chapter 7java: an introduction to computer science & programming - walter savitch 1 chapter 7 l...

Chapter 7 Java: an Introduction to Computer Science & Programming - Walter Savitch 1

Chapter 7

Event-Driven Programming» GUIs and the AWT

Simple Window Interfaces Components, Containers, and Layout

Managers Panels and Text Components Adding Menus Inner Classes

Event-Driven Programming Using the AWT


Event-Driven Programming

A different approach to program design Programs react to events, e.g.

» mouse click

» key press

» menu selection

» button selection

» message sent from printer

» etc. GUI: graphical user interface

» windows, scroll bars, menus, radio buttons, etc.

» a special case of event-driven programming

» the only type of event we will be concerned with in this chapter An event is an object


AWT

Abstract Window Toolkit Implements GUI A standard part of Java Not fancy, but adequate Makes extensive use of inheritance Based on events and event handlers Firing an event: when an object generates an event Listener object

» every object that can fire an event, such as a button, can have one or more listener objects

» Listener objects have methods (called event handlers) that perform actions depending on the event

» You the programmer write these event handlers


WARNING!

Programs that use the AWT can hang the computer

Save your work before running

a program using the AWT!

Chapter 7 Java: an Introduction to Computer Science & Programming - Walter Savitch 5Chapter 6 Java: an Introduction to Computer Science & Programming - Walter Savitch 5

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

public class FirstWindow extends Frame

Example: include the AWT library & extend Frame

The 1st two lines are needed when you use the AWT library

3rd line: starts the class definition

FirstWindow is derived from AWT Frame class

Frame is derived from the more basic class AWT Window class» Frame has a window, border,

title bar, and close button Frame will be modified by

adding properties to meet the program requirements

The first few lines of FirstWindow(Display 7.1/page 329):


A word about displays: pixels

Displays are organized as a 2-dimensional grid of pixels

pixel: picture element

» the smallest "addressable" unit of a screen

"Address" = coordinates

» (width, height) The origin has coordinates (0,0)

and is in upper left

» increasing width is to right

» increasing height is down

(0, 0)

Increasing width (x direction)

Increasing height (y direction)

(75, 100)


public void paint(Graphics g) { g.drawString("Please, don't click that button!", 75, 100); }

Example: define paint method; Graphics class and drawString

FirstWindow inherits properties of Frame and adds paint method» paint is called by AWT, not by FirstWindow

One argument, g, of type Graphics, is passed» think of it as a portion of the screen in which the object is to be painted

Every Graphics object has a drawString method» 75 and 100 are the coordinates of the start of the window

– they depend on the programming environment– you may have to adjust the numbers– some things are done by trial and error: try it and see

Additional method paint (Display 7.1/page 329):


public class FirstWindow extends Frame{ public static final int WIDTH = 300; public static final int HEIGHT = 200; … public static void main(String[] args) { <program statements to define main> }

<program statements to define paint method>

}

Example:define system constants

Conventional style of declaring systemconstants (Display 7.1/page 329):

The window width and height are system constants that specify the window size in pixels

» may adjust for different systems public allows other classes to read

the values static final prevents any other

class from changing the values Using variables instead of "hard

coded" numbers simplifies maintenance

» just change these two lines and recompile to change the window size

» avoids looking through the entire file for every occurrence of the numbers


public static void main(String[] args){ FirstWindow myWindow = new FirstWindow();1) myWindow.setSize(WIDTH, HEIGHT);

2) WindowDestroyer listener = new WindowDestroyer();3) myWindow.addWindowListener(listener);

4) myWindow.setVisible(true);}

Example: display window;define and register listeners

The main method(Display 7.1/page 329):

1) setSize: a Frame method to set the size of the window

2) listener: an object of the windowDestroyer class

» an object that receives events from an object

» should close window if "close window" button is clicked

» must be defined (written)

3) Registering the listener» associates the listener object with

an event-firing object» the listener will listen for events

fired by the associated object

4) setVisible: a Frame method to make the window visible


import java.awt.*;import java.awt.event.*;…public class WindowDestroyer extends WindowAdapter{ public void windowClosing(WindowEvent e) { System.exit(0); }}

