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The Java Language and Environment

The Java Language and Environment. Cecilia Bastarrica, Anupama Vadali, and Prof. Steven A. Demurjian, Sr. Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Box U-255 Storrs, CT 06269-2155. steve@engr.uconn.edu http://www.engr.uconn.edu/~steve

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The Java Language and Environment

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  1. The Java Language and Environment Cecilia Bastarrica, Anupama Vadali, and Prof. Steven A. Demurjian, Sr. Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Box U-255 Storrs, CT 06269-2155 steve@engr.uconn.edu http://www.engr.uconn.edu/~steve http://www.engr.uconn.edu/cse (860) 486 - 4818

  2. Overview of Presentation • Motivation and Introduction to Java • Designing and Developing Applets in Java • The Java User Interface - GUI with AWT • Designing and Developing Java Classes • Inheritance and Interfaces in Java • Polymorphism and Object Serialization

  3. Motivation and Introduction to Java • Java is Emerging as the OO Language of Choice • Java’s Utilization in … • Distributed Internet-Based Applications of All Types • Legacy/COTS Integration for Enterprise Computing • General-Purpose, Single-CPU Development • Significant Dissemination on WWW: • http://www.javasoft.com • http://www.gamelan.com • We’ll Overview Key Features and Capabilities

  4. An Overview of Java • Java is a Third Generation, General Purpose, Platform Independent, Concurrent, Class-Based, Object-Oriented Language and Environment • Java Composed of JDK and JRE • Java Language • Java Packages (Libraries) • javac Compiler to Bytecode (p-code) • JDB Java Debugger • Java Interpreter - Platform Specific • JDK: Java Development Environmenthttp://www.javasoft.com/products/jdk/1.2/ • JRE: Java Runtime Environmenthttp://www.javasoft.com/products/jdk/1.2/jre/index.html

  5. What is Java?Software Releases and IDEs • Java is Free! • Current Releases • Version 2 for Win95, Win98, NT • Version 2 (Early Access) for Solaris • Third-Party Ports to All Conceivable HW/SW Platforms from Micros to Mainframes http://www.javasoft.com/cgi-bin/java-ports.cgi • Integrated Development Environments (IDEs) • Commercial Products, Freeware, Visual IDEs • Visual J++, Visual Café, Kawa, Jpad, Javelin

  6. Java Virtual Machine (JVM) • JVM is a Platform Specific Program which Interprets and Executes Java Code • JVM Interprets and Executes Bytecodes • JVM Targeted as Small/Efficient - Embeddable within Consumer Electronics • JVM Stack Based Machine - Simulates Real Processor CA FE BA BE 00 03 00 2D 00 3E 08 00 3B 08 00 01 08 00 20 08

  7. Java Visualization

  8. Packages In Java • Allows Related Classes to be Grouped into a Larger Abstraction • Similar to Ada95 Packages • Unavailable in C++ • Utilization of Packages for SW Design and Development • Components, Modularization, Groupings • Enforcement by Compiler of Package Rules • Overall, Packages Enhance the Control and Visibility to Fine-Tune • Who Can See What When

  9. The Java API Packages • Application Programming Interface (API) • Java Defined - Building Blocks/Libraries • Java Platform 1.2/2 Core API java.applet java.rmi java.awt java.rmi.dgc java.awt.datatransfer java.rmi.registry java.awt.event java.rmi.server java.awt.image java.security java.beans java.security.acl java.io java.security.interfaces java.lang java.sql java.lang.reflect java.text java.math java.util java.net java.util.zip • Power of Java Contained with APIs

  10. The Java Language • Overview of Non-OO Capabilities • Based on C/C++ • No includes, typedefs, structures, groups • Unicode Character Set - 34,168 Characters • Automatic Coercions Not Supported • Strongly-Type Language • Variables in Java • Primitive Types: ints, floats, booleans, Unicode chars • Reference Types: arrays, classes, interfaces • No Physical Pointers in Java!

