F27SB2 Software Development 2

Lecture 2: Java GUIs 1

Overview

• Java GUIs based on 2 main class libraries• AWT - Abstract Windowing Toolkit– original Java GUI classes

• Swing– subset of JFC - Java Foundation Classes– extends AWT– subtle but crucial differences with AWT...

• we will use Swing but need to be aware of AWT concepts

AWT

import java.awt.*;• contains original AWT GUI classes • Component– main AWT class of GUI elements• e.g Label, Button, TextField

Container

• class of Components that can have other Components inside them

• e.g. Frame – top-level displayable window

• e.g. Panel – Container with no display

• LayoutManager– interface class– control positions of nested Components

Container methods

• all Containers have rectangular size • smallest enclosing rectangle– width– height

public void setSize(int width,int height)

• width and height are in pixels• pixel – one point on screen

Container methods

• size of pixel depends on– screen’s physical size• diagonal in inches e.g. 15”; 17”; 21”

– screen resolution• number of columns/rows in pixels e.g. 1680*1050

• NB make sure Container is big enough to hold everything!

• NB size of Container will change depending on properties of LayoutManager - more later

Container methods

• Containers may be visible or invisiblepublic void setVisible(boolean b)

• make visiblesetVisible(true)

• make invisiblesetVisible(false)

• use to change Component availability • e.g. turn available buttons on/off

Container methods

Containers have:– background• what things are drawn on e.g. paper

– foreground• what things are drawn in e.g pen

• e.g. Label  

Container methods

public void setBackground(Color c)

• set Container’s background colour to cpublic void setForeground(Color c) 

• set Container’s foreground colour to c • use these to change foreground/background

dynamically at run-time

Container methods

public class Colorpublic static final Color black, blue, cyan, darkGrey, gray, green lightGray, magenta, orange, pink, red, white,yellow

• must preface colour identifier with Color • e.g. Color.blue• NB to make things in foreground disappear– set foreground to same colour as background

Basic Swing classes

import javax.swing.*; • many Swing classes extend AWT classes public class JComponent extends Container 

• main Swing GUI class NB JComponents may be:• opaque – can’t see through background• transparent – can see through background

Basic Swing classes

• opacity depends on look and feel• change opacity with:public void setOpaque(Boolean isOpaque)

• may need to setOpaque(true)

Frames

public class JFrame extends Frame

• top-level window• with: title-bar; menu-bar; cursor; icon; etc• NB not a JComponentpublic void setTitle(String title) 

• set title on Frame’s title bar to title

Frames

• to create and open a simple JFrame:class class-name extends JFrame{ ... public class-name(...){ ... }}

• frame is set up in a class called class-name which is an extended JFrame

• need an constructor for a class-name to– initialise an instance of the JFrame– hold JComponents that make up the JFrame

Frames

class program-name{ ... class-name frame-name; ...}

• in the class for the main program • need an instance of class-name called frame-name

Frames

public static void main(String [] args){ frame-name = new class-Name(); frame-name.setTitle(“title”); frame-name.setSize(width,height); frame-name.setVisible(true);}

• call the constructor to instantiate frame-name to an instance of class-name

• give JFrame an appropriate title

Frames

public static void main(String [] args){ frame-name = new class-Name(); frame-name.setTitle(“title”); frame-name.setSize(width,height); frame-name.setVisible(true);}

• set the JFrame’s size to width and height– NB must allow size for title bar when specifying

Frame height e.g. 20 pixels• make the JFrame visible

Frames

• in general, cannot work directly on JFrame• must manipulate the JFrame’s ContentPane• ContentPane is a Container • has BorderLayout as default– more later

• to access ContentPane use:public Container getContentPane()

Frames

• e.g. 200*220 white windowimport java.awt.*;import javax.swing.*;class White extends JFrame{ public White() { getContentPane(). setBackground(Color.white); } }