Example: windowAdapter abstract class and System.exit

windowDestroyer definition(Display 7.2/page 332):

windowAdapter» Often the parent for windows

(descendents of Frame class)» an abstract class: it has methods

but they are undefined, so you need to define whichever methods you will use

» See Display 7.3/page 334 for windowAdapter methods

"close window" in the only event in this case

» so windowClosing method must be defined

System.exit(0) ends the Java program

» "0" is the conventional value used to indicate normal termination


Useful methods in Frame class

setTitle inserts a string in the title bar of the window setBackground(Color.blue) specifies the background

color of the window» Display 7.6/page 343 lists other predefined color values

addWindowListener registers a listener for events setSize and paint have already been described etc. - see Display 7.7/page 344


Two principle ways tocreate new classes from old ones

One method: Inheritance Use extend to create a custom

window based on an existing class

For example:

… FirstWindow extends Frame

Another: Container class Use a container class Use the add method to place

items in it

» Items placed in a container class are called components

Three kinds of objects to deal with:

1. The container class itself (usually a window)

2. The components to add to it

3. The layout manager


AWT container class

AWT container classes: Window, Panel, Frame» All three are descendents of the class Container

Custom windows are built by adding components to a container class, usually in the constructor

Use the add method inherited from Container to place items in the new (container) class

Components: items added to a container» for windows: buttons, menus, text fields, etc.» Component is also a class

The AWT class hierarchy with Container and Component classes is shown on the next slide (Display 7.9/page 350)


Hierarchy of AWT classes

Display 7.9/page 350Object

MenuComponentComponent

Container Button Menubar MenuItem

Window

Label

TextComponentPanel

Menu

Frame TextArea TextField

Class

Abstract Class

Key: If there is a line between two classes, then the lower class is a derived class of the higher class. Not all AWT components are shown.

AWT


Example: creating a window using a container class and components

ButtonDemo(Display 7.8/page 347)

The constructor is shown in box at right

Note the code to create and add buttons (shown in blue)

public ButtonDemo() { setSize(WIDTH, HEIGHT);

addWindowListener(new WindowDestroyer()); setTitle("Button Demonstration"); setBackground(Color.blue);

setLayout(new FlowLayout());

Button stopButton = new Button("Red"); stopButton.addActionListener(this); add(stopButton);

Button goButton = new Button("Green"); goButton.addActionListener(this); add(goButton); }

ButtonDemo constructor:


Example:drawString and repaint

The String instance variable theText is used as an argument in drawString so the text inside the window can be changed easily

actionPerformed method determines which event occurred (which button was clicked) and changes the value of theText accordingly

repaint()is called to update the window with the new value of theText

public void paint(Graphics g) { g.drawString(theText, 75, 100); }

public void actionPerformed(ActionEvent e) { <pseudocode to select which action to take> if(red) theText = "Stop!"; else if(green) theText = "Go!"; else theText = "Error in button interface.";

repaint(); //force color and text change }

private String theText = "Watch me!";}

Using a String instance variablein drawString:


Example: action listeners

Different kinds of components require different kinds of listeners Buttons fire events of type ActionEvent which require action listeners ActionListener is not a class, it is a property (or interface) of a class To give the ActionListener property to a class:

1. Add implements ActionListener to the beginning of the class definition, e.g.:public class ButtonDemo extends Frame implements ActionListener

{ <statements to define class> }

2. Define a method named actionPerformed Use addActionListener to add an action listener for a button, e.g.:

Button stopButton = new Button("Red");

stopButton.addActionListener(this);

See definition of ButtonDemo, Display 7.8/page 347


actionPerformed method

Usually a selection (branching) statement that determines which action event fired and performs the appropriate action

getActionCommand reads the text value assigned to the whichever button was clicked

» the text value was assigned when the Button object was created

public void actionPerformed(ActionEvent e) { if (e.getActionCommand().equals("Red")) { setBackground(Color.red); theText = "STOP"; } else if (e.getActionCommand().equals("Green")) { setBackground(Color.green); theText = "GO"; } else theText = "Error in button interface.";

repaint(); //force color and text change }

actionPerformed method for ButtonDemo:


Layout managers

Three basic layout managers provided with AWT:1. FlowLayout2. BorderLayout3. GridLayout

There are others but they are not covered in this text


FlowLayout

To create a FlowLayout object:setLayout(new FlowLayout());

Arranges components in the order they are added» starting in upper left of window (or other object) and

proceeding to the right» when the top line is full it goes to the next line, etc.

