Chapter 3: Widgets for a GUI

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Chapter 3 Widgets for a GUI

• General Component methods
• Useful widgets classes
• Text classes
• Label
• TextField
• TextArea
• Active widgets
• Button
• Checkbox
• Choice
• List

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Building a GUI

• Graphical User Interfaces composed of widgets
  (buttons, text fields, etc.)
• Various useful classes defined in AWT
• Part of Component class (abstract class)

3

• Component methods (inherited by all)
• Point location() - x,y position
• Dimension size() - width,height of object
• Rectangle bounds() - both location, size info
• void move (int x, int y) - set x,y position
• void resize (int w, int h) - set w,h of object
• void resize (Dimension d) - similar
• void reshape (int x, int y, int w, int h) - set
  rectangle
• Dimension minimumSize() - determined by system,
  smallest size needed
• Dimension preferredSize() - similar to minimum

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• Component methods (cont)
• Color getBackground() - return background color
• Color getForeground() - return foreground color
• void setBackground(Color c) - set background
  color
• void setForeground(Color c) - set foreground
  color
• Font getFont() - return current font
• void setFont(Font f) - set current font
• boolean isShowing() - true if Component on screen
• boolean isVisible() - true if object should be
  visible
• void hide() - turns component invisible
• void show() - makes component visible

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• Component methods (cont)
• Graphics getGraphics() - returns the Graphics
  object associated with the current component,
  useful for doing graphical operations on this
  component
• void paint(Graphics g) - method that shows
  component, should not be called by user
  (overridden)
• void repaint() - informs the computer that this
  component needs to be repainted
• void repaint(int x, int y, int width, int height)
  - similar to repaint() but indicates only the
  portion falling in the rectangle needs to be
  redone
• void update(Graphics g) - how component redone
  (generally box erased, repainted), useful to
  avoid flicker, should not be called by user
  (overridden)

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Text Classes

• Label - simple text statements, generally one
  line with no editing expected
• TextComponent - abstract class with methods used
  by
• TextField - one line text, with some editing
• TextArea - generally for more than one line of
  text with some editing

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• Label class
• constructors
• Label() - no text, center alignment
• Label(String label) - center-aligned label with
  text
• Label(String label, int alignment) - label with
  text
• aligned based on arg, vals Label.LEFT, CENTER,
  RIGHT
• methods
• String getText() - return string associated with
  Label
• void setText(String label) - set string
• int getAlignment() - return Labels current
  alignment
• void setAlignment(int alignment) - set alignment

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• TextComponent class
• abstract class (parent of TextField, TextArea)
• methods
• String getText() - return the text associated
  with object
• void setText(String text) - set text of object
• boolean isEditable() - flag indicating text can
  be edited
• void setEditable(boolean canEdit) - set editable
  flag
• String getSelectedText() - return highlighted
  text
• int getSelectionStart() - location where
  selection starts
• int getSelectionEnd() - location where selection
  ends
• void select (int start, int end) - select text
  start-end
• void selectAll() - select all of text

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• TextField class - one line, editing
• constructors
• TextField() - empty (no text), default (system)
  width
• TextField(int columns) - no text, columns wide
• TextField(String text) - set text, use it for
  width
• TextField(String text, int columns) - set text,
  width
• methods
• int getColumns() - return column width of object
• void setColumns(int c) - set column width
• char getEchoChar() - return echo char (if any)
• void setEchoChar(char c) - set echo char
• boolean echoCharIsSet() - check if echo char set
• Dimension minimumSize() - minimum needed size
• Dimension preferredSize() - preferred size

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• TextArea class - gt 1 line, editing
• look and feel depends on system (may include
  scrollbars)
• constructors
• TextArea() - empty (no text), default (system)
  size
• TextArea(int rows, int cols) - no text, rows,
  cols size
• TextArea(String text) - set text, default sized
• TextField(String text, int rows, int cols) - set
  text, size
• methods
• int getColumns() - column width of object
• int getRows() - row height of object

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• TextArea class (cont)
• methods
• void appendText(String str) - add string to the
  end of the current text
• void insertText(String str, int pos) - insert
  string at position pos
• void replaceText(String str, int start, int end)
  - insert string replacing chars from start to end
• Dimension minimumSize() - minimum required size
• Dimension preferredSize() - preferred size

