Subclass Vs Concrete class

Hi,
What is the exact difference between subclass and concrete class?
What is the need in subclassing abstract class to again abstract class?
Thanks in Advance,
venu.

Hi,
What is the exact difference between subclass
and concrete class?"Subclass" defines a class' relationship to another class.
class A extends BB is a subclas of A.
Every class in Java except Object is a subclass of some other class.
A concrete class is one that is not abstract.
class A {}
abstract class B{}Cass A is concrete. Class B is not.
A class can be both a subclass and concrete. In fact, every single concrete class except Object is a subclass.
What is the need in subclassing abstract class to
again abstract class?You would do that when you're able to provide concrete implementations for some but not all of the abstract methods

Similar Messages

Abstract classes, Interfaces, and concrete classes

I have another technical interview tomorrow and somehow I keep getting asked the same question and I feel my answer is not really up to par. The question is:
"What is the advantage of subclassing an abstract class versus concrete class?"
"What is the difference of using an interface versus an abstract class, which is better to use?"
For the first question, I usually answer performance is the advantage because you don't have to instantiate the class.
For the second question, I usually say that you can put implementation in an abstract class and you can't in an interface. I really can't answer the second part to this question.
Any ideas?

For the first question, I usually answer performance
is the advantage because you don't have to instantiate
the class. Try invoking the class B in the following somewhere in another class.
abstract class A{
A(){
System.out.println("abstract instantiated");
class B extends A{
B(){super();}
}

Concrete classes

hey can someone show me the code on how to write a concrete class for automobiles because ive written the main abstract superclass which should contain the commonalities of serialnumber, and quantity, but im not sure how to write out the first concrete subclass and what it should contain if someone can provide the code thatd be cool thanks.
{ from EdenMonaro }
"I'd start with a pencil and a blank sheet of paper. An abstract Asset class and concrete child classes seem to be a good place to start. What information is common to all your assets? For example, asset number, asset description, purchase date, purchase price, accumulated depreciation, current location.
What information is unique to the child classes? Serial numbers on automobiles and electronic gear? Automobiles have lots of identification numbers: engine numbers, VINs, registration, insurance. CDs have a title and artist(s) with tracks. Furniture is different again, type (desk, chair, etc), colour, finish.
You need another class with collections of assets, perhaps AssetRegister. This is the class you put methods in to list and search assets. "

Implement the asset tracking program. allow the user
to add, modify, and delete electronics, automobiles,
furniture, and cd's. Allow the user to list the assets
by category and search for an asset by its serial
number.
Write an add method, a modify method, a delete method, and your search methods. Make these generic so that can be used for all categories. For a GUI are you going to use a servlet/jsp/html or an application/applet? You need to start writing the pieces that I mentioned and come back with specific implementation questions.

How to improve speed of queries that use ORM one table per concrete class

Hi,
Many tools that make ORM (Object Relational Mapping) like Castor, Hibernate, Toplink, JPOX, etc.., have the one table per concrete class feature that maps objects to follow structure:
CREATE TABLE ABSTRACTPRODUCT (
    ID VARCHAR(8) NOT NULL,
    DESCRIPTION VARCHAR(60) NOT NULL,
    PRIMARY KEY(ID)
CREATE TABLE PRODUCT (
    ID VARCHAR(8) NOT NULL REFERENCES ABSTRACTPRODUCT(ID),
    CODE VARCHAR(10) NOT NULL,
    PRICE DECIMAL(12,2),
    PRIMARY KEY(ID)
CREATE UNIQUE INDEX iProduct ON Product(code)
CREATE TABLE BOOK (
    ID VARCHAR(8) NOT NULL REFERENCES PRODUCT(ID),
    AUTHOR VARCHAR(60) NOT NULL,
    PRIMARY KEY (ID)
CREATE TABLE COMPACTDISK (
    ID VARCHAR(8) NOT NULL REFERENCES PRODUCT(ID),
    ARTIST VARCHAR(60) NOT NULL,
    PRIMARY KEY(ID)
there is a way to improve queries like
SELECT
    pd.code CODE,
    abpd.description DESCRIPTION,
    DECODE(bk.id,NULL,cd.artist,bk.author) PERSON
FROM
    ABSTRACTPRODUCT abpd,
    PRODUCT pd,
    BOOK bk,
    COMPACTDISK cd
WHERE
    pd.id = abpd.id AND
    bk.id(+) = abpd.id AND
    cd.id(+) = abpd.id AND
    pd.code like '101%'
or like this:
SELECT
    pd.code CODE,
    abpd.description DESCRIPTION,
    DECODE(bk.id,NULL,cd.artist,bk.author) PERSON
FROM
    ABSTRACTPRODUCT abpd,
    PRODUCT pd,
    BOOK bk,
    COMPACTDISK cd
WHERE
    pd.id = abpd.id AND
    bk.id(+) = abpd.id AND
    cd.id(+) = abpd.id AND
    abpd.description like '%STARS%' AND
    pd.price BETWEEN 1 AND 10
think in a table with many rows, then exists something inside MaxDB to improve this type of queries? like some anotations on SQL? or declare tables that extends another by PK? on other databases i managed this using Materialized Views, but i think that this can be faster just using PK, i'm wrong? the better is to consolidate all tables in one table? what is the impact on database size with this consolidation?
note: with consolidation i will miss NOT NULL constraint at database side.
thanks for any insight.
Clóvis

