represent the applet size.

12. More Information about Swing Components Adding JComponents to a JApplet Step 1. Import classes (as needed) from packages java.awt.* (for Container) and javax.swing.* (for JApplet and the needed JComponents). Step 2. Write the header for a class that extends JApplet. Step 3. Declare and define any JComponents that you will add to your applet. Step 4. In method init( ), call method getContentPane( ) to serve as a Container object for holding swing components in a JApplet. Step 5. Select a Layout manager and with method setLayout( ), install this choice as the Layout manager for the ContentPane. Optionally you may also set the background color for the ContentPane. (default is lightGray) Step 6. Add all the declared JComponents to the ContentPane. To provide functionality to these components you will have to add event listeners, but this will be deferred until a bit later.

13. Step 1. Import classes from packages java.awt and javax.swing Step 2. Programmer’s class extends JApplet Step 3. Declare and define JComponents Step 4. Use getContentPane( ) to provide a container for components Step 5. Provide a Layout manager for the ContentPane, and optionally a background color (default is lightGray) Step 6. Add components to the ContentPane Swing Components Example – Create an Applet with a JLabel and a JTextField import java.awt.*; import javax.swing.*; publicclass LabelDemo extends JApplet { private JLabel nameLabel = new JLabel(“name”); private JTextField theName = new JTextField(25); public void init( ) { Container cp = getContentPane( ); cp.setLayout(new FlowLayout( )); cp.setBackground(Color.cyan); cp.add(nameLabel); cp.add(theName); } } Layout managers will be discussed in the next lecture. FlowLayout is the simplest to use, but provides the least programmer control over placement of components. LabelDemo

14. Creates a JLabel object with center alignment an no text Creates a JLabel with default center alignment Inherited from JComponent Swing Components Common methods in JLabel Constructors JLabel( ); JLabel(String textTo Appear); JLabel(String textToAppear, int alignment); Alignment is either JLabel.LEFT, JLabel.CENTER, or JLabel.RIGHT Other JLabel methods include String getText( ); void setText(String textToAppear); setOpaque(boolean); setBackground(Color c); setForeground(Color c); setFont(Font f); Hides container window color

15. Text field with default width determined by system Blank text field with specified number of columns Width of text field = length of string Sets both width and contents of text field Blank text area with default length and width Blank text area with r rows and c columns Text area of default size containing the string Text area of set size with initial string Swing Components Common TextComponent methods Constructors JTextField( ); JTextField(int cols); JTextField(String textToEnter); JTextField(String, int); JTextField objects are used to enter or display text on a single line. JTextArea objects are used to enter or display more than one line of text. The constructors for a JTextArea are similar to those for the JTextField. JTextArea( ); JTextArea(int r, int c); JTextArea(String textToEnter); JTextArea(String, int, int);

16. Declarations creating a JTextArea and a JScrollPane The JScrollPane object is added to the ContentPane Swing Components Other JTextComponent methods include: void setText(String textToEnter); String getText( ); A JTextArea is often enclosed in a JScrollPane. Using the constructor for a JScrollPane that takes only the component to be shown in this window, the scroll bars will be invisible until they are needed. The statements needed to place a JTextArea in a JScrollPane are as follows: private JTextArea theText = new JTextArea(3, 10); private JScrollPane sp = new JScrollPane(theText); //in method init( ) Container cp = getContentPane( ); cp.add(sp); The JScrollPane is created holding the JTextArea object

17. Without this statement, the background of the label will not be set to white, but will remain lightGray, the default color of the ContentPane Swing Components JLabels can have their own Font and foreground and background color. Suppose we want to add a JLabel object named textLabel to a ContentPane cp, and we want the font to be bold, sansSerif, and 20 point, and we want the label to appear blue on a white background. We would use the following statements in method init( ): textLabel.setFont(new Font(“sansSerif”, Font.BOLD, 20) ); textLabel.setOpaque(true); textLabel.setForeground(Color.blue); textLabel.setBackground(Color.white); cp.add(textLabel); This applet illustrates the points stated on the last few slides about JLabel and JTextComponent objects. Notice what happens when we comment out the setOpaque(true) statement for the addressField. Enter text in the text fields and text areas and see what happens when you exceed the space provided. Component Demol

