Java Lecture 6

Frequently used Listeners/Events

ActionListener:

• actionPerformed(ActionEvent)

ItemListener:

• itemStateChanged(ItemEvent)

MouseListener:

• mouseEnter(MouseEvent): entered component area
• mouseExit(MouseEvent): exited component area
• mouseClick(MouseEvent): button click
• mousePress(MouseEvent): button press (preceeds drag)
• mouseRelease(MouseEvent): button release (follows drag)

MouseMotionListener:

• mouseMove(MouseEvent): no buttons down
• mouseDrag(MouseEvent): 1 or more buttons down

KeyListener:

• keyTyped(KeyEvent): normal key event
• keyPressed(KeyEvent): key held down
• keyReleased(KeyEvent): held key released

/*
 * <applet code=TextFieldDemo width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class TextFieldDemo
        extends java.applet.Applet
        implements ActionListener
{
    TextField t;

    public void init() {
        t = new TextField();
        t.addActionListener(this);
        t.setText("Maybe");

        add(new Label("Answer: "));
        add(t);

    }
    public void actionPerformed(ActionEvent e){
        if (e.getSource() == t) {
            System.out.println("Answer is: " + t.getText());
            System.out.println("same as: " + e.getActionCommand());
        }
        return;
    }
}

/*
 * <applet code=ChoiceDemo width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class ChoiceDemo
        extends java.applet.Applet
        implements ItemListener
{
    Choice c;

    public void init() {
        c = new Choice();
        c.addItemListener(this);

        c.add("Yes");
        c.add("No");
        c.add("Maybe");
        add(new Label("Answer: "));
        add(c);

    }
    public void itemStateChanged(ItemEvent e) {
        if (e.getSource() == c) {
            System.out.println("Answer is: " + c.getSelectedItem());
            System.out.println("same as: " + e.getItem());
        }
        return;
    }
}

/*
 * <applet code=CheckboxDemo width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class CheckboxDemo
        extends java.applet.Applet
        implements ItemListener
{
    Checkbox c1, c2;
    Checkbox cg1, cg2, cg3, cg4;
    CheckboxGroup myGroup;

    public void init() {
        c1 = new Checkbox("Box 1");
        c1.addItemListener(this);
        c2 = new Checkbox("Box 2", null, true);
        c2.addItemListener(new ItemListener() {
          public void itemStateChanged( ItemEvent e) {
              if (e.getStateChange() == ItemEvent.SELECTED)
                  System.out.println("Box 2 selected");
          }
         });
        add(c1);
        add(c2);

        myGroup = new CheckboxGroup();
        cg1 = new Checkbox("Option 1");
        cg1.setCheckboxGroup(myGroup);
        cg1.addItemListener(this);
        cg2 = new Checkbox("Option 2", myGroup, false);
        cg2.addItemListener(this);
        cg3 = new Checkbox("Option 3", myGroup, true);
        cg3.addItemListener(this);
        cg4 = new Checkbox("Option 4", myGroup, true);
        cg4.addItemListener(this);
        Panel p = new Panel();
        p.setLayout(new GridLayout(2, 2));
        p.add(cg1);
        p.add(cg2);
        p.add(cg3);
        p.add(cg4);
        add(p);
    }

    public void itemStateChanged(ItemEvent e) {
        if (e.getSource() == c1) {
            if (e.getStateChange() == ItemEvent.SELECTED)
                System.out.println("Box one selected");
            else
                System.out.println("Box one de-selected");
            return;
        }
        if (e.getSource() instanceof Checkbox){
            Checkbox target = (Checkbox) e.getSource();

            if (target.getCheckboxGroup() == myGroup)
            {
            // target is always equal to
            //   myGroup.getSelectedCheckbox()
                System.out.println( "Current option: "
                        + target.getLabel());
                return;
            }
        }
        return;
    }
}

import java.awt.*;
import java.awt.event.*;

public class FrameDemo
        extends Frame
        implements ActionListener
{
  public static void main(String args[]) {
    FrameDemo f = new FrameDemo();
    f.pack(); // layout components
    f.show(); // display on screen
  }

  List l;

  public FrameDemo() {
    super("The Frame test");

    l = new List();
    l.addActionListener(this);

    l.add("abc");
    l.add("defg");
    l.add("hijk");
    add(new Label("Select one: "));
    add(l);
  }


  public void actionPerformed(ActionEvent ae) {
      if (ae.getSource() == l) {
          System.out.println("Selected string:"
                + ae.getActionCommand());
          System.out.println("Selected index:"
                + l.getSelectedIndex());
    }
  }
}

Overview of Components

Canvas

Common Methods to use:
none
Useful Methods to override:
• output event handlers (paint(), update())
• all input event handlers (mouseUp(), etc.)
Events generated:
none

