Question on bounding generic with Enum

Consider an enum that implements some interface:
public enum Blah implements Bleh
//I can now make a class like this:
public class Foo<T extends Enum<T>>
// The compiler has no problem so far.
// But if I do this:
public class Foo<T extends Enum<T> & bleh>
The compiler gives this error:
classes cannot directly extend java.lang.Enum
What have I done wrong?
-Eric

It takes more than 10 minutes to show up. More like a
week. Be patient. I just cut and pasted this thread.You responded so fast I thought the bug was already known. I hope this
can be fixed in the next release because I think it will be a very useful
bounding technique for enums.
-Eric

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Problems with enums

Hi, I am trying to build a card class where each card has a suit, rank and prime number. I am trying to do it with enums, but I am having problems initialising the class from my Deck. The Card class is like so:
import java.util.Comparator;
public class Card
     private final Rank rank;
     private final Suit suit;
     private final PrimeVal primeVal;
     public enum Rank
          Deuce(0),Three(1),Four(2),Five(3),Six(4),Seven(5), Eight(6),
          Nine(7), Ten(8), Jack(9), Queen(10), King(11), Ace(12);
          private final Integer rankValue;
         Rank(int rankValue)
             this.rankValue = rankValue;
         public Integer faceValue(){ return rankValue; };
     public enum PrimeVal
          Deuce(2),Three(3),Four(5),Five(7),Six(11),Seven(13), Eight(17),
          Nine(19), Ten(23), Jack(29), Queen(31), King(37), Ace(41);
          private final Integer primeVal;
          PrimeVal (int primeVal)
             this.primeVal = primeVal;
         public Integer primeVal(){ return primeVal; };
     public enum Suit
          Spades(0x1000), Hearts (0x2000), Diamonds(0x4000), Clubs(0x8000);
          private final Integer suitValue;
          Suit(int suitValue)
               this.suitValue = suitValue;
          public Integer suitValue(){ return suitValue; };
     public Card(Rank rank, Suit suit, PrimeVal primeVal)
        this.rank = rank;
        this.suit = suit;
        this.primeVal = primeVal;
    public Rank rank() { return rank; }
    public Suit suit() { return suit; }
    public String toString() { return rank + " of " + suit; }
}and my initialisation procedure in Deck is:
static
         for (Card.Suit suit : Card.Suit.values())
            for (Card.Rank rank : Card.Rank.values() )
                Deck.add(new Card(rank, suit));
    }This code is predominantly from a enum tutorial off of this site, my problem is, I am just learning, I don't know to keep the features and add in the prime number and get it to initialise correctly... which is why the constructor is wrong. Is this even a good way of doing it?
Many thanks, Ron

Kaj, thanks, i know I need a constructor, I just
don't know how to add it in! That was the question,
how to add it?
Thanks, RonAre you kidding me?
Here's your original code, trying to invoke the constructor:
Deck.add(new Card(rank, suit)); You're passing 2 arguments to the Card constructor, but you need 3.
Think.

What's the deal with enums?

Hello,
I'm trying to familiarise myself with the new Java features that project Tiger (Java 1.5) will introduce. I come from a C++ programming background. I very briefly studied enums whilst I learned C++ and never used them in my programs, so I was, quite frankly, relieved when I learned that Java didn't support them.
But now it does.
I'm wondering, from a software design and engineering perspective, what benefit enums will add to the Java language and software engineering in general. I have read some pleas and arguments for enums. As I understand those arguments, proponents feel that minor ledgibility improvements to switch statements (which some OO programmers don't use on the basis that it weakens the object-orientedness through its goto-esque break statements) necessitate the addition of a whole other structure.
Out of curiosity, wouldn't it be more advantageous for Sun to put their efforts into supporting constant strings in switches in addition to primitive data types if people are so concerned with clarity?
I fail to understand what enums will do that constant string arrays or constant values cannot. I would like to think that Sun wouldn't add C++ features unless if there was a reason to do so, so I believe that there are good reasons to introduce this feature.
Please help clarify this confusion.
Thank you!

