Chapter 6: Events in Java 1.0 Model –Event class –selecting component to handle –handleEvent method –helper methods (action, keyUp, mouseUp, etc.) 1.1 Model –Listeners - connected to Event generatorsEvent types: ActionEvent, ItemEvent, etc. –corresponding Listeners: ActionListener, etc. –connecting generator to listener –Interfaces and Adapters

Abstract Classes Class may be abstract by adding modifier in class header public abstract class MyClass Resulting class cannot be instantiated Used to create general classes (Car) that you expect to specialize

Interfaces A class may only inherit from one parent Sometimes want to create specialized class, combines several capabilities Interface is a list of methods to instantiate public interface Drawable { public void draw() {} // empty body } Class header, indicate implements interface public class Rect extend GObj implements Drawable; must implement all methods of interface may have more than one interface

Event-Driven Programming Input “events” by the user caught by Java –move mouse, click button, scroll window, type key For each, Java creates an Event object Object added to queue, your program is then expected to “handle” the events

1.0 Event Model Event object: instance vars: Object target - which object generated the event int id - a value indicating the type of event Object arg - useful info about the event (dependent) long when - time of event int x - x location of event int y - y location of event int clickCount - number of times button clicked int key - key pressed (for key events) int modifiers - for key and mouse events

Event id Constants ACTION_EVENT - Buttons, Checkboxes, Choices, Lists, MenuItems, TextFields KEY_PRESS, KEY_RELEASE, KEY_ACTION, KEY_ACTION_RELEASE MOUSE_DOWN, MOUSE_UP, MOUSE_DRAG, MOUSE_MOVE, MOUSE_ENTER, MOUSE_EXIT - mouse LIST_SELECT, LIST_DESELECT - List events SCROLL_ABSOLUTE, SCROLL_LINE_UP, SCROLL_LINE_DOWN, SCROLL_PAGE_UP, SCROLL_PAGE_DOWN WINDOW_EXPOSE, WINDOW_ICONIFY, WINDOW_DEICONIFY, WINDOW_DESTROY, WINDOW_MOVED LOAD_FILE, SAVE_FILE GOT_FOCUS, LOST_FOCUS

Handling Events Java does lots of stuff for you (keys in Text objects, window moves, etc.) You add handling events for things you want handled a particular way, or when no obvious method (what to do when button pushed)

Source of the Event Java sends event to Component it thinks is responsible Component should –handle event and stop –pass event its parent class –say it can’t handle event (pass to parent in GUI) –handle event and pass on (rare) Java will automatically pass event to parent in GUI hierarchy if not handled –for Button, Comp (e.g. Panel) contains Button

Event-Handling Methods Every component has handleEvent method public boolean handleEvent(Event e) { if (e.id == Event.WINDOW_DESTROY) { System.exit(0); return true; }} Return value indicates if handled –true - event handled, terminate –false - event not completely handled, try someone else (parent in GUI hierarchy)

Passing On Events Sometimes we handle some events, want to pass it on to others: public boolean handleEvent(Event e) { if (e.id == Event.WINDOW_DESTROY) { … } else return super.handleEvent(e); } Give the next Component up in the class hierarchy the chance to handle event

Helper Methods handleEvent rarely used Java provides “helper” methods specific to particular types of events –can be implemented instead of handleEvent –called after handleEvent tried –action events - actions –key events - keyDown, keyUp –mouse events - mouseDown, mouseUp, mouseDrag, mouseMove, mouseEnter, mouseExit

action Method form: public boolean action (Event e, Object arg) {} used for many widgets: –Button e.target is button obj, arg is string name of button –Checkbox e.target is Cb obj, arg is check value (true or false) –Choice, List e.target is choice obj, arg is string of choice made