Label

Common Methods to use:
• Label(String s): creates a label
• Label(String s, int align): creates a label
• setText(String s): sets the text of the label.
• setAlignment(int align): sets the alignment (LEFT, CENTER, RIGHT).
Events generated:
none

Button

Common Methods to use:
• Button(String s): creates a button
• setLabel(String s): sets the text of the label.
• setEnable(boolean en): disabling greys out button, prevents clicks.
Events generated
• ActionEvent when button released
     event source is the button instance
     action command is the button label

Choice

Common Methods to use:
• Choice(): creates a Choice
• addItem(String s): adds another string to the list of choices (added to the end).
• select(String s): make s the currently selected choice (it must be an existing choice).
• String getSelectedItem(): returns the currently selected string.
Events generated:
• ItemEvent when choice is made (even if choice doesn't change)
     event source is the choice instance
     event item is the selected choice name

Checkbox

Common Methods to use:
• Checkbox(String s): creates a checkbox
• Checkbox(String s, CheckboxGroup g, boolean state): creates a checkbox in the specified group
• setLabel(String s): sets the text of the label.
• setState(boolean b): checks or unchecks the checkbox. If in a group, other checkboxs are unchecked if b is true.
• boolean getState(): returns true if the checkbox is currently checked.
Events generated:
• ItemEvent when a box is checked or unchecked. If box is in a group, only one action is generated and that is the selection of one of the boxes.
     event source is the checkbox instance

List

Common Methods to use:
• List(int nrows, boolean multi): creates a list with nrows; if multi is true several items can be selected at once
• addItem(String s): adds a string to the list (at the end).
• addItem(String s, int i): inserts a string into the list at the specified index.
• setMultipleMode(boolean b): set whether several items can be selected at once.
• select(int i): selects string at index i.
• String[] getSelectedItems(): returns an array of the currently selected strings.
• int getSelectedIndex(): return the index of the currently selected item.
Events generated:
• ActionEvent when item is double clicked
• ItemEvent when item is selected/deselected

TextField

Common Methods to use:
• TextField(String s, int cols): creates a TextField with the specified initial text and size.
• setText(String s): sets the text that is being displayed.
• String getText(): returns the text that is currently in the TextArea.
• setEditable(boolean b): sets whether the user can change the displayed text.
Events generated:
• ActionEvent when user hits return
     action event's command is the current value of the text
• TextEvent when text is changed

TextArea

Common Methods to use:
• TextArea(String s, int rows, int cols): creates a TextArea with the specified initial text and size.
• setText(String s): sets the text that is being displayed.
• String getText(): returns the text that is currently in the TextArea.
• setEditable(boolean b): sets whether the user can change the displayed text.
Events generated:
• TextEvent when text is changed

Overview of Layout Managers

FlowLayout

To create:
• FlowLayout(): creates a FlowLayout with CENTER alignment
• FlowLayout(int align): creates a FlowLayout with specified alignment (LEFT, CENTER, RIGHT)
To add a component:
add(Component c): adds the component after any existing components
Notes:
• lays things out in a row; when row is full, starts on the next row
• each row is justified (compoents of a row are bunched together)
• rows are placed starting at the top (not centered vertically)
• each component is given its natural size (if there is room)

BorderLayout

To create:
• BorderLayout()
To add a component:
add(String where, Component c): adds the component in the specified location (Center, East, South, West, North)
Notes:
• only one component in a location
• if a location is empty, the space is given to the Center.
• if a Center is empty, there is a empty space in the middle
• each component is sized to fit the available space

GridLayout

To create:
• GridLayout(int nrows, int ncols): create a grid of the specified size.
To add a component:
add(Component c): adds the component to the next available slot (fills from left to right, top to bottom)
Notes:
• if not enough components are added, number of columns is decreased
• number of rows or columns is increased as needed
• each component is sized to fit the available space

CardLayout

To create:
• CardLayout()
To add a component:
add(String name, Component c): adds the component to the card deck, component is given the specified name
Notes:
• only one component is displayed at a time
• the next(), previous(), and show() methods can be used to control which component is displayed
• each component is sized to fit the available space

Graphics Class

• Color getColor()
• void setColor(Color c)

• Font getFont()
• void setFont(Font f)

• void drawLine(int fromX, int fromY, int toX, int toY)
• void drawRect(int ulx, int uly, int width, int height)
• void drawString(String s, int baselineX, y)
• void drawOval(int ulx, int uly, int width, int height)
• void drawArc(int ulx, int uly, int width, int height, int startAngle, int angleDeg)

• void clearRect(int ulx, int uly, int width, int height)
• void fillRect(int ulx, int uly, int width, int height)
• void fill...(...)

• void drawImage(Image img, int ulx, int uly, ImageObserver ob)