18. Add to end of menu Menu items may be String (text) or icons Allows the user to input text if “true” Swing Components JComboBox A JComboBox describes a menu with several choices, one of which can be selected. When the user is not accessing the JComboBox object, only the currently selected choice (and a down arrow) appears in the menu. The class JComboBox is similar to class Choice in AWT. Methods in JComboBox include: void addItem(Object alternative); void insertItemAt(Object alternative, int loc); void removeItem(Object alternative); void removeItemAt(int loc); Object getSelectedItem( ); int getSelectedIndex( ); Object getItemAt(int loc); int getItemCount( ); void setEditable(boolean);

19. Create a menu with the above menu items Create an empty menu …. …and add items individually Swing Components Constructing a Menu A menu can be displayed in a JComboBox or a JList. Both of these classes have a constructor that creates its menu from an array of menu items. The JComboBox class also has a constructor that creates an empty menu. String [ ] menuItems = {“open”, “close”, “save”, “delete”, “quit”} JComboBox jcb1 = new JComboBox(menuitems); JComboBox jcb2 = new JComboBox( ); jcb2.addItem(“open”); jcb2.addItem(“close”); etc. The JList class only has the first type of constructor. JList myJList = new JList(menuItems);

20. Wrap the JList inside a JScrollPane object Add the JScrollPane object to the ContentPane Swing Components A JList menu may contain more items than those that are visible at any one time. The programmer should set the number of rows that are visible to the desired value, then wrap the JList object inside of a JScrollPane. JList myList = new JList (menuItems); JScrollPane jsp = new JScrollPane(myList); //in method init( ) myList.setVisibleRowCount(3); cp.add(jsp); A JComboBox provides for only a single selection. A JList allows multiple selections. There are three selection modes the programmer can choose from: myList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION); myList.setSelectionMode(ListSelectionModel.SINGLE_INTERVAL_SELECTION); myList.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION); default If one of the multiple selection modes is chosen, the user selects several choices by using Shift and Control keys in combination with mouse clicks.

21. Swing Components JButton A JButton has a label and/or an icon. The programmer can set the Font and background and foreground color of the button in a manner similar to that which was done for the JLabel. JButton saveButton = new JButton(“Save”); //in method init( ) saveButton.setFont(new Font(“sansSerif”, Font.BOLD+Font.ITALIC, 16)); saveButton.setOpaque(true); saveButton.setBackground(Color.cyan); saveButton.setForeground(Color.white);

22. Add buttons to the ButtonGroup Add buttons to the ContentPane Democrat Independent Republican Swing Components ButtonGroup Objects of class JRadioButton or JCheckBox can be grouped together into a ButtonGroup. If ungrouped, each object these classes can be toggled “on” or “off” separately. If part of a ButtonGroup, when one object from either of these classes is asserted, the previously asserted object is toggled “off”. private JRadioButton b1 = new JRadioButton(“Democrat”,false); private JRadioButton b2 = new JRadioButton(“Independent”,true); private JRadioButton b3 = new JRadioButton(“Republican”, false); private ButtonGroup voters = new ButtonGroup( ); //in method init( ) after getting a ContentPane cp voters.add(b1); voters.add(b2); voters.add(b3); cp.add(b1); cp.add(b2); cp.add(b3); Labels and initial settings Select option

23. JScrollBar.HORIZONTAL or JScrollBar.VERTICAL Choosing 0 sets indicator to the default size Swing Components JScrollBar The constructor for a JScrollBar has the following parameters • Orientation • Initial value • Size of the indicator • Minimum value • Maximum value JScrollBar scrollbar = new JScrollBar(JScrollBar.HORIZONTAL, 10, 0, 1, 20);

24. Swing Components JPanel A JPanel is a component capable of holding other components. It is used as: • A surface for drawing text or shapes • A container for grouping components A JPanel may have its own Layout Manager (different from the Layout Manager used for the ContentPane in which it resides). A JPanel object is transparent. It will show the color of the container in which it resides. JPanel objects will be used extensively to group components for placement purposes when we discuss Layout Managers in the next lecture

25. Review To review the swing components discussed in this lecture, we again refer to the applet below. Refer to the applet code to observe the effect of each statement. Be sure you understand how the picture of the components on the applet corresponds to the code. Display of common swing components