  11. The Java Language • Statements in Java - Resembles C and C++ • Assignment/Expressions and Precedence • for, while, do-while • if-then, switch-case • break, continue, label, return • Exception Handling • Similar to C++ • try, throws, catch Blocks • Strongly Integrated Throughout APIs

  12. The Java LanguageMotivating the Class Concept • Conceptually, Classes are Structures/Records with Functions that are Encapsulated structure Item { int UPC, OnShelf, InStock, ROLimit; char* Name; float RCost, WCost; Status Create_New_Item(int UPC, ...); NameCost* Get_Item_NameCost(int UPC); void Modify_Inventory(int UPC, int Delta) ; Boolean Check_If_On_Shelf(int UPC); Boolean Time_To_Reorder(int UPC); }; NameCost *nc; Item I1, I2; I1.Create_New_Item(...); nc = I2.Get_Item_NameCost(UPC);

  13. The Java LanguageObject-Oriented Features • Class - similar to C++ Class • Classes have Members (Methods and Variables) • Members Tagged Using Keywords • private: Typically, Inaccessible • public: Potential to be Accessible • protected: Accessible via Inheritance • package: Accessible within Package • Involve Visible Between Classes, Within Packages, and Due to Inheritance

  14. Classes in Java • A Supermarket Item • Keywords must be Utilized for Each Attribute or Method Declaration class Item { private String UPC, Name; private int Quantity; private double RetailCost; protected double WholeCost; public Item() { ... }; public void finalize() { ... }; public boolean Modify_Inventory(int Delta){...}; public int Get_InStock_Amt() {return Quantity;}; };

  15. Classes in Java class Item { private String UPC, Name; private int Quantity; private double RetailCost; protected double WholeCost; public Item() { ... }; public void finalize() { ... }; public boolean Modify_Inventory(int Delta) { int i = Get_InStock_Amt (); if (Delta >= 0) { Quantity += Delta; return true; } else { return false;} }; public int Get_InStock_Amt() {return Quantity;}; public double Compute_Item_Profit() {...}; protected boolean Modify_WholeSale(); {...}; };

  16. Visibility of Attributes/Methods • Class Members (Attributes and Methods) • Visibility Tags for Members • Private: Visible only to Class • Protected: Visible to Class and Other Classes within Package • Public: Visible to Class, Other Classes within Package, and if Class is Public, Visible to Other Packages/Classes • No Tag: Visible Only to Other Classes within Defining Package • Java's Controlled Sharing within/between Packages not Supported in C++ • Abstraction/Encapsulation Superior in Java!

  17. Inheritance - Two Roles • Controlled Sharing Between Classes • Generalization vs. Specialization • Treat Instances of Different Classes in a Uniform Fashion • Polymorphism and Dynamic Binding • Inheritance in Java Item / \ DeliItem ProduceItem | SaladItem class DeliItem extends Item { ... }; class SaladItem extends DeliItem { ... }; class ProduceItem extends Item { ... };

  18. Designing and Developing Applets in Java • Applets Small Independent Programs Intended for Embedding into WWW Pages and Executable via Java-Enabled Browser (Netscape or IE) • Applets Operate Under Severe Security Limits: • Can’t Execute Local Programs • Can’t Communicate with Host Other than one from Which Downloaded • Can’t Read/Write to Local File System • Can’t Find Information on Local System Except Java/OS Versions and Character/Line Separators

  19. Designing and Developing Applets in Java • An Applet is a Java Program that Executes as part of an HTML Page applet A.java HTML file ... <APPLET CODE = “A.class”> ... javac applet A.class

  20. Applets Inheritance Structure java.lang.Object java.awt.Component java.awt.Container java.applet.Applet your applet Everything in Java inherits from the Object class Event-handling and drawing capabilities Ability to hold components Limits what an Applet can and cannot do Every Applet is a subclass of the Applet class.

  21. Applet Methods Eligible for Overriding Methods for Milestones • init - initializes an applet when it is loaded • start - (re)starts applet’s execution • stop - stops applet’s execution • destroy - final cleanup before unloading Methods for Drawing • paint • update The applet subclass must override at least one of these methods: init, start or paint.

  22. Applets Handle Events by Implementing the Corresponding interface import java.awt.event.MouseListener; import java.awt.event.MouseEvent; … public class Simple extends Applet implements MouseListener { … public void init() { addMouseListener(this); … } public void mouseClicked(MouseEvent event) { addItem(“click…”); } … } Handling Events mouseClicked mouseEntered mouseExited mousePressed mouseReleased Protocol of behavior All methods in the interface must be implemented

  23. Running an Applet in HTML <APPLET CODE = “AppletSubclass.class” WIDTH = anInt HEIGHT = anInt> </APPLET> The Browser: • Reserves a Display Area for the Applet • Loads the bytecode • Creates an Instance of the Subclass • Calls the init and start Methods