Helper Calling Parent As with handleEvent, if helper fails to handle event, should call superclass action: –return super.action(e,arg) Return values have same meaning as in handleEvent

key Methods form: public boolean keyUp (Event e, int key) {} public boolean keyDown(Event e, int key) {} key is char pressed –modifiers field indicates if Shift, Alt, Meta, or Control held while pressed –check with constants SHIFT_MASK, META_MASK, CONTROL_MASK (e.modifiers & Event.SHIFT_MASK) or e.shiftDown() also has metaDown and controlDown, but none for Alt –other keys (Functions like F1) use Action Key events, Java has constants for such (Event.F1)

mouse Methods form: public boolean method(Event e, int x, int y) {} methods: mouseUp, mouseDown, mouseMove, mouseDrag, mouseEnter, mouseExit x, y are locations of occurrence –modifiers used to indicate if left, right, center –no modifiers: left –meta set: right –alt set: center

import java.applet.*; import java.awt.*; public class TestAction extends Applet { Button button = new Button("AButton"); Checkbox cbox1 = new Checkbox("CB1"); Checkbox cbox2 = new Checkbox("CB2"); CheckboxGroup cbg = new CheckboxGroup(); Choice choice = new Choice(); Panel panel = new Panel(); MyCanvas canvas = new MyCanvas(); public void init () { panel.setLayout(new GridLayout(2,2)); cbox1.setCheckboxGroup(cbg); cbox2.setCheckboxGroup(cbg); choice.addItem("Item1"); choice.addItem("Item2"); choice.addItem("Item3"); panel.add(cbox1); panel.add(cbox2); panel.add(button); panel.add(choice); panel.layout(); setLayout(new GridLayout(2,1)); add(panel); add(canvas); }

public boolean action (Event e, Object arg) { if (e.target == button) { canvas.setMessage(arg + " pressed"); } else if ((e.target == cbox1) || (e.target == cbox2)) { Checkbox tempcb = (Checkbox) e.target; canvas.setMessage(tempcb.getLabel() + " checked"); } else if (e.target == choice) { canvas.setMessage(arg + " chosen"); } return true; } class MyCanvas extends Canvas { private String theMessage = “”; public MyCanvas() { resize(500,40); } public void paint(Graphics g) { g.drawString(theMessage,10,30); } public void setMessage(String s) { theMessage = s; repaint(); }

1.0 to the 1.1 Delegation Model Event generators tend to be scattered in 1.0 Handlers often centralized Difficult to write good general handlers Idea: connect generators with appropriate handlers

The Delegation Model (1.1) Any object can be a source of an Event Any object can listen for events –implement (register) an appropriate listener interface –listeners stay close to generators Need to import java.awt.event.*

General methods Object getSource() - generator of event, from EventObject int getID() - constant for type of event (useful for some Mouse or Key events) - from AWTEvent

Handling Events For different types of events we have –an Event class (with useful methods) –an EventListener interface with required methods that must be implemented as part of listener –after creating instance of class that can generate events, connect to appropriate listener interface use addListenerType(Object generator) methods

Action Events click Button, double-click member in List, select MenuItem, press in TextField ActionEvent methods: –String getActionCommand() - string for source button - label list, menuitems - text of selected item textfield - content of textfield –int getModifiers() - use to check if modifiers set Listener: ActionListener –methods: void actionPerformed(ActionEvent e)

Item Events Select Checkbox, CheckboxMenuItem, Choice item, or single-click List item ItemEvent methods: –Object getItem() - item selected (usually String) –int getStateChange() - returns value of state ItemEvent.SELECTED or ItemEvent.DESELECTED –ItemSelectable getItemSelectable() - return ItemSelectable object that generated event Listener: ItemListener –methods: void itemStateChanged(ItemEvent e)

Input Events Abstract class for Key and Mouse Events Has constants for masks: ALT_MASK, etc. Also has boolean methods: –boolean isAltDown() –boolean isControlDown() –boolean isMetaDown() –boolean isShiftDown() Other method: int getModifiers()

Mouse Events MouseEvent, MouseMovedEvent methods: –int getX() - x location of mouse event –int getY() - y location of mouse event –Point getPoint() - x,y location –int getClickCount() - number of clicks Listeners: –MouseListener - for click, press, enter, exits methods: –void mouseClicked(MouseEvent e) –mousePressed, mouseReleased, mouseEntered, mouseExited –MouseMotionListener - for drag, move methods: –void mouseMoved(MouseMovedEvent e) –void mouseDragged(MouseMovedEvent e)