Hi Lars,
i dont understand because the optimizer get that Index for TM at execution plan, and because dont use the join via KEY column, note the WHERE clause is "TM.OID = MF.MY_TIPO_MOVIMENTO" by the key column, and the optimizer uses an INDEX that the indexed column is ID_SYS, that isnt and cant be a primary key, because its not UNIQUE, follow the index columns:
indexes of TipoMovimento
INDEXNAME     COLUMNNAME          SORT     COLUMNNO     DATATYPE     LEN     INDEX_USED     FILESTATE     DISABLED
ITIPOMOVIMENTO     TIPO               ASC     1          VARCHAR          2     220546          OK          NO
ITIPOMOVIMENTO     ID_SYS               ASC     2          CHAR          6     220546          OK          NO
ITIPOMOVIMENTO     MY_CONTA_DEBITO          ASC     3          CHAR          8     220546          OK          NO
ITIPOMOVIMENTO     MY_CONTA_CREDITO     ASC     4          CHAR          8     220546          OK          NO
ITIPOMOVIMENTO1     ID_SYS               ASC     1          CHAR          6     567358          OK          NO
ITIPOMOVIMENTO2     DESCRICAO          ASC     1          VARCHAR          60     94692          OK          NO
after i create the index iTituloCobrancaX7 on TituloCobranca(OID,DATA_VENCIMENTO) in a backup instance and get surprised with the follow explain:
OWNER     TABLENAME     COLUMN_OR_INDEX          STRATEGY                    PAGECOUNT
     TC          ITITULOCOBRANCA1     RANGE CONDITION FOR INDEX          5368
               DATA_VENCIMENTO               (USED INDEX COLUMN)
     MF          OID               JOIN VIA KEY COLUMN               9427
     TM          OID               JOIN VIA KEY COLUMN               22
                              TABLE HASHED
     PS          OID               JOIN VIA KEY COLUMN               1350
     BOL          OID               JOIN VIA KEY COLUMN               497
                                   NO TEMPORARY RESULTS CREATED
     JDBC_CURSOR_19                    RESULT IS COPIED   , COSTVALUE IS     988
note that now the optimizer gets the index ITITULOCOBRANCA1 as i expected, if i drop the new index iTituloCobrancaX7 the optimizer still getting this execution plan, with this the query executes at 110 ms, with that great news i do same thing in the production system, but the execution plan dont changes, and i still getting a long execution time this time at 413516 ms. maybe the problem is how optimizer measure my tables.
i checked in DBAnalyser that the problem is catalog cache hit rate (we discussed this at [catalog cache hit rate, how to increase?|;
) and the low selectivity of this SQL command, then its because of this that to achieve a better selectivity i must have an index with, MF.MY_SACADO, MF.TIPO and TC.DATA_VENCIMENTO, as explained in previous posts, since this type of index inside MaxDB isnt possible, i have no choice to speed this type of query without changing tables structure.
MaxDB developers can develop this type of index? or a feature like this dont have any plans to be made?
if no, i must create another schema, to consolidate tables to speed queries on my system, but with this consolidation i will get more overhead, i must solve the less selectivity because i think if the data on tables increase, the query becomes impossible, i see that CREATE INDEX supports FUNCTION, maybe a   FUNCTION that join data of two tables can solve this?
about instance configuration it is:
Machine:
Version:       '64BIT Kernel'
Version:       'X64/LIX86 7.6.03   Build 007-123-157-515'
Version:       'FAST'
Machine:       'x86_64'
Processors:    2 ( logical: 8, cores: 8 )
data volumes:
ID     MODE     CONFIGUREDSIZE     USABLESIZE     USEDSIZE     USEDSIZEPERCENTAGE     DROPVOLUME     TOTALCLUSTERAREASIZE     RESERVEDCLUSTERAREASIZE     USEDCLUSTERAREASIZE     PATH
1     NORMAL     4194304          4194288          379464          9               NO          0               0               0               /db/SPDT/data/data01.dat
2     NORMAL     4194304          4194288          380432          9               NO          0               0               0               /db/SPDT/data/data02.dat
3     NORMAL     4194304          4194288          379184          9               NO          0               0               0               /db/SPDT/data/data03.dat
