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Java Overview

Java Overview. About me. Peter Kriens Work as a consultant (mainly for for ERICSSON) Finnasandsvagen 22 43933 Onsala, Sweden +46 705950899 Peter.Kriens@aQute.se. History. 1990 SUN starts crash team under Patrick Naughton Carte blanche to clean up the software mess at SUN

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Java Overview

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  1. Java Overview

  2. About me • Peter Kriens • Work as a consultant (mainly for for ERICSSON) • Finnasandsvagen 22 • 43933 Onsala, Sweden • +46 705950899 • Peter.Kriens@aQute.se

  3. History • 1990 SUN starts crash team under Patrick Naughton • Carte blanche to clean up the software mess at SUN • Gosling created OAK for embedded devices (!) • Products all failed • 1993 Mosaic, the first web browser was born • Somebody realized the combination: Applets • Language became the product

  4. History • 1996 JDK 1.0 released • Focused on Applets, lousy graphics • 8 packages, 4Mb download • 1997 JDK 1.1 released • Better graphics, reflection, security (try), beans, RMI, ZIP files • 22 Packages, 9 Mb download

  5. History • 1999 JDK 1.2 SE released • Lightweight UI (!), collections, security (again), JAR files, Native interface change… • 59 packages, 20 Mb download • 2000 JDK 1.3 SE in beta • Improvements ... • 77 packages (so far), 25 Mb download

  6. Quick Tour: Hello World • Getting started • Content of file: HelloWorld.java • /** * Small hello world example class. * */public class HelloWorld { public static void main( String args[] ) { System.out.println( "Hello world" + (args.length >= 1 ? args[0] : "whoever") ); }} • javac HelloWorld.java • java HelloWorld peterHello world peter • That is all ...

  7. Quick Tour: Objects • Creating a class • Content of file: Point.java • /** * A Simple point class. */public class Point { double _x; // Instance variable double _y; public Point() { this(0,0); } public Point( double x, double y ) { _x=x; _y=y; } public double getX() { return _x; } public double getY() { return _y; } public double getLength() { return Math.sqrt( getX()*getX() + getY() * getY() ); } public Point translate( Point t ) { return new Point(getX() + t.getX(), getY() + t.getY() );}}

  8. Quick Tour: Not to be an Object • Not all variables are objects like in Smalltalk • int, char, boolean, long, double, float, byte are <primitive> types. • Each primitive type is represented by a class in java.lang • public class HelloWorld { public static void main( String args[] ) { int length = args.length; Point p = new Point( length, length ); }}

  9. Quick Tour: Inner class • Everything had to be a class. Callbacks require their own class • Too cumbersome to specify in other file, so special syntax was wrought (ugly!): • public static void main( String args[] ) { Point p = new Point() { public double getX() { return super.getX() * 2; } };

  10. Quick Tour: Garbage Collection • Java cleans up after you • /* GC */...Point p = new Point();p = p.translate( new Point(10,10) );... • Previous code creates 3 new objects • No need for destructor. No need for delete

  11. Quick Tour: Inheritance • Classes can be extended • /* Extending point class */public class Point3 extends Point { double _z; public Point3() { _z=0; super(0,0); } public double getZ() { return _z; } public double getLength() { return Math.sqrt( getX()*getX() + getY()*getY() + getZ() * getZ() ); } ...} • Only use it for an "is-a" relationship • Powerful but easy to overdo

  12. Quick Tour: Inheritance • Single inheritance only • All classes inherit from class Object • A Class is a an instance of class Class • So the class Class is an instance of Class (!) • Class objects can be used as normal objects • Reflection • ClassLoaders • "super" keyword to access super class

  13. Quick Tour: Interfaces • Used for specifications • /* Interfaces */public interface Compare { public int compare( Compare t );}class CPoint extends Point implements Compare { public int compare( Compare t ) { Point tt = (Point) t; return getLength() - t.getLength(); }} • Decouples using from implementation • Very popular with specifications

  14. interface Quick Tour: Interfaces public interface Log { public void log(String s);} client uses Log public class SimpleLog { public void log(String s) { System.out.println( s ); }} implements Simple Log IBM Log Motorola Log

