SCJP Java Programming Exam Cram Notes : components.

10. Components

Java's building blocks for creating GUIs.

All non-menu related components inherit from java.awt.Component, that provides basic support for event handling, controlling component size, color, font and drawing of components and their contents.

Component class implements ImageObserver, MenuContainer and Serializable interfaces. So all AWT components can be serialized and can host pop-up menus.

Component methods:

Controls	Methods / Description
Size	Dimension getSize() void setSize(int width, int height) void setSize(Dimension d)
Location	Point getLocation() void setLocation(int x, int y) void setLocation(Point p)
Size and Location	Rectangle getBounds() void setBounds (int x, int y, int width, int height) void setBounds (Rectangle r)
Color	void setForeground(Color c) void setBackground(Color c)
Font	void setFont(Font f) void setFont(Font f)
Visibility and Enabling	void setEnabled(boolean b) void setVisible(boolean b)

Container class extends Component. This class defines methods for nesting components in a container.

Component add(Component comp)

Component add(Component comp, int index)

void add(Component comp, Object constraints)

void add(Component comp, Object constraints, int index)

void remove(int index)

void remove(Component comp)

void removeAll()

The following are the containers:

Container	Description
Panel	Provides intermediate level of spatial organization and containment. Not a top-level window Does not have title, border or menubar. Can be recursively nested. Default layout is Flow layout.
Applet	Specialized Panel, run inside other applications (typically browsers) Changing the size of an applet is allowed or forbidden depending on the browser. Default layout is Flow layout.
Window	Top-level window without a title, border or menus. Seldom used directly. Subclasses (Frame and Dialog) are used. Defines these methods: void pack() - Initiates layout management, window size might be changed as a result void show() - Makes the window visible, also brings it to front void dispose() - When a window is no longer needed, call this to free resources.
Frame	Top-level window (optionally user-resizable and movable) with a title-bar, an icon and menus Typically the starting point of a GUI application. Default layout is Border layout.
Dialog	Top-level window (optionally user-resizable and movable) with a title-bar. Doesn't have icons or menus. Can be made modal. A parent frame needs to be specified to create a Dialog. Default layout is Border layout.
ScrollPane	Can contain a single component. If the component is larger than the scrollpane, it acquires vertical / horizontal scrollbars as specified in the constructor. SCROLLBARS_AS_NEEDED - default, if nothing specified SCROLLBARS_ALWAYS SCROLLBARS_NEVER

Top-level containers (Window, Frame and Dialog) cannot be nested. They can contain other containers and other components.

GUI components:

Component	Description	Constructors	Events
Button	A button with a textual label.	new Button("Apply")	Action event.
Canvas	No default appearance. Can be sub-classed to create custom drawing areas.		Mouse, MouseMotion, Key events.
Checkbox	Toggling check box. Default initial state is false. getState(), setState(boolean state) - methods Can be grouped with a CheckboxGroup to provide radio behavior. Checkboxgroup is not a subclass of Component. Checkboxgroup provides these methods: getSelectedCheckbox and setSelectedCheckbox(Checkbox new)	Checkbox(String label) Checkbox(String label, boolean initialstate) Checkbox(String label, CheckBoxGroup group)	Item event
Choice	A pull-down list Can be populated by repeatedly calling addItem(String item) method. Only the current choice is visible.		Item event
FileDialog	Subclass of Dialog Open or Save file dialog, modal Dialog automatically removed, after user selects the file or hits cancel. getFile(), getDirectory() methods can be used to get information about the selected file.	FileDialog(Frame parent, String title, int mode) Mode can be FileDialog.LOAD or FileDialog.SAVE
Label	Displays a single line of read-only non-selectable text Alignment can be Label.LEFT, Label.RIGHT or Label.CENTER	Label() Label(String label) Label(String label, int align)	None
List	Scrollable vertical list of text items. No of visible rows can be specified, if not specified layout manager determines this. Acquires a vertical scrollbar if needed. List class methods: addItem(String), addItem(String, int index) getItem(int index), getItemCount() getRows() - no of visible rows int getSelectedIndex() int[] getSelectedIndexes() String getSelectedItem() String[] getSelectedItems()	List() List(int nVisibleRows) List(int nVisibleRows, boolean multiSelectOK)	Item event - selecting or deselecting Action event - double clicking
Scrollbar	With the last form of constructor, calculate the spread as maxvalue - minvalue. Then the slider width is slidersize / spread times of scrollbar width.	Scrollbar() - a vertical scrollbar. Scrollbar(int orientation) Scrollbar(int orientation, int initialvalue, int slidersize, int minvalue, int maxvalue) Orientation can be Scrollbar.HORIZONTAL Scrollbar.VERTICAL	Adjustment event
TextField	Extends TextComponent Single line of edit / display of text. Scrolled using arrow keys. Depending on the font, number of displayable characters can vary. But, never changes size once created. Methods from TextComponent: String getSelectedText() String getText() void setEditable(boolean editable) void setText(String text)	TextField() - empty field TextField(int ncols) - size TextField(String text) - initial text TextField(String text, int ncols) - initial text and size	Text event Action event - Enter key is pressed.
TextArea	Extends TextComponent Multiple lines of edit/display of text. Scrolled using arrow keys. Can use the TextComponent methods specified above. Scroll parameter in last constructor form could be TextArea.SCROLLBARS_BOTH, TextArea.SCROLLBARS_NONE, TextArea.SCROLLBARS_HORIZONTAL_ONLY TextArea.SCROLLBARS_VERTICAL_ONLY	TextArea() - empty area TextArea(int nrows, int ncols) - size TextArea(String text) - initial text TextArea(String text, int nrows, int ncols) - initial text and size TextArea(String text, int nrows, int ncols, int scroll)	Text event

Pull-down menus are accessed via a menu bar, which can appear only on Frames.

All menu related components inherit from java.awt.MenuComponent

Steps to create and use a menu

Create an instance of MenuBar class
Attach it to the frame - using setMenubar() method of Frame
Create an instance of Menu and populate it by adding MenuItems, CheckboxMenuItems, separators and Menus. Use addSeparator() method to add separators. Use add() method to add other items.
Attach the Menu to the MenuBar. Use add() method of Menubar to add a menu to it. Use setHelpMenu to set a particular menu to appear always as right-most menu.

Menu(String label) - creates a Menu instance. Label is what displayed on the Menubar. If this menu is used as a pull-down sub-menu, label is the menu item's label.

MenuItems generate Action Events.

CheckboxMenuItems generate Item Events

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