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import java.applet. import java.awt. public
class TestClass extends Applet Label myLabel1
new Label("Test label 1",Label.LEFT) Label
myLabel2 new Label() TextField myTextField1
new TextField("Init text",20) TextField
myTextField2 new TextField("Start text")
TextArea myTextArea new TextArea("Initial
text",3,10) public void init ()
resize(500,400) myLabel1.setText("TLab1")
myLabel2.setText("TLab2") myTextField1.setFore
ground(Color.green) myTextField2.setEchoCharac
ter('') myTextArea.insertText("silly ",8)
add(myLabel1) add(myTextField1)
add(myTextArea) add(myLabel2)
add(myTextField2)
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Active Widgets

• Various classes provide mechanisms for users to
  indicate info
• Button - labeled object that can be clicked
• Checkbox - labeled object, can be checked/un
• Choice - object providing multiple choices with
  one showing
• List - object providing multiple choices with all
  showing

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• Button class
• constructors
• Button() - unlabeled button
• Button(String label) - button with label
• methods
• String getLabel() - return label of object
• void setLabel(String label) - set label of Button
• We will learn later how to check if the mouse has
  been used to access button

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• Checkbox class
• constructors
• Checkbox() - no label, initially unchecked
• Checkbox(String label) - labeled, initially
  unchecked
• Checkbox(String label, CheckboxGroup group,
  boolean state)
• methods
• String getLabel() - return label of object
• void setLabel(String label) - set label of
  Checkbox
• boolean getState() - determine if
  checked/unchecked
• void setState(boolean state) - set the state of
  the Checkbox
• CheckboxGroup getCheckboxGroup() - returns group

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• CheckboxGroup class
• Checkbox objects added using Checkbox methods
• constructors
• CheckBoxGroup() - empty group
• methods
• Checkbox getCurrent() - Checkbox of group that is
  checked
• void setCurrent(Checkbox c) - set c as checked
  member

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import java.applet. import java.awt. public
class TestButton extends Applet Button
myButton new Button("AButton") Checkbox
myCheckBox1 new Checkbox("CB1") Checkbox
myCheckBox2 new Checkbox("CB2") CheckboxGroup
myCheckBoxGroup new CheckboxGroup() Checkbox
myCBG1 new Checkbox("Big") Checkbox myCBG2
new Checkbox("Bigger") Checkbox myCBG3 new
Checkbox("Biggest") public void init ()
resize(500,400) myCheckBox2.setState(true)
myCBG1.setCheckboxGroup(myCheckBoxGroup)
myCBG2.setCheckboxGroup(myCheckBoxGroup)
myCBG3.setCheckboxGroup(myCheckBoxGroup)
myCheckBoxGroup.setCurrent(myCBG1)
add(myCheckBox1) add(myButton)
add(myCBG1) add(myCBG2) add(myCBG3)
add(myCheckBox2)
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• Choice class
• constructor
• Choice() - initially empty set of choices
• methods
• void addItem(String item) - add the named item to
  the choices
• int getSelectedIndex() - return num of selected
  item
• String getSelectedItem() - return string
  associated with item
• String getItem(int index) - returns string of
  index item
• void select(int index) - select the index item
  (starting at 0)
• void select(String item) - select item with
  string name
• int countItems() - return number of items

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• List class
• constructors
• List() - default-size list in single-selection
  mode
• List(int rows, boolean multSel) - list with rows,
  if multSel true can make multiple selections
• methods
• void addItem(String item) - add the named item to
  the choices
• void addItem(String item, int index) - add item
  at loc index
• void delItem(int index) - remove the index item
• void delItems(int start, int end) - remove items
  from start to end
• void clear() - remove all items
• void replaceItem(String item, int index) -
  replace item at index
• int countItems() - count of the number of items
  in List

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• methods
• void getSelectedIndex() - num of selected index
• -1 if multiple or none
• boolean isSelected(int index) - true if index
  selected
• void select(int index) - set index as selected
• void deselect(int index) - unselect index
• void makeVisible(int index) - scroll so index
  visible
• int getVisibleIndex() - index of last makeVisible
  call
• boolean allowsMultipleSelections() - true if list
  allows multiple selections
• void setMultipleSelections(boolean m) - set
  multiple selection flag to m

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• methods
• int getRows() - number of rows in object
• Dimension minimumSize() - min size needed
• Dimension minimumSize(int rows) - min size for
  rows
• Dimension preferredSize() - preferred size
• Dimension preferredSize(int rows) - preferred by
  rows

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import java.applet. import java.awt. public
class TestLists extends Applet Choice myChoice
new Choice() List myList new
List(4,true) public void init ()
resize(500,400) myChoice.addItem("Small")
myChoice.addItem("Medium") myChoice.addItem("L
arge") myChoice.select("Medium")
myList.addItem("Left") myList.addItem("Right")
myList.addItem("Straight")
myList.select(0) myList.select(2)
add(myChoice) add(myList)