  15. Quick Tour: Exceptions • Exceptions are used to separate normal flow of program from error cases • Runtime exceptions: • public class HelloWorld { public static void main( String args[] ) { System.out.println( "Hello world" + args[0] ); }}

  16. Quick Tour: Exceptions • Checked exceptions: • public class HelloWorld { public static void main(String args[]) throws IOException { FileOutputStream out = new FileOutputStream( "temp" ); out.write( args[0].getBytes() ); out.close(); }} • Exceptions can be very messy

  17. Quick Tour: Exceptions • Handling exceptions: • public class HelloWorld { public static void main( String args[] ) { FileOutputStream out; try { out = new FileOutputStream( "temp" ); out.write( args[0].getBytes() ); out.close(); } catch( IOException e ) { System.out.println( "Exception " + e ); e.printStackTrace(); } finally { try { out.close(); } catch(Exception ee) {} } }} • Never ignore exceptions: catch(...) {} • Easy to overdo exceptions

  18. Quick Tour: Packages • Name spaces for Java • package = add a class to a package • import = use classes from a package • Encapsulate a number of related classes • Used for access control • Content of file: ../se/aQute/plane/Point.java • package se.aQute. plane;import se.aQute.basictest.*;public class Point { double _x; double _y; public Point() { clear(); } ...}

  19. Quick Tour: Equality • Equal and identical are different concepts • Identity check is == • Class Object has method equals(Object o) method that is identity • 3 == 3; // YES • "three" == new String("three"); // NO • "three".equals( new String("three") ); // YES • Classes can override equals(Object o) for their semantics • Watch hashCode() when overriding equals() !!!

  20. Quick Tour: toString() • toString() is a useful method for debugging • Each object inherits a default implementation from Object • Overriding can be very, very useful during debugging • public class Point { . . . public String toString() { return getX() + "," + getY(); } . . .}

  21. The Java VM • Java is compiled to byte codes • Byte codes are interpreted by the VM File: HelloWorld.java public class HelloWorld { public static void main( String args[] ) { System.out.println( "Hello world" ); } } File: HelloWorld.class javac compiler 0xCA 0xFE 0xBA 0xBE 0x01 0xF7 0x76 0x41 0x23 . . . rt.jar or classes.zip java VM Hello world

  22. Native code interface • Native code interface through JNI • Native interface defined in Java class • class Native { int var; public native int foo();} • Translated via javah into C header file • #include <native.h>typedef struct ClassNative { long var; } ClassNative;HandleTo(Native);extern long Native_foo(struct HNative *);

  23. The Java VM: The good news • Portable format allows execution on many different computer types • Hundreds of VMs available from different vendors • Optimized for certain applications • Inherently safe

  24. The Java VM: The bad news • Interpretation requires CPU cycles • Instruction set not optimized for target machines • Byte format is rather verbose

  25. Is Java Cool? • Nothing particularly innovative • Portable object code from Pascal (P-code!) • Syntax from C++ • Object model from Smalltalk • Garbage Collection from Lisp • Reasons for success: • People were getting fed up with C++ • Java looked much simpler • Applets kick started it (but no longer drive it)

  26. The runtime library • The runtime library is Java's best asset • Single implementation of common code • Some implementations are pretty bad (AWT!) • Library has grown very hard • From 8 packages to 77 packages! • Need for profiles • There is an amazing amount of code to be found on the net

  27. The runtime library: java.lang • Basic package which is always included • Contains primitive type classes for Boolean, Integer, Double, Float, Byte, Character, Void • number <-> string conversions • number <-> number conversions • Max/Min values • Used in reflection

  28. The runtime library: java.lang • Process, Runtime and System for access to system resources • Running of external processes and linking of external libraries • Debugging • Memory interface • Time • System properties • Utiltity functions like arraycopy

  29. The runtime library: java.lang • Thread, ThreadGroup for threads • A Thread is like an internal process • Run multiple threads at the same time • Combine threads in a group for security control • Monitors are used to manage shared resources • Math = Math library • Contains mathematical routines • sqrt, cos, sin, log, ln ...