Key Events KeyEvent methods: –char getKeyChar() - char of key pressed –boolean isActionKey() - true if action key –int getKeyCode() - code for action keys –void setKeyChar(char c) - set char in event –void setKeyCode(int keyCode) - set keyCode Listener: KeyListener –Methods: void keyPressed(KeyEvent e) void keyReleased(KeyEvent e) void keyTyped(KeyEvent e)

Text Events Changes to TextField or TextArea Listener: TextListener –methods: void textValueChanged(TextEvent e)

Adapters To implement an interface you must implement ALL methods of interface –even if only an empty method Alternate solution: extend an Adapter (interface with empty methods): –MouseAdapter –MouseMotionAdapter –KeyAdapter

General Use of Listener in declaring generator: private Widget myWidget = new Widget(); private ApropListClass myListener; in init or constructor: myListener = new ApropListClass(myWidget); myWidget.addApropListener(myListener) new class: class ApropListClass implements ApropListener { private Widget copyWidget; public ApropListClass(Widget w) { copyWidget = w; } public void apropListenerMethod(Item e) { // implement }

import java.applet.*; import java.awt.*; import java.awt.event.*; public class TestAction extends Applet { Button button = new Button("AButton"); private AButtonListener myButtonListener; Checkbox cbox1 = new Checkbox("CB1"); private ACheckboxListener myCB1Listener; Checkbox cbox2 = new Checkbox("CB2"); private ACheckboxListener myCB2Listener; CheckboxGroup cbg = new CheckboxGroup(); Choice choice = new Choice(); private AChoiceListener myChoiceListener; Panel panel = new Panel(); MyCanvas canvas = new MyCanvas();

public void init () { myButtonListener = new AButtonListener(button,canvas); button.addActionListener(myButtonListener); myCB1Listener = new ACheckboxListener(cbox1,canvas); cbox1.addItemListener(myCB1Listener); myCB2Listener = new ACheckboxListener(cbox2,canvas); cbox2.addItemListener(myCB2Listener); myChoiceListener = new AChoiceListener(choice,canvas); choice.addItemListener(myChoiceListener); panel.setLayout(new GridLayout(2,2)); cbox1.setCheckboxGroup(cbg); cbox2.setCheckboxGroup(cbg); choice.addItem("Item1"); choice.addItem("Item2"); choice.addItem("Item3"); panel.add(cbox1); panel.add(cbox2); panel.add(button); panel.add(choice); panel.validate(); setLayout(new GridLayout(2,1)); add(panel); add(canvas); }

class AButtonListener implements ActionListener { Button buttonCopy; MyCanvas canvasCopy; public AButtonListener(Button bcopy, MyCanvas ccopy) { buttonCopy = bcopy; canvasCopy = ccopy; } public void actionPerformed(ActionEvent e) { canvasCopy.setMessage(buttonCopy.getLabel() + " pressed"); } class ACheckboxListener implements ItemListener { Checkbox checkboxCopy; MyCanvas canvasCopy; public ACheckboxListener(Checkbox cbcopy, MyCanvas ccopy) { checkboxCopy = cbcopy; canvasCopy = ccopy; } public void itemStateChanged(ItemEvent e) { canvasCopy.setMessage(checkboxCopy.getLabel() + ” checked"); }

class AChoiceListener implements ItemListener { Choice choiceCopy; MyCanvas canvasCopy; public AChoiceListener(Choice chcopy, MyCanvas ccopy) { choiceCopy = chcopy; canvasCopy = ccopy; } public void itemStateChanged(ItemEvent e) { canvasCopy.setMessage(choiceCopy.getSelectedItem() + ” chosen"); } class MyCanvas extends Canvas { private String theMessage = ""; public MyCanvas() { setSize(500,40); } public void paint(Graphics g) { g.drawString(theMessage,10,30); } public void setMessage(String s) { theMessage = s; repaint(); }