Example: ButtonDemo (Display 7.8/page 347) uses a flow layout


BorderLayout

To create a BorderLayout object:setLayout(new BorderLayout());

Components can be placed in any one of five regions» "North"» "South"» "East"» "West"» "Center"


Example: BorderLayout

In the constructor for some container object:setLayout(new BorderLayout());add(new Button("Up"),"North");add(new Button("Down"),"South");add(new Button("Right"),"East");add(new Button("Left"),"West");add(new Button("Straight ahead"),"Center");

Up

Down

Left RightStraight ahead

The result:


GridLayout

Arranges components in a two-dimensional grid (rows and columns)

Each entry is the same size For a grid with two rows and three columns:

setLayout(new GridLayout(2,3));» the first argument is the number of rows» the second argument is the number of columns» creates a layout like this:


Summary for creating simple window interface

1. Derive a window object from Frame and add components.

2. You must register a window listener for the close-window button» e.g., by adding the following to the GUI class definition within a constructor:

addWindowListener(new WindowDestroyer());» See Display 7.2/page 332 for a definition of WindowDestroyer.

3. Some components, e.g. buttons, generate action events, so the GUI (or some other class) must be made an action listener.

» Every component that generates an action event should have an action listener registered with it.

» Use addActionListener to register an action listener.

4. Use implements ActionListener at the beginning of the class definition to make your GUI (or other class) an action listener.

» You also need to add a definition of the method actionPerformed to the class.

5. There are other ways, but this one is simple and commonly used.


Programming tips

1. Don't reinvent the wheel: copy a similar program and modify it.» Obviously there is a lot of detail to deal with, so start with a

program similar to what you need - the sample programs in each chapter are often good starting points for doing the Programming Exercises at the end of the chapter.

However, it is NOT ok to copy another student's work!

2. See Display 7.11/page 360 for an outline of the code to create a simple GUI class. You may want to create this file and use it as a template: copy it and modify the copy to create a new GUI.


WindowListener options

1. Make the window itself the button listener by implementing the ActionListener interface» we did this when we placed buttons in a window

2. Make the window listener a separate class» we did this for the window-closing button by defining WindowDestroyer

3. Make the window itself the window listener by implementing the WindowListener interface


More about interfaces

Interfaces are just a collection of undefined methods A Java class can be derived from only one base class but can

implement more than one interface All methods in an interface must be defined

» unused methods can be defined with empty bodies, e.g.public void windowDeiconified(WindowEvent e){}

ActionListener has only one method (ActionPerformed), but WindowListener has seven methods» if you do not need the other methods, using ActionListener instead of WindowListener avoids having to write the empty method definitions


The Panel class

A container class to group objects Used to subdivide a Frame into different areas For example, if you want two buttons in the south area of a

border layout, put the two objects in a panel and place the panel in the south position

Typical program organization:

Create new panel

set panel attributes, e.g. background color and panel layout

create and add objects such as buttons

Establish the overall layout and place the panel in the layout Example code: Display 7.13/page 366

Example: Panel public PanelDemo() { setTitle("Panel Demonstration"); setSize(WIDTH, HEIGHT); setBackground(Color.blue); addWindowListener(new WindowDestroyer());

Panel buttonPanel = new Panel(); buttonPanel.setBackground(Color.white);

buttonPanel.setLayout(new FlowLayout());

Button stopButton = new Button("Red"); stopButton.setBackground(Color.red); stopButton.addActionListener(this); buttonPanel.add(stopButton);

Button goButton = new Button("Green"); goButton.setBackground(Color.green); goButton.addActionListener(this); buttonPanel.add(goButton);

setLayout(new BorderLayout()); add(buttonPanel, "South"); }


create panel

place panel in GUI

PanelDemo constructorfrom Display 7.13/page 366


Resulting GUI for PanelDemo

Button Demonstration X

Watch me!

Red Green


TextArea and TextField classes

TextArea: defines an area to display text» specifies how many lines and how many characters per line to display

TextField: also defines an area to display text» but only the number of characters in the line is specified

» the field has only one line For both classes:

» The user can enter text in the field» getText reads text entered by the user» setText writes text to the field

» more text can be written to the field than the specified size, but only an amount equal to the size will be displayed (the scroll bar must be used to see the remaining text)

» initial text can be specified when the object is created with new


Example: TextArea

The constructor for TextAreaDemo (Display 7.14/page 369) creates a panel with a text area 10 lines by 40 characters

The panel color is set to blue and the background of the text area to white

The panel is placed in the center of the window

textPanel = new Panel();textPanel.setBackground(Color.blue);theText = new TextArea(10, 40);theText.setBackground(Color.white);textPanel.add(theText);add(textPanel, "Center");