4     NORMAL     4194304          4194288          379624          9               NO          0               0               0               /db/SPDT/data/data04.dat
5     NORMAL     4194304          4194288          380024          9               NO          0               0               0               /db/SPDT/data/data05.dat
log volumes:
ID     CONFIGUREDSIZE     USABLESIZE     PATH               MIRRORPATH
1     51200          51176          /db/SPDT/log/log01.dat     ?
parameters:
KERNELVERSION                         KERNEL    7.6.03   BUILD 007-123-157-515
INSTANCE_TYPE                         OLTP
MCOD                                  NO
_SERVERDB_FOR_SAP                     YES
_UNICODE                              NO
DEFAULT_CODE                          ASCII
DATE_TIME_FORMAT                      ISO
CONTROLUSERID                         DBM
CONTROLPASSWORD
MAXLOGVOLUMES                         2
MAXDATAVOLUMES                        11
LOG_VOLUME_NAME_001                   /db/SPDT/log/log01.dat
LOG_VOLUME_TYPE_001                   F
LOG_VOLUME_SIZE_001                   6400
DATA_VOLUME_NAME_0005                 /db/SPDT/data/data05.dat
DATA_VOLUME_NAME_0004                 /db/SPDT/data/data04.dat
DATA_VOLUME_NAME_0003                 /db/SPDT/data/data03.dat
DATA_VOLUME_NAME_0002                 /db/SPDT/data/data02.dat
DATA_VOLUME_NAME_0001                 /db/SPDT/data/data01.dat
DATA_VOLUME_TYPE_0005                 F
DATA_VOLUME_TYPE_0004                 F
DATA_VOLUME_TYPE_0003                 F
DATA_VOLUME_TYPE_0002                 F
DATA_VOLUME_TYPE_0001                 F
DATA_VOLUME_SIZE_0005                 524288
DATA_VOLUME_SIZE_0004                 524288
DATA_VOLUME_SIZE_0003                 524288
DATA_VOLUME_SIZE_0002                 524288
DATA_VOLUME_SIZE_0001                 524288
DATA_VOLUME_MODE_0005                 NORMAL
DATA_VOLUME_MODE_0004                 NORMAL
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DATA_VOLUME_GROUPS                    1
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MAXVOLUMES                            14
LOG_IO_BLOCK_COUNT                    8
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BACKUP_BLOCK_CNT                      64
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MAX_LOG_QUEUE_COUNT                   0
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CACHE_SIZE                            262144
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XP_DATA_CACHE_RGNS                    0
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KERNELDIAGSIZE                        800
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EXTERNAL_DUMP_REQUEST                 NO
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__PARAM_VERIFIED__                    2008-05-03 23:12:55
DIAG_HISTORY_NUM                      2
DIAG_HISTORY_PATH                     /var/opt/sdb/data/wrk/SPDT/DIAGHISTORY
_DIAG_SEM                             1
SHOW_MAX_STACK_USE                    NO
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LOG_SEGMENT_SIZE                      2133
_COMMENT
SUPPRESS_CORE                         YES
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FORMAT_DATAVOLUME                     YES
OFFICIAL_NODE
UKT_CPU_RELATIONSHIP                  NONE
HIRES_TIMER_TYPE                      CPU
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LOAD_BALANCING_DIF                    10
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HS_STORAGE_DLL                        libhsscopy
HS_SYNC_INTERVAL                      50
USE_OPEN_DIRECT                       YES
USE_OPEN_DIRECT_FOR_BACKUP            NO
SYMBOL_DEMANGLING                     NO
EXPAND_COM_TRACE                      NO
JOIN_TABLEBUFFER                      128
SET_VOLUME_LOCK                       YES
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SHAREDSQL_CLEANUPTHRESHOLD            25
SHAREDSQL_COMMANDCACHESIZE            262144
MEMORY_ALLOCATION_LIMIT               0
USE_SYSTEM_PAGE_CACHE                 YES
USE_COROUTINES                        YES
FORBID_LOAD_BALANCING                 YES
MIN_RETENTION_TIME                    60
MAX_RETENTION_TIME                    480
