Command-Line Applications. GUI Libraries GUI-related classes are defined primarily in the java.awt and the javax.swing packages.

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1 CS257 Computer Science I Kevin Sahr, PhD Lecture 14: Graphical User Interfaces Command-Line Applications 2 The programs we've explored thus far have been text-based applications A Java application

Graphical User Interfaces (GUIs) Applications can be written to use a GUI GUI Libraries GUI-related classes are defined primarily in the java.awt and the javax.

1 1 CS257 Computer Science I Kevin Sahr, PhD Lecture 14: Graphical User Interfaces Command-Line Applications 2 The programs we've explored thus far have been text-based applications A Java application is a stand-alone program with a main method Our programs have been command-line applications, which interact with the user using text input and output Now we will examine some Java programs that have Graphical User Interfaces (GUIs) Applications can be written to use a GUI GUI Libraries GUI-related classes are defined primarily in the java.awt and the javax.swing packages 3 The Abstract Windowing Toolkit (AWT) was the original Java GUI package The Swing package provides additional and more versatile components a J is added to Swing class names to distinguish them from existing AWT classes Both packages are needed to create a Java GUI-based program GUI Libraries 4 The new JavaFX library is intended to eventually replace Swing as the primary GUI library for Java Currently Swing remains more polished and full-featured Swing will be supported for some time to come

2 Applets 5 A Java applet is a program that is intended to be executed using a web browser Applets can also be executed using the appletviewer tool of the Java SDK (invoked for applets by the Eclipse Run button) Since a web browser does not have a command-line, applets must use GUIs In this class we will do our GUI development as applets Applets and HTML 6 An applet is embedded into an HTML file using a tag that references the compiled bytecode file of the applet The bytecode version of the program is transported across the web and executed by a Java interpreter that is part of the browser The Eclipse Run button will automatically generate an html file for your applet program and launch it using the appletviewer tool Applets and HTML Here is a simple html file that will launch an applet called AppTest <html> 7 <head> <title>simple Swing Applet Example</title> </head> <body> <applet code=apptest.class width=100 height=75> </applet> </body> </html> Java Applets 8 A class that defines an applet extends the JApplet class (in package javax.swing) The JApplet class handles all the details of applet creation, execution, and interaction with the web browser Our applet classes only have to deal with issues that specifically relate to what our particular applet will do

3 The init method 9 An applet doesn't have a main method Instead, it must override JApplet methods that serve specific purposes The init method is invoked when the applet is first created The program s GUI should be set-up in the init method GUI Components 10 A GUI component is an object that represents a screen element, such as a button or a text field All components share some basic attributes, such as a background color A color in a Java program is represented as an object created from the Color class The background color of a component can be set using the method setbackground: compa.setbackground(somecolor); Colors 11 The Color class contains several predefined color constants, including: Color.black Color.blue Color.cyan Color.orange Color.white Color.yellow Color.red Color.green So we could set the background color of a component to blue using: compa.setbackground(color.blue); Labels 12 A label is a GUI component that displays a line of text Labels are usually used to display information or identify other components in the interface The following creates a label with the text HELLO JLabel labela = new JLabel( HELLO );

4 Labels 13 The text displayed on a label can be changed by invoking the method settext: labela.settext( GOODBYE ); By default the label background is transparent We can make it opaque using: labela.setopaque(true); GUI Containers 14 A GUI container is a component that is used to hold and organize other components The Container class is derived from the Component class, so containers have all component attributes (e.g., a background color) and methods (e.g., setbackground) Containers that contain other components make up the containment hierarchy of an interface Applet Containment Hierarchy All JApplet objects have a main container called the Content Pane that is automatically created for them The content pane is the root of the containment hierarchy for an applet All of the GUI components (including other containers) must be added to the content pane in order to be displayed The JApplet method getcontentpane allows you to access your applet s content pane: Container cp = getcontentpane(); 15 Panels 16 A panel (class JPanel) is a container that cannot be displayed on its own but is used to organize other components A panel must be added to another container to be displayed We can use panels to help organize the layout of our GUI The add method is used to add a component to a container: containera.add(somecomponent);

