1. J2SE 5.0 Generics

• Carol McDonald

• Java Technology Architect

• • Sun Microsystems, Inc.

2. Speakers Qualifications

CarolcDonald:

• Java Architect at Sun Microsystems

• Before Sun, worked on software development of:

• • Application tomanage car LoansforToyota(>10 million loans)

• • PharmaceuticalIntranet( RocheSwitzerland)

• • TelecomNetwork Mgmt( DigitalFrance)

• • X.400Email Server( IBMGermany)

3. Java SE 1.5 Language Changes 4. Seven Major New Features

Generics

Autoboxing/Unboxing

Enhanced for loop (foreach)

Type-safe enumerations

Varargs

Static import

Metadata

5. Generics 6. Is there a problem in here? Vector v = new Vector(); v.add(newInteger (4)); OtherClass.expurgate(v); ... static void expurgate(Collection c) { for (Iterator it = c.iterator(); it.hasNext();) if ((( String )it.next()).length() == 4) it.remove(); } 7. The Problem (Pre-J2SE 5.0) Vector v = new Vector(); v.add(newInteger (4)); OtherClass.expurgate(v); ... static void expurgate(Collection c) { for (Iterator it = c.iterator(); it.hasNext();) /*ClassCastException */ if ((( String )it.next()).length() == 4) it.remove(); } 8. Generics

Problem: Collection element types

• Compiler is unable toverify types

• Assignment must use thecastoperator

• This can generate runtime errors ( ClassCastException )

Solution:

• Tell the compiler what typethe collection is

• Let the compiler fill in thecast

9. Using Generic Classes

Instantiate agenericclass to create type specific object

Example

Vector x = new Vector ();

x.add(new Integer(5)); // Compiler error!

Vector y = new Vector ();

10. Wildcards

Method to print contents of any Collection?

Wrong!

Passing aCollection of type Stringwill give acompiler error

void printCollection(Collection c) { for (Object o : c)System.out.println( o ); } 11. Wildcards

Correct way:

?is thewildcardtype

Collection means Collection ofunknown

void printCollection( Collectionc) { for (Object o : c)System.out.println(o); } 12. Bounded Wildcards

A wildcard can be specified with anupper bound

public voiddrawAll (List< ? extends Shape >s) { ... } List c = getCircles(); drawAll(c) ; List t = getTriangles(); drawAll(t) ; 13. Autoboxing & Unboxing 14. Autoboxing/Unboxing of Primitive Types

Problem: (pre-J2SE 5.0)Conversion betweenprimitivetypes andwrappertypes (and vice-versa)

• must manually convert a primitive type to a wrapper type before adding it to a collection

• inti= 22;

• List l = new LinkedList();

• l.add( new Integer(i) );

15. Autoboxing/Unboxing of Primitive Types

Solution: Let the compiler do it

• Integer intObj=22 ;// Autoboxing conversion

• int i=intObj; // Unboxing conversion

• ArrayList< Integer >al= new ArrayList();

• al .add( i );//Autoboxing conversion

16. Enhanced for Loop 17. Enhanced for Loop (foreach)

Problem: (pre-J2SE 5.0)

• Iterating over collections is tricky

• Often, iterator only used to get an element

• Iterator is error prone(Can occur three times in a for loop)

Solution: Let the compiler do it

• New for loop syntax for (variable : collection)

• Works for Collections and arrays

18. Enhanced for Loop Example

Oldcode pre-J2SE 5.0

• void cancelAll(Collection c) {

• for ( Iterator i = c.iterator(); i.hasNext() ; ){

• • TimerTask task =(TimerTask) i.next();

• • task.cancel();

• • }

• }

NewCode

• void cancelAll(Collection c) {

• • for ( TimerTask task : c )

• • task.cancel();

• }

Iterating over collections,tricky, error prone

New for loop syntax:

Let the compiler do it

Works for Collections and arrays

19. Type-safe Enumerations 20. Type-safe Enumerations

Problem: (pre-J2SE 5.0)to define an enumeration:

• Defined a bunch of integer constants:

• public static final int SEASON_WINTER = 0;

• public static final int SEASON_SPRING = 1;

Issues of using Integer constants

• Not type safe (any integer will pass),Brittleness (how do add value in-between?), Printed values uninformative (prints just int values)