• constructor explicitly locates ContentPane and sets background to white

• however, many methods will work implicitly on ContentPane e.g. add,setLayout

Frames

class TestWhite{ static White w; public static void main(String [] args) { w = new White(); w.setTitle(“White”); w.setSize(200,220); w.setVisible(true); }}

• main constructs instance of White and sets up title, size and visibility

Events and listeners

• not yet enough to display window...• need to provide further details for the

operating system– what to do when window is opened or closed or

iconified etc• event– something that happens outside of a program

• listener– code that responds to an event

Events and listeners

• selecting window menu bar icons causes events

• events are detected by the Java system• Java system:– stops our program– calls the appropriate listener

Events and listeners

• we need to...– import AWT Java event classesimport java.awt.event.*;

– tell the system that we have a listener for window events

public void addWindowListener(WindowListener l)

– build a WindowListener to respond to events

Events and listeners

interface WindowListener

• package provides framework for building a WindowListener– abstract method• defined by method signature • type + name + parameters

– no method body

Events and listeners

• interface– specification of abstract methods that

implementation class will provide• implementation class must provide method

details

Events and listenerspublic interface WindowListener extends EvenListener{ public void windowActivated(WindowEvent e); public void windowClosed(WindowEvent e); public void windowClosing(WindowEvent e); public void windowDeactivated(WindowEvent e); public void windowDeiconified(WindowEvent e); public void windowIconified (WindowEvent e); public void windowOpened(WindowEvent e);}

Events and listeners

• specific events when window is:– activated/deactivated or opened/closed or

iconified/deiconified• system:– calls the appropriate method from interface– provides a WindowEvent parameter to provide

details of the event

Events and listeners

• all methods in WindowListener are abstract• our class must – implement WindowListener– provide details for all methods

• we could implement details of all the interface methods ourselves...

Events and listeners

• better to:– subclass a supplied implementation of the interface– override methods we are concerned about

public class WindowAdapter implements WindowListener

• provides empty versions of all above methods• call WindowAdapter constructor– with body that defines our versions of methods– to return a WindowListener

Example

• e.g. 200*220 window with white background import java.awt.*;import java.awt.event.*;import javax.swing.*; class White extends JFrame{ public White() { setBackground(Color.white); }}

Exampleclass testWhite{ public static void main(String [] args) { White w; w = new White(); w.setTitle(“White”); w.setSize(200,220); w.setVisible(true); ...

Example ... w.addWindowListener (new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); } }

• call addWindowListener – with a new WindowListener• built from WindowAdapter–with a new windowClosing to exit from White

Example

Example: flashing window

• window flashes black to white and back every 0.5 sec

• repeatedly change background• need some way of pausingimport java.util.*;

• Calendar classCalendar Calendar.getInstance()

• returns a Calendar object which holds a snapshot of the current date and time

Example: flashing window

long getTimeInMillis()

• returns time in milliseconds from Calendar object

• NB to update time, must create new Calendar object

• to pause: – find start time– repeatedly • compare current time with start time

– until required time has passed

Example: flashing window

void pause(long millisecs){ long startTime = Calendar.getInstance(). getTimeInMillis(); while(Calendar. getInstance(). getTimeInMillis()- startTime<millisecs);}

Example: flashing window

import java.awt.*; import java.awt.event.*;import javax.swing.*; import java.util.*; class Blackwhite extends JFrame{ public Blackwhite() { getContentPane(). setBackground(Color.black); }  private void pause(long millisecs){...}

Example: flashing window

  public void flash() { while(true) { pause(500); getContentPane(). setBackground(Color.white); pause(500); getContentPane(). setBackground(Color.black); } }}

Example: flashing window

class TestBlackwhite{ public static void main(String [] args) { static Blackwhite b; b = new Blackwhite(); b.setTitle(“Black and white”); b.setSize(200,220); b.setVisible(true);

Example: flashing window

b.addWindowListener (new WindowAdapter() { public void windowClosing (WindowEvent e) { System.exit(0); } }); b.flash(); }}

Example: flashing window