Creating a text area (excerpt from Display 7.14/page 369):


Example: setText and getText

Using the same example, TextAreaDemo (Display 7.14/page 369):

Clear the text area:

theText.setText("");

Read the text entered by the user into String instance variable memo1:

memo1 = theText.getText();


Label class

Used to label text fields or other components

Typical program organization to attach a label to a text field:

Create new panel

set panel attributes, e.g. background color and panel layout

add text field and label

Place the panel in the layout


Example: Label

Excerpt from LabelDemo (Display 7.15/page 374)name = new TextField(20);

namePanel.add(name, "South");

Label nameLabel = new Label("Enter your name here:");

namePanel.add(nameLabel, "Center");

Resulting GUI: Name Tester X

Test Clear

Enter your name here:

A very good name!A label


Inputting and outputting numbers in the AWT

Problem: the numbers read from or written to a text area are String type but we want numeric types» all AWT input and output is String

Conversion methods are needed» to convert from numeric types to String, and

» to convert String to numeric types


Inputting numbers: methods, methods, methods, ...

Numeric wrapper classes have a valueOf method that converts text numbers to the wrapper numeric type (Integer, Double, etc.) …

… but we often want the corresponding primitive type, so we also need to use the appropriate "Value" method (intValue, doubleValue, etc.) …

… and we want to ignore white space before and after the input, so the trim method is also used


Example:reading an integer from a text area

Combining all these results in a rather long statement, but it should make sense on careful analysis:

Input exampleRead an integer from text field inputOutputField:

int n = Integer.valueOf(inputOutputField.getText().trim()).intValue();


Outputting numbers

Not so many methods need to be strung together Just use toString to convert a numeric value to String,

and setText to write it to the text area.

Output exampleWrite an integer to text field inputOutputField:

int n = 31;

inputOutputField.setText(Integer.toString(n));


Adding menus

In the AWT menus are not a component but are added in ways similar to adding components

Three AWT classes are used» Menu» Menu» MenuItem

The basic approach is to use an add method to» put MenuItems in a Menu and» put Menus in a Menu.

Example:creating a menu

Menu memoMenu = new Menu("Memos");MenuItem m;m = new MenuItem("Save Memo 1");m.addActionListener(this);memoMenu.add(m);

m = new MenuItem("Save Memo 2");m.addActionListener(this);memoMenu.add(m);

m = new MenuItem("Get Memo 1");m.addActionListener(this);memoMenu.add(m);

m = new MenuItem("Get Memo 2");m.addActionListener(this);memoMenu.add(m);

m = new MenuItem("Clear");m.addActionListener(this);memoMenu.add(m);

m = new MenuItem("Exit");m.addActionListener(this);memoMenu.add(m);

MenuBar mBar = new MenuBar();mBar.add(memoMenu);setMenuBar(mBar);


Excerpt from MenuDemo constructor(Display 7.18/page 386)

The resulting GUI before and afterclicking Memos is shown on page 388

Note: the method to add a menu bar is setMenuBar, not add


Inner classes

An inner class is a class that is defined inside another class A full description is beyond the scope of this text However, one situation has already been described:

using the AWT to create GUIs Another simple situation is common and is also described:

helping classes Moving the definition of a helping class inside the class

definition has two advantages:» it allows the helping class to access all instance variables,

including private ones» it improves the level of information hiding

For these reasons inner classes are often used as listeners to handle events fired within the outer class


Summary ... GUIs are written using event-driven programming In event-driven programming, a user action, like a mouse click,

generates an event and the that event is automatically passed to an event-handling program to perform the appropriate action.

Two main ways to build a GUI with the AWT:

1. Use inheritance to create a derived class from a preexisting one

2. Add components to a container A window is defined as a derived class of Frame A button is an object of the class button The add method is used to add components to a container Components in a container are arranged by a layout manager object


… summary, continued

A panel is a container to organize objects inside a larger container. Text fields and text areas are used by GUIs for input and output of text,

including numbers in text format

» methods exist to convert numeric types to and from text MenuBars are created by adding Menus and MenuItems in a manner

similar to putting components in containers, but they are not members of the classes Container or Components

Both buttons and menu items fire action events and so should have an ActionListener registered with them to respond to the event

chapter 7java: an introduction to computer science & programming - walter savitch 1 chapter 7 l...

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