  24. Loading an Applet • Finding an Applet • CODEBASE • Relative/Absolute Address CODEBASE=“example/” CODEBASE=“http://someServer/…/otherDirectory/” • Bringing the Applet • Class by Class/Archives otherDirectory example HTML file HTML file Class file Class file

  25. The <APPLET> tag <APPLET [CODEBASE = codebaseURL] (CODE = appletFile | OBJECT = serializedApplet) [ARCHIVE = archivesList] [ALT = alternateText] [NAME = appletInstanceName] WIDTH = pixelsHEIGHT = pixels [ALIGN = alignment] [VSPACE = pixels] [HSPACE = pixels] > [<PARAMNAME = appletParameter1VALUE = value>] [<PARAMNAME = appletParameter2VALUE = value>] … [alternateHTML] </APPLET>

  26. Security Issues • An Untrusted Applet Cannot: • Load Libraries or Define Native Methods • Read or Write Files on the Host that is Executing the Applet • Make Network Connections Except to the Host from Which it was Loaded From • Start any Program on the Host that is Executing the Applet • Get Many System Properties

  27. Example of a GUI Applet /* http://java.sun.com/docs/books/tutorial/ui/components/example/ ButtonDemo.java */ import java.awt.*; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; import java.applet.Applet; public class ButtonDemo extends Applet implements ActionListener { Button b1, b2, b3; static final String DISABLE = "disable"; static final String ENABLE = "enable"; public void init() { b1 = new Button(); b1.setLabel("Disable middle button"); b1.setActionCommand(DISABLE); b2 = new Button("Middle button"); b3 = new Button("Enable middle button"); b3.setEnabled(false); b3.setActionCommand(ENABLE);

  28. Example of a GUI Applet (Continued) //Listen for actions on buttons 1 and 3. b1.addActionListener(this); b3.addActionListener(this); //Add Components to the Applet, using the default FlowLayout. add(b1); add(b2); add(b3); } public void actionPerformed(ActionEvent e) { String command = e.getActionCommand(); if (command == DISABLE) { //They clicked "Disable middle button" b2.setEnabled(false); b1.setEnabled(false); b3.setEnabled(true); } else { //They clicked "Enable middle button" b2.setEnabled(true); b1.setEnabled(true); b3.setEnabled(false); } } }

  29. The Java User Interface - GUI with AWT • UI Refers to the Communications Between a Program and a User • Java Abstract Windowing Toolkit (AWT) and Swing Contains Complete Set of Classes for Writing GUI Programs • AWT Classes Categorized into: • GUI Components • Containers • Layout Managers • Drawing • Event Handling

  30. AWT Components • Button • CheckBoxes • Choices • Lists • Menus • Textfields • Text Areas • Labels

  31. Containers • The Java AWT Provides Three Types of Containers Implemented as Subclasses of Container Class: • Window • Frame - creates a normal full fledged window to contain components • Dialog - provides a window that is dependent on another window • FileDialog - helps the user to open and save file • Panel - Groups Components within an Area of an Existing Window • ScrollPane -Like Panel, Used to Display Large Component in a Limited Amount of Space

  32. Inheritance Hierarchies for Components

  33. Layout Managers • Layout Manager Controls the Size and Position of Components in a Container • By Default Every Container Object has an Associated LayoutManger • Panel Objects - FlowLayout • Window Object - BorderLayout

  34. Layout Managers - Simple • FlowLayout • Default for Panel Objects • Lays out Components from Left to Right Starting New Rows if Needed • GridLayout • Displays the Components in Equal Size in the Requested Number of Rows and Columns

  35. Layout Managers - Special Purpose • BorderLayout • Default for Window Objects • Uses 5 Areas to hold Components: North, South, East, West, and Center • CardLayout • One Area Contains Different Components at Different Times

  36. Layout Managers - Flexible • GridBagLayout • Aligns Components by Placing them in a Grid of Cells • Allows some Components to Span more than one Cell • The Rows and Columns have Different Heights and Widths

  37. Example of a GUI applet TextField Name TextArea Address Label Order Now CheckBox Size Choices Color OK Cancel Quit Button

  38. Excerpts for GUI Applet Example(Note: Many Lines Omitted!!) import java.applet.*; import java.awt.*; import Message; public class Sample extends Applet { Panel p1, p2; Label l1, l2, l3, l4, l5; TextField name; TextArea address; Checkbox order; Choice size, color; Button ok, clear, quit ; Message message;