Solution: New type of class declaration

• enumtype has public, self-typed members for each enum constant

• enum Season { WINTER, SPRING, SUMMER, FALL }

21. Enumeration Example: public classCard{ publicenum Suit{ spade, diamond, club, heart }; publicenum Rank{ace, two, three, four, five,six, seven, eight, nine, ten,jack, queen, king }; privateCard (Rank rank, Suit suit) { this.rank = rank; this.suit = suit; } } List< Card >deck= new ArrayList< Card >(); for ( Suitsuit :Suit.values ()) for ( Rankrank :Rank.values ()) deck.add(new Card(rank, suit)); Think how much JDK1.4 code this would require! 22. Varargs 23. Before Varargs Example//example method that takes a variable number ofparameters int sum( Integer[] numbers ) {for(int i: numbers) // do something } // Code fragment that calls the sum method sum( new Integer[] {12,13,20} ); http://www.javaworld.com/javaworld/jw-04-2004/jw-0426-tiger1.html

Problem: (in pre-J2SE 5.0)To have a method that takes avariable number of parameters

• Can be done with an array, but caller has to create the array first

24. Varargs Example(Cont) //example method that takes a variable number of parameters int sum ( Integer... numbers ){ for(int i: numbers) // do something }// Code fragment that calls the sum method sum( 12,13,20 ); http://www.javaworld.com/javaworld/jw-04-2004/jw-0426-tiger1.html

Solution: Let the compiler do it for you:

• String format (String fmt,Object... args);

• Java now supports printf(...)

25. Varargs examples

APIs have been modifiedso that methods accept variable-length argument listswhere appropriate

• Class.getMethod

• Method.invoke

• Constructor.newInstance

• Proxy.getProxyClass

• MessageFormat.format

New APIs do this too

• System.out. printf (%d + %d = %d , a, b, a+b);

26. Static Imports 27. Static Imports

Problem: (pre-J2SE 5.0)

• Having to fully qualify every static referenced from external classes

Solution: New import syntax

• importstaticTypeName.Identifier;

• importstaticTypename.*;

• Also works for static methods and enums

• e.gMath.sin(x)becomessin(x)

28. Formatted I/O 29. Simple Formatted I/O

Printfis popular with C/C++ developers

• Powerful, easy to use

Finally adding printf to J2SE 5.0 (using varargs)

• out.printf(%-12s is %2d long, name, l);

• out.printf(value = %2.2F, value);

30. Annotations 31. Annotations Metadata (JSR-175)

Provide standardised way ofadding annotationsto Java code

• public@Remotevoid foo() {}

Annotations areused by toolsthat work with Java code:

• Compiler

• IDE

• Runtime tools

Used togenerateinterfaces, deployment descriptors...

32. Annotations Example: JAX-RPC

Old Code

public interfacePingIFimplements java.rmi.Remote{

• public void foo()throws java.rmi.RemoteException;

• }

• public class Pingimplements PingIF{

• public void foo() {...}

• }

New Code

• public class Ping {

• public@Remotevoid foo() {}

• }

33. Resources and Summary 34. For More Information (1/2)

Memory management white paper

• http://java.sun.com/j2se/reference/whitepapers/

Destructors, Finalizers, and Synchronization

• http://portal.acm.org/citation.cfm?id=604153

Finalization, Threads, and the Java Technology Memory Model

• http://developers.sun.com/learning/javaoneonline/2005/coreplatform/TS-3281.html

Memory-retention due to finalization article

• http://www.devx.com/Java/Article/30192

35. For More Information (2/2)

FindBugs

• http://findbugs.sourceforge.net

Heap analysis tools

• Monitoring and Management in 6.0

• • http://java.sun.com/developer/technicalArticles/J2SE/monitoring/

• Troubleshooting guide

• • http://java.sun.com/javase/6/webnotes/trouble/

• JConsole

• • http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html

36. Resources

http://java.sun.com/javase

Java.net

http://jdk.dev.java.net

37. Stay in Touch with Java SE

http://java.sun.com/javase

JDK Software Community

• planetjdk.org

• community.java.net/jdk

JDK 6

• http://jdk6.dev.java.net/

• http://jcp.org/en/jsr/detail?id=270

JDK 7

• http://jdk7.dev.java.net/

• http://jcp.org/en/jsr/detail?id=277

38. Thank You!

Carol McDonald

• Java Technology Architect

• • Sun Microsystems, Inc.