Apr 19, 2023

Components

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Types of Components

Button Canvas Checkbox Choice Label

List Scrollbar TextComponent

TextArea TextField

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Button

new Button() constructs a Button with no text new Button(String) constructs a Button with the specified text

void addActionListener(ActionListener) void removeActionListener(ActionListener)

String getLabel() void setLabel(String)

String actionEvent.getActionCommand() Returns the “action command” associated with this Button By default, the action command = the text on the Button

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A Button listener

button.addActionListener (new MyButtonListener ());

class MyButtonListener implements ActionListener { public void actionPerformed (ActionEvent event) { showStatus ("Button clicked."); }}

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Label constructors

new Label() constructs a Label with no text new Label(String) constructs a Label with the

specified text new Label(String, alignment) constructs a label with

the specified text and alignment: Label.LEFT Label.RIGHT Label.CENTER (These are int constants defined in the Label class)

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Label methods

String getText() void setText(String) int getAlignment() void setAlignment(int)

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Scrollbar constructors

new Scrollbar() creates a vertical Scrollbar new Scrollbar(int orientation)

Scrollbar.HORIZONTAL or Scrollbar.VERTICAL new Scrollbar(int orientation, int initialValue, int

bubbleSize, int minimum, int maximum) bubbleSize: size of the bubble (sometimes called elevator) min value of Scrollbar = minimum max value of Scrollbar = maximum - bubbleSize

(value of scrollbar is given by left or top edge of bubble) initialValue: the initial position setting of the bubble

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Scrollbars

bubbleSize

maximum

maximum — bubbleSize

minimum

The units for bubbleSize are the same as those for minimum and maximum The scrollbar reading is taken

from the left edge of the bubble

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Scrollbar methods

void addAdjustmentListener(AdjustmentListener) void removeAdjustmentListener(AdjustmentListener)

int getMinimum() void setMinimum(int)

int getMaximum() void setMaximum(int)

int getValue() void setValue(int)

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A Scrollbar listener

scrollbar.addAdjustmentListener(new MyScrollbarListener ());

class MyScrollbarListener implements AdjustmentListener { public void adjustmentValueChanged (AdjustmentEvent event) { showStatus ("Scrollbar: " + scrollbar.getValue ()); }};

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Checkbox constructors

new Checkbox() new Checkbox(String) new Checkbox(String, boolean) new Checkbox(String, boolean, CheckboxGroup) new Checkbox(String, CheckboxGroup, boolean)

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Checkbox methods

void addItemListener(ItemListener) void removeItemListener(ItemListener)

CheckboxGroup getCheckboxGroup() void setCheckboxGroup(CheckboxGroup)

String getLabel() void setLabel(String)

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A Checkbox listener

checkbox.addItemListener (new MyCheckboxListener ());

class MyCheckboxListener implements ItemListener { public void itemStateChanged (ItemEvent event) { showStatus ("Checkbox status is now " + (event.getStateChange () == ItemEvent.SELECTED)); }};

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CheckboxGroup methods

Checkbox getSelectedCheckbox() Suggested use:

myCG.getSelectedCheckbox().getLabel()

void setSelectedCheckbox(Checkbox)

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A CheckboxGroup listener

No such thing—listen to individual Checkboxes

checkbox1.addItemListener (myCheckboxGroupListener); checkbox2.addItemListener (myCheckboxGroupListener);

class MyCheckboxGroupListener implements ItemListener { public void itemStateChanged (ItemEvent event) { Checkbox d = checkboxGroup.getSelectedCheckbox (); showStatus ("CheckboxGroup: " + d.getLabel ()); }}

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Choice constructor

new Choice()

After constructing a Choice, use add(String) to add items to it

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Choice methods

addItemListener(ItemListener) removeItemListener(ItemListener)

void add (String) void remove(String) void removeAll()

String getSelectedItem() void select(String)

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A Choice listener

choice.addItemListener (new MyChoiceListener ());

class MyChoiceListener implements ItemListener { public void itemStateChanged (ItemEvent event) { showStatus ("Choice: " + choice.getSelectedItem ()); }};

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List constructors

new List() constructs an empty list new List(int)

int: the number of items visible (a scrollbar is automatically added if needed)

new List(int, boolean) boolean: true if multiple selection is allowed

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List methods I

void addItemListener(ItemListener) void removeItemListener(ItemListener)

void add(String) void remove(String) void removeAll()

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List methods II

String getSelectedItem() String[] getSelectedItems()

int getSelectedIndex() int[] getSelectedIndexes()

void select(int) // can only select by index There are other methods for adding, removing, etc. by

index

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A List listener

list.addItemListener (new MyListListener ());

class MyListListener implements ItemListener { public void itemStateChanged (ItemEvent event) { showStatus ("List: " + list.getSelectedItem ()); }};

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TextComponent

No constructors Instead, construct a TextField or TextArea

void addTextListener(TextListener) void removeTextListener(TextListener)

String getText() void setText(String)

boolean isEditable() void setEditable(boolean)

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TextField constructors

new TextField() new TextField(int)

int: the approximate desired number of columns new TextField(String)

String: the initial text in the TextField new TextField(String, int)

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TextField methods

void addActionListener(ActionListener) void removeActionListener(ActionListener)

The ActionListener for a TextField responds only when the Enter key is pressed

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TextArea constructors

new TextArea() new TextArea(int rows, int columns) new TextArea(int rows, int columns, int scroll)

TextArea.SCROLLBARS_VERTICAL_ONLY TextArea.SCROLLBARS_HORIZONTAL_ONLY TextArea.SCROLLBARS_BOTH TextArea.SCROLLBARS_NONE

new TextArea(String) new TextArea(String, int rows, int columns)

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TextComponent listeners

textField.addActionListener (new MyTextFieldListeners ()); textField.addTextListener (new MyTextFieldListeners ());

class MyTextFieldListeners implements ActionListener,TextListener { public void actionPerformed (ActionEvent event) { showStatus ("TextField (ActionListener): " + textField.getText ()); } public void textValueChanged (TextEvent event) { showStatus ("TextField (TextListener): " + textField.getText ()); }};

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Suggestion: Don’t use text listeners!

Text fields are intended to be passive; that is, nothing should happen immediately as a result of typing into them Consequently, I’ve seen a lot of programs in which text

listeners do their work at the wrong time It usually works much better to ignore text fields until

an active control (such as a Button) is used, and then do getText() to find the current value in the text field

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Canvas

This is a rectangular blank area that you can paint on Constructor: new Canvas() Don't use Canvas directly; instead, subclass it and

override its void paint(Graphics) method Or even simpler: Paint directly on a Panel

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The End