Session 6 Lecture Notes for First Course in Java
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Responding to User Input (Day 11)

Listening to Events

So that your class can respond to user events, such as mouse clicks, you implement event listeners, which the java.awt.event package provides as interfaces such as the following.
http://java.sun.com/j2se/1.3/docs/api/java/awt/event/package-summary.html

In the following sample, 

1. Line 6-7 create two buttons, which are JButton components
2. Lines 12 and 13 add action listeners for the buttons using "this" current ChangeTitle class as the even listener object
3. Lines 15-16 add the buttons, complete with their action listeners, to the pane. 
4. Lines 31-36 get the source of the user's action and respond accordingly.

Line 31 implements the actionPerformed() method of the ActionListener interface:
http://java.sun.com/j2se/1.3/docs/api/java/awt/event/ActionListener.html
The listener interface for receiving action events. The class that is interested in processing an action event implements this interface, and the object created with that class is registered with a component, using the component's addActionListener method. When the action event occurs, that object's actionPerformed method is invoked.

Action Events

The code above handles action events on JButtons. You also use the actionPerformed() method for other components: JCheckBox, JComboBox, JTextField, and JRadioButton.

http://java.sun.com/j2se/1.3/docs/api/java/awt/event/ActionEvent.html

An action event object inherits the getSource() method from the EventObject class. Therefore, line 32 can use the getSource() method of the action event instance evt to determine the cause (source) of the user's action.

Adjustment Events

How do you make a scroll bar that adjusts itself in response to the user?

http://java.sun.com/docs/books/tutorial/uiswing/components/scrollpane.html
Example of an application that adjusts the size of the scroll pane to fit whatever the drawing area displays.

Line 36 uses a null string ("") as a shortcut to convert the newValue integer into a string.

Focus Events

A text field has the focus when it is active for user input. Typically, the cursor blinks in the text field with focus.

Like the ActionListener, the FocusListener is a subinterface of the EventListener interface.
Just as you can call getSource() on an action event, so you can call getSource on a focus event.
http://java.sun.com/j2se/1.3/docs/api/java/awt/event/FocusListener.html

Item Events

How you make the GUI respond to the user selecting an item from a pick list?

Key Events

http://java.sun.com/docs/books/tutorial/uiswing/events/KeyEventDemo.html
This applet displays as text whatever key event you cause.

http://java.sun.com/docs/books/tutorial/uiswing/events/keylistener.html
Description of the key event listener, including this code to handle keyboard events:

Mouse Events

The MouseListener interface provides five methods.
http://java.sun.com/j2se/1.4/docs/api/java/awt/event/MouseListener.html

http://java.sun.com/docs/books/tutorial/uiswing/events/MouseEventDemo.html
The applet above responds to the user's mouse events by display descriptive text. 
http://java.sun.com/docs/books/tutorial/uiswing/events/mouselistener.html
Here is the code to handle mouse events.

.

Mouse Motion Events

http://java.sun.com/j2se/1.4/docs/api/java/awt/event/MouseMotionListener.html

Window Events

http://java.sun.com/j2se/1.3/docs/api/java/awt/event/WindowListener.html

http://java.sun.com/docs/books/tutorial/uiswing/events/WindowEventDemo.html

http://java.sun.com/docs/books/tutorial/uiswing/events/windowlistener.html

Line 45 uses the getWindow method of the WindowEvent class.
http://java.sun.com/j2se/1.3/docs/api/java/awt/event/WindowEvent.html#getWindow()

Designing Layout

The SwingColorTest application has three panels:

1. JFrame (outermost panel), which contains three panels:
   • Color Panel
   • sRGB Panel
   • HSB Panel

The constructor defines the layout manager (line 13), sets the controls, and adds the controls to the pane. 

Defining Subpanels

Line 88 creates an array for the text fields.
Lines 94-100 use a for loop to add each text field in the array.

GUI Workflow

Before you begin writing code for the logic of HOW the GUI works, you should first verify that the GUI has the correct LOOK, that all the necessary components are in the proper place. 

The workflow is as follows:

1. The user updates the value of a text field in either the RGB column or the HSB column.
2. In response to the event, both the color box and the other text column reflect the new values.

The update() method tests which column is the source of the update:

RGB to HSB

To handle the conversion from RGB to HSB, use the RGBtoHSB() method of the Color class.

http://java.sun.com/j2se/1.3/docs/api/java/awt/Color.html#RGBtoHSB(int, int, int, float[])
public static float[] RGBtoHSB(int r,
                               int g,
                               int b,
                               float[] hsbvals)

Converts the components of a color, as specified by the default RGB model, to an equivalent set of values for hue, saturation, and brightness that are the three components of the HSB model.

Line 65 casts the floats in the HSB array to integers so that the valueOf() method of the String class can convert the values to strings. 

HSB to RGB

To handle the conversion from HSB to RGB, use the getHSBColor() method of the Color class.

http://java.sun.com/j2se/1.3/docs/api/java/awt/Color.html#getHSBColor(float, float, float)
public static Color getHSBColor(float h,
                                float s,
                                float b)

Creates a Color object based on the specified values for the HSB color model.

Line 70 casts the values in the text fields to floats because getHSBColor expects arguments of type float.

Handling User Events

The example includes three classes:

• SwingColorTest (lines 5-82) contains the application window--including the update()method (line41) that updates text fields, and the main()method (lines 27-35) that handles the creation of the frame and exiting the application.
• SwingColorControls class (lines 84-118) is responsible both for the GUI and for the event-handling that is logically distinct from the GUI implementation. This class also uses an update()method (line 109)--in this case to update the color.
• ExitWindow (lines 120-123) closes the application

Homework Assignments due 27 April:  

1. p. 296, 2nd bullet

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