  30. The runtime library: java.lang • Throwable • Base class for Exceptions and Error • SecurityManager for security • Performs security checks (when installed) • Access to call stack

  31. The runtime library: java.lang • String • Unicode! • 16 bit char = 65536 possible characters • Functions • String can be concatenated with + • System.out.println( "File : " + file + " : " + exception ); • substring, indexOf, trimming • Conversion from number to String • Symbols (unique value with intern() )

  32. The runtime library: java.lang • StringBuffer • Used to concatenate strings • Expensive: • String concat = "";for ( int i=0; i<10; i++ ) concat = concat + i; • Less Expensive • StringBuffer sb = new StringBuffer();for ( int i=0; i<10; i++ ) sb.append( "" + i );String concat = sb.toString();

  33. The runtime library: java.io • Streams are used for byte access to files • Readers/Writers are used for Unicode access • Streams, Readers, Writers can be used as pipes • Buffering • Data access (e.g. getInt(), getShort() ) • Between threads (PipeXXXXX) • Conversion from stream to reader/writer • ObjectStreams

  34. The runtime library: java.net • Access to the net (TCP/IP) • InetAddress • Stream connections: Socket, ServerSocket • Extendable: SocketImplFactory • URL, URLConnection • Extendable: URLStreamHandlerFactory • Datagrams • Unicast/Multicast

  35. The runtime library: java.util • Useful classes • Collections: Vector, Hashtable, BitSet, Properties, Stack, Enumeration, Map, Array, Iterator • Time: Date, Calendars, TimeZones • Locale: Locale, ResourceBundles • Random • Observable • StringTokenizer • EventObject, EventListener

  36. The runtime library: other • java.rmi • Remote method invocation support • java.math • Big Integer (unlimited digits) for private/public key calculations

  37. The runtime library: more other • java.security • Classes for certificates, principals, permissions • java.sql • Access support to SQL databases • java.text • Support for language independent messages • java.util.zip • Access to zip files which are the preferred delivery vehicle for java applications.

  38. The runtime library: more other • java.util.jar • Access to jar files and their manifest • java.lang.reflect • Access to objects in runtime via reflection on their interfaces • java.beans • Support for Java beans

  39. The runtime libraries: GUI • Original AWT in 1.0 was … well, awful • JDK 1.1 at start 1997 improved significantly • Netscape released lightweight GUI called IFC at that time • SUN decided to develop their own lightweight GUI at the end of 1997 • Netscape joined them SUN team and stopped support for IFC

  40. GUI: AWT • Uses peer model • A widget controls a native widget • Supports "native look and feel" • Event model: • first based on single dispatching method handleEvent • Today based on listeners • Layout managers • LightWeight components

  41. GUI: AWT looks

  42. GUI: IFC • Derived from NeXT, adopted by Netscape • Lightweight components • Very clean code, small, reliable and included in Netscape Communicator • Internal windows, drag & drop • Event handling via strings • Powerful GUI builder called Constructor • Available (including source) but not maintained

  43. GUI: IFC example, full control

  44. GUI: JFC • Derived from IFC (same people [@ start]) • Lightweight components, listeners • JButton top = new JButton("Top");top.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { System.out.println("top"); }} ); • Compare Smalltalk, there it would be:Button top = Button new: 'top'.top action: [ Transcript print: 'top' ]. • HUGE • Based on Model-View paradigm & Pluggable UI • Easier to use than it looks

  45. GUI: JFC complexity, the button L&F ... Button UI Abstract Button Button Model Default Button Model JButton JToggle Button JMenu Item JRadio Button JCheck Box JCheckBox MenuItem JMenu JRadioButton MenuItem

  46. JFC: The good and changing looks

  47. Components: Beans • Components are wrapped objects • Access via reflection • Allows runtime composition of systems (via end users?): Visual Programming • Properties can be set via strings Bean Methods Events Properties

  48. Java Beans • Events follow the listener model • SomeBean bean = new SomeBean();bean.addTickEventListener(new TickEventListener() { public void tick(TickEvent e) { System.out.println("tick");}}); • Properties are defined via methods that start with get/set • E.g. property "tickCount"public class SomeBean { public long getTickCount() { … } public void setTickCount() { … }} • Bean programmer can override defaults with an Introspector

  49. The Bean Box

  50. Enterprise Java Beans • SUN's attempt to enter the enterprise computing market • Mainframe connectivity • Application Servers • Message Queues • Transactions EJB client

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