  39. Excerpts for GUI Applet Example (Note: Many Lines Omitted!!) p1.setLayout(new GridLayout(0,2,2,2)); p1.add(l1); p1.add(name); p1.add(l2); p1.add(address); p1.add(l3); p1.add(order); p1.add(l4); p1.add(size); p1.add(l5); p1.add(color); p2.setLayout(new FlowLayout()); p2.add(ok); p2.add(clear); p2.add(quit); setLayout(new BorderLayout()); add("North", p1); add("South", p2); } public void init() { p1 = new Panel(); p2 = new Panel(); l1 = new Label("Name"); l2 = new Label("Address"); // … other labels omitted name = new TextField(30); address = new TextArea(3, 30); order = new Checkbox(); size = new Choice(); size.addItem("Small"); size.disable(); ok = new Button("OK"); clear = new Button("Clear"); quit = new Button("Quit");

  40. Excerpts for GUI Applet Example (Note: Many Lines Omitted!!) public boolean action(Event e, Object o) { if(e.target == ok) { // You must provide the actions/code } if(e.target == clear) { // You must provide the actions/code } if(e.target == quit) { // You must provide the actions/code } if(e.target == order) { if(order.getState() == true) { // You must provide the actions/code } else if(order.getState() == false) { // You must provide the actions/code } } return true; } }

  41. Event Handling • When a User Acts on a Component, the AWT Detects the Event and Notifies the Event Listeners • A Class Implements the Event Listener and Every Class Instance can Register as Event Listeners • Steps for Implementing and Registering: • Declare that a Class Implements a Listener Interface in the Class declaration • Implement the Listener methods in the Class • Register an Instance of the Class as a Listener on One or More Components

  42. Example of Event Handling public class Beeper ... implements ActionListener { ... // where initialization occurs: button.addActionListener(this); ... public void actionPerformed(ActionEvent e) { // Make a beep sound ... } }

  43. The AWT Events • Action Events • Adjustment Event • Component Events • Container Events • Focus Events • Item Events • Key Events • Mouse Events • Mouse-Motion Events • Text Events • Window Event

  44. Action Listeners • Easiest Event Handlers to Implement • Generated by: • Clicking a Button • Double Clicking a List Item • Choosing a Menu Item • Pressing Return in a Text Field • Listener Interface - ActionListener • Methods - actionPerformed(ActionEvent)

  45. Action Listeners - ActionEvent • Parameter to the actionPerfomed Method • ActionEvent Defines Two Useful Methods • String setActionCommand • Associates a string to the action • String getActionCommand • Returns the string associated with this action

  46. Item Listeners • Generated by Components that Maintain State, Generally on/off State • Components that Generate ItemEvents: • Checkboxes • Choices • Lists • Listener Interface - ItemListener • Methods - itemStateChanged(ItemEvent)

  47. ItemListeners - ItemEvent • Parameter to the itemStateChanged Method • ItemEvent Defines Two Useful Methods: • Object getItem() • Returns the component specific object associated with the item whose state has changed • int getStateChange() • Returns the new state of the item • SELECTED = 1 • DESELECTED = 0

  48. Excerpts for GUI Applet with Listener(Note: Many Lines Omitted!!) import java.applet.*; import java.awt.*; import java.awt.event.*; import Message; public class Sample1 extends Applet implements ActionListener, ItemListener { // Declarations similar to previous example static final String OK = "ok"; public void init() { // Init actions similar to previous example, // except for code shown below for checkbox and buttons order = new Checkbox(); order.addItemListener(this); ok = new Button("OK"); ok.setActionCommand(OK); ok.addActionListener(this); }

  49. Excerpts for GUI Applet with Listener(Note: Many Lines Omitted!!) public void actionPerformed(ActionEvent e) { String command = e.getActionCommand(); if(command == OK) { // You must provide the actions/code } if(command == CLEAR) { // You must provide the actions/code } if(command == QUIT) { // You must provide the actions/code } } public void itemStateChanged(ItemEvent e1) { if(e1.getStateChange() == ItemEvent.SELECTED) { // You must provide the actions/code } else { // You must provide the actions/code } } }

  50. Designing and Developing Java Classes • Encapsulation Capabilities of Java • Classes (Data + Operations) - ADTs • Packages - Collections of “Related” Classes • Class Declaration: • Access Modes and Variable Members • Constructors and Class Bodies • Method Declaration and Access Modes • Class and Instance Members • Garbage Collection

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