MAX_SINGLE_HASHTABLE_SIZE             512
MAX_HASHTABLE_MEMORY                  5120
ENABLE_CHECK_INSTANCE                 YES
RTE_TEST_REGIONS                      0
HASHED_RESULTSET                      YES
HASHED_RESULTSET_CACHESIZE            262144
CHECK_HASHED_RESULTSET                0
AUTO_RECREATE_BAD_INDEXES             NO
AUTHENTICATION_ALLOW
AUTHENTICATION_DENY
TRACE_AK                              NO
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TRACE_INSERT                          NO
TRACE_LOCK                            NO
TRACE_LONG                            NO
TRACE_OBJECT                          NO
TRACE_OBJECT_ADD                      NO
TRACE_OBJECT_ALTER                    NO
TRACE_OBJECT_FREE                     NO
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TRACE_OPTIMIZE                        NO
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TRACE_PAGES                           NO
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TRACE_DATATREE                        0
TRACE_DATAINDEX                       0
TRACE_DBPROC                          0
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TRACE_FILEDIR                         0
TRACE_FRAMECTRL                       0
TRACE_IOMAN                           0
TRACE_IPC                             0
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TRACE_LOGTRANS                        0
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TRACE_MESSAGES                        0
TRACE_OBJECTCONTAINER                 0
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TRACE_OMS_CONTEXT                     0
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TRACE_OMS_FLUSHCACHE                  0
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TRACE_OMS_KEYRANGE                    0
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TRACE_OMS_MEMORY                      0
TRACE_OMS_NEWOBJ                      0
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TRACE_PAGER                           0
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OPTIMIZE_AGGREGATION                  YES
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OPTIMIZE_JOIN_HASH_MINIMAL_RATIO      1
OPTIMIZE_OPERATOR_JOIN_COSTFUNC      YES
OPTIMIZE_JOIN_PARALLEL_MINSIZE        1000000
OPTIMIZE_JOIN_PARALLEL_SERVERS        0
OPTIMIZE_JOIN_OPERATOR_SORT           YES
OPTIMIZE_QUAL_ON_INDEX                YES
DDLTRIGGER                            YES
SUBTREE_LOCKS                         NO
MONITOR_READ                          2147483647
MONITOR_TIME                          2147483647
MONITOR_SELECTIVITY                   0
MONITOR_ROWNO                         0
CALLSTACKLEVEL                        0
OMS_RUN_IN_UDE_SERVER                 NO
OPTIMIZE_QUERYREWRITE                 OPERATOR
TRACE_QUERYREWRITE                    0
CHECK_QUERYREWRITE                    0
PROTECT_DATACACHE_MEMORY              NO
LOCAL_REDO_LOG_BUFFER_SIZE            0
FILEDIR_SPINLOCKPOOL_SIZE             10
TRANS_HISTORY_SIZE                    0
TRANS_THRESHOLD_VALUE                 60
ENABLE_SYSTEM_TRIGGERS                YES
DBFILLINGABOVELIMIT                   70L80M85M90H95H96H97H98H99H
DBFILLINGBELOWLIMIT                   70L80L85L90L95L
LOGABOVELIMIT                         50L75L90M95M96H97H98H99H
AUTOSAVE                              1
BACKUPRESULT                          1
CHECKDATA                             1
EVENT                                 1
ADMIN                                 1
ONLINE                                1
UPDSTATWANTED                         1
OUTOFSESSIONS                         3
ERROR                                 3
SYSTEMERROR                           3
DATABASEFULL                          1
LOGFULL                               1
LOGSEGMENTFULL                        1
STANDBY                               1
USESELECTFETCH                        YES
USEVARIABLEINPUT                      NO
UPDATESTAT_PARALLEL_SERVERS           0
UPDATESTAT_SAMPLE_ALGO                1
SIMULATE_VECTORIO                     IF_OPEN_DIRECT_OR_RAW_DEVICE
COLUMNCOMPRESSION                     YES
TIME_MEASUREMENT                      NO
CHECK_TABLE_WIDTH                     NO
MAX_MESSAGE_LIST