Comments on the previous posting inline. Note: I'll go off on a few tangents, but try to always return to the issue of enums. The fact that I go off on tangents (related initially to an enum type example) should show that enums help eliminate numerous problems.
Fair enough, but wouldn't this accomplish the same thing in
a slightly wordier wayPretty much. But you still have the validation code in there yourself:
currentDirection = ( direction > 0 && direction < 5 ? direction : 0 );People can still accidentally enter an invalid value, and it won't get caught by the compiler. So, I could call set(Compass.WEST) and hope/expect that is equivalent to set(Direction.LEFT). This will only occur if both Compass.WEST and Direction.LEFT are the same int value. If they are not, then we are in trouble. Worse, still if Compass.WEST and Direction.RIGHT are the same value, the code will compile, and run without any illegal value flagging, but our traveller will go off in the wrong direction.
Error handling tangent: Given the implementation, the error handling is not performed in set, rather the illegal value is captured as a zero, which must be dealt with at a later time - say in a switch statement that calls get. This is not a good idea - it increases the "distance" between the error occuring (bad call to set) and the error being detected (if xInt != 0). The larger the distance, the harder debugging is. If you throw an exception in set the bug is immediately flagged. With enums, none of this should be necessary.
Implementation tangent: set, get and currentDirection are static. This is not a good idea - it only allows one direction. If you have multiple travellers, each would have a Direction object, but (sticking with constants) UP, etc. would still be static.
I keep the switch statement in my enum workaround, but I believe that
the second reply suggested that enum could altogether eliminate the
use for switch statements. How exactly would it do that?enum in its simplest form would not remove switch. However, using an extension of the type-safe enum idiom (i.e. the current Java approach to enums), you can embed direction specific logic in each of the "enum" members. If I understand the enum-spec correcly, this will also be possible with the Java 1.5 enum language facility.
So, for an example, sticking with the domain: Consider a class Location, with x and y elements, and a Direction. The non-OO, non-enum approach would be as follows:
class Location {
    int x,y;
    void move (final int direction) {
        switch (direction) {
            case UP:
                --y;
                break;
            case DOWN:
                ++y;
                break;
            case LEFT;
                --x;
                break;
            case RIGHT:
                ++x;
                break;
            default;
                throw new RuntimeException("Invalid direction " + direction);
class Direction {
    public static final int UP = 1;
    public static final int DOWN = 2;
    public static final int LEFT = 3;
    public static final int RIGHT = 4;
}Note that I've deliberately been loose with my visibility. All logic is in Location, and Direction is dumb. If we go with a type safe-enum idiom, we get:
class Location {
    int x,y;
    void move (final Direction direction) {
        if (direction == Direction.UP) {
            --y;
        } else if (direction == Direction.DOWN) {
            ++y;
        } else if (direction == Direction.LEFT) {
            --x;
        } else if (direction == Direction.RIGHT) {
            ++x;
        } else { // direction is null
            throw new NullPointerException("null direction");
class Direction {
    public static final Direction UP = new Direction();
    public static final Direction DOWN = new Direction();
    public static final Direction LEFT = new Direction();
    public static final Direction RIGHT = new Direction();
    private Direction () {} // the ctor is private to limit instances
}This code is a rewrite of the int approach, but is "type-safe". We've moved from switch to if as we can't switch on object references. Note: there is still error handling code, as you're not "null-safe" (I don't know what the Java 1.5 spec says about this)
Little else has changed - the responsibilities are still the same. We can address this by going OO, which removes the if (formerly a switch) statement, but does couple the classes.
class Location {
    int x,y;
    void move (final Direction direction) {
        direction.move(this);
class Direction {
    private final int xOffset, yOffset;
    private Direction (final int xOffset, final int yOffset) {
        this.xOffset = xOffset;
        this.yOffset = yOffset;
    public static final Direction UP = new Direction(0,1);
    public static final Direction DOWN = new Direction(0,-1);
    public static final Direction LEFT = new Direction(-1,0);
    public static final Direction RIGHT = new Direction(1,0);
    void move(final Location location) {
        location.x += xOffset;
        location.y += yOffset;
}This removes any conditional logic, embedding (constructor parameterised) logic within Direction. It also eliminates the "null-safe" check, as direction.move(this) will throw an NPE if a null is passed.
Also, I've read some things saying that enum reduces 'boilerplate'
code. What exactly does that and 'boilerplate' mean?The Direction class is an example of the type-safe enum approach currently used in Java. It has been enhanced with logic for movement. Even if I dropped this logic, there is still functionality worth adding to the Direction class - a toString method that returns one of "UP", "DOWN", etc (shown next); a way to serialize the object and retain reference equality - you have to write a readResolve method (not shown).
class Direction {
    private final String name;
    private Direction (final String name) {
        this.name = name;
    public static final Direction UP = new Direction("UP");
    public static final Direction DOWN = new Direction("DOWN");
    public static final Direction LEFT = new Direction("LEFT");
    public static final Direction RIGHT = new Direction("RIGHT");
    public String toString() {
        return name;
}Once you've written this code for one type-safe enum, you've written them for them all. You copy-cut-paste, and rename the class/elements. All this code is predictable. It is "boiler plate". Now, if you gave me a compiler that generated all this code for me, when I present it with
enum Direction {UP, DOWN, LEFT, RIGHT}; then I've got something that reduces my boiler plate code to zip.... Enter the Java 1.5 compiler.