5 A Simple Applet 17 Let's look at a simple applet that organizes two labels in a panel and displays that panel in a frame See President.java Event Driven Programming 18 Traditional program flow-of-control is linear the order of statement execution is dictated by the programmer GUI programs are usually event driven the order of statement execution is based on events that occur at run-time events might include mouse clicks, graphical button presses, incoming chat messages, etc. Event Driven Programming 19 Event driven programs involve two basic steps: 1.setup objects that handle events 2.enter an event loop, that waits for events to occur and passes the events to appropriate objects until the program terminates Events 20 In Java, an event is an object that represents some activity to which we may want to respond The Java standard class library contains several classes that represent typical events Many components, such as a graphical button, can generate (or fire ) an event

6 Buttons 21 A push button is a component that allows the user to initiate an action event by pressing a graphical button using the mouse A push button is defined by the JButton class The following creates a button labeled PRESS ME JButton button = new JButton( PRESS ME ); Pressing a button generates an action event, which is an object of class ActionEvent Listeners 22 In Java, a listener object "waits" for a specific event to occur and responds accordingly When an appropriate event occurs, a specific method in the listener object is invoked the method is passed an event object that describes what occurred We can design our listener objects to take whatever actions are appropriate when an event occurs ActionListeners An action listener is a listener that can handle action events Action listeners are created by implementing the ActionListener interface The ActionListener interface contains a single abstract method: public void actionperformed(actionevent e) 23 the actionperformed method defines how this action listener will handle an action event Listening for an Event 24 Any object that is-a ActionListener can request to receive events from a button using: button.addactionlistener(this); When the button is pressed the actionperformed method is invoked on all objects that are listening for events from that button

7 Handling an Event 25 If a listener is registered for events from multiple objects, it may need to check which object generated the event The source object can be determined by invoking getsource on the event object argument: public void actionperformed(actionevent e) { Object src = e.getsource(); if (src == button)...do something... Disabled Components 26 Components can be disabled if they should not be used A disabled component is "grayed out" and will not respond to user interaction The status is set using the setenabled method: button.setenabled(false); // disables...do some stuff... button.setenabled(true); // re-enables GUI Design 27 Good GUI design can preempt invalid user inputs Use setenabled to keep the user from performing actions that are not allowed by the current state of the program Layout Managers 28 A layout manager is an object that determines the way that components are arranged in a container There are several layout managers defined in the Java standard class library: Flow Layout Border Layout Card Layout Grid Layout GridBag Layout Box Layout Overlay Layout Defined in the AWT Defined in Swing

8 Layout Managers 29 Every container has a default layout manager, but we can explicitly set the layout manager for a container as well Each layout manager has its own particular rules governing how the components will be arranged Some layout managers pay attention to a component's preferred size or alignment, while others do not A layout manager attempts to adjust the layout as components are added and as containers are resized Layout Managers 30 We can use the setlayout method of a container to change its layout manager JPanel panel = new JPanel(); panel.setlayout(new BorderLayout()); Flow Layout Flow layout puts as many components as possible on a row, then moves to the next row Rows are created as needed to accommodate all of the components Components are displayed in the order they are added to the container Each row of components is centered horizontally in the window by default, but could also be aligned left or right Flow layout is the default for panels 31 Border Layout 32 A border layout defines five areas into which components can be added North West Center East South

9 Border Layout 33 Each area displays one component (which could be a container such as a JPanel) Each of the four outer areas enlarges as needed to accommodate the component added to it If nothing is added to the outer areas, they take up no space and other areas expand to fill the void The center area expands to fill space as needed Border Layout The area in which a component should be placed in a boarder layout is designated when The you possible add it areas are: 34 BorderLayout.NORTH BorderLayout.SOUTH BorderLayout.EAST BorderLayout.WEST BorderLayout.CENTER EX: somepanel.setlayout(new BorderLayout()); somepanel.add(button, BorderLayout.NORTH); An Example 35 The example program AppTest.java illustrates everything discussed in this lecture Lecture 14 Vocabulary 36 command-line application Graphical User Interface (GUI) Abstract Windowing Toolkit (AWT) Swing applet GUI component GUI container containment hierarchy content pane event driven programming event listener layout manager flow layout border layout

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