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Hi All,
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>
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>
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>
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HTML and/or DW Tutorials, and other information links that I have saved
http://validator.w3.org/
http://www.visibone.com/
http://www.w3schools.com/
http://www.hotscripts.com/
http://webdesignledger.com/
http://www.projectseven.com/
http://www.adobe.com/devnet/
http://www.scriptarchive.com/
http://www.htmldog.com/guides/
http://www.htmlcodetutorial.com/
http://alistapart.com/topics/code
http://www.how-to-build-websites.com/
http://css.maxdesign.com.au/floatutorial/
Download User Guide as PDF for easy search
http://www.adobe.com/support/documentation
http://www.ianr.unl.edu/internet/mailto.html
http://lynda.com/ Hours of videos. (must pay)
http://apptools.com/examples/pagelayout101.php
http://www.thesitewizard.com/archive/css.shtml
http://www.projectseven.com/tutorials/index.htm
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http://livedocs.adobe.com/en_US/Dreamweaver/9.0/
Nate's Forms http://www.mindpalette.com/scripts/
Customizing the layouts that come with CS3 (VIDEO)
http://www.adobe.com/designcenter/video_workshop/?id=vid0155
FormMail http://www.bebosoft.com/products/formstogo/index.php
For those using MySQL - Installing PHP and MySQL on Windows XP
http://www.webassist.com/professional/products/solutionrecipes.asp
Community MX lessons http://www.communitymx.com/abstract.cfm?cid=3D074
http://www.adobe.com/cfusion/designcenter/search.cfm?product=Dreamweaver&go=Go
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Please help me on this because presently i am working UK porject when i am executing transaction code S_P00_07000134 getting no list genarated
Please give me some inputs
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Sent!! chk u r mail for the notes.
Regrads,
shashikanth,
Asssign points for usefull answer.

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A question about a method with generic bounded type parameter

Hello everybody,
Sorry, if I ask a question which seems basic, but
I'm new to generic types. My problem is about a method
with a bounded type parameter. Consider the following
situation:
abstract class A{     }
class B extends A{     }
abstract class C
     public abstract <T extends A> T someMethod();
public class Test extends C
     public <T extends A> T someMethod()
          return new B();
}What I want to do inside the method someMethod in the class Test, is to
return an instance of the class B.
Normally, I'm supposed to be able to do that, because an instance of
B is also an instance of A (because B extends A).
However I cannot compile this program, and here is the error message:
Test.java:16: incompatible types
found   : B
required: T
                return new B();
                       ^
1 errorany idea?
many thanks,

Hello again,
First of all, thank you very much for all the answers. After I posted the comment, I worked on the program
and I understood that in fact, as spoon_ says the only returned value can be null.
I'm agree that I asked you a very strange (and a bit stupid) question. Actually, during recent months,
I have been working with cryptography API Core in Java. I understood that there are classes and
interfaces for defining keys and key factories specification, such as KeySpec (interface) and
KeyFactorySpi (abstract class). I wanted to have some experience with these classes in order to
understand them better. So I created a class implementing the interface KeySpec, following by a
corresponding Key subclass (with some XOR algorithm that I defined myself) and everything was
compiled (JDK 1.6) and worked perfectly. Except that, when I wanted to implement a factory spi
for my classes, I saw for the first time this strange method header:
protected abstract <T extends KeySpec> T engineGetKeySpec
(Key key, Class<T> keySpec) throws InvalidKeySpecExceptionThat's why yesterday, I gave you a similar example with the classes A, B, ...
in order to not to open a complicated security discussion but just to explain the ambiguous
part for me, that is, the use of T generic parameter.
The abstract class KeyFactorySpi was defined by Sun Microsystem, in order to give to security
providers, the possibility to implement cryptography services and algorithms according to a given
RFC (or whatever technical document). The methods in this class are invoked inside the
KeyFactory class (If you have installed the JDK sources provided by Sun, You can
verify this, by looking the source code of the KeyFactory class.) So here the T parameter is a
key specification, that is, a class that implements the interface KeySpec and this class is often
defined by the provider and not Sun.
stefan.schulz wrote:
>
If you define the method to return some bound T that extends A, you cannot
return a B, because T would be declared externally at invocation time.
The definition of T as is does not make sense at all.>
He is absolutely right about that, but the problem is, as I said, here we are
talking about the implementation and not the invocation. The implementation is done
by the provider whereas the invocation is done by Sun in the class KeyFactory.
So there are completely separated.
Therefore I wonder, how a provider can finally impelment this method??
Besides, dannyyates wrote
>
Find whoever wrote the signature and shoot them. Then rewrite their code.
Actually, before you shoot them, ask them what they were trying to achieve that
is different from my first suggestion!
>
As I said, I didn't choose this method header and I'm completely agree
with your suggestion, the following method header will do the job very well
protected abstract KeySpec engineGetKeySpec (Key key, KeySpec key_spec)
throws InvalidKeySpecException and personally I don't see any interest in using a generic bounded parameter T
in this method header definition.
Once agin, thanks a lot for the answers.

Question about reading generic tables with keys

Hello
I'm wondering if it's possible to read a generic table with key statement:
FIELD-SYMBOLS:
    <lv_key>         TYPE zconf_key,
    <lt_data_bi>     TYPE INDEX TABLE,
    <lt_data_ai>     TYPE INDEX TABLE,
    <ls_data_bi>     TYPE any,
    <ls_data_ai>     TYPE any.
* create data types
CREATE DATA lt_data_bi TYPE (ls_node-data_table_type).
ASSIGN lt_data_bi->* TO <lt_data_bi>.
CREATE DATA lt_data_ai TYPE (ls_node-data_table_type).
ASSIGN lt_data_ai->* TO <lt_data_ai>.
* do check
LOOP AT <lt_data_ai> ASSIGNING <ls_data_ai>.
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    READ TABLE <lt_data_bi> WITH KEY key = <lv_key>
    ASSIGNING <ls_data_bi> BINARY SEARCH.
I receive the message: "The specified type has no structure and therefore no component called KEY" in the read statement above.
Does anyone has an idea what I can do here?
Regards,
Mathias

Mathias Glockner wrote:
>     READ TABLE <lt_data_bi> WITH KEY key = <lv_key>
In your structure ls_data_bi there is no field called KEY . please check whether you have field KEY in structure
a®s

Question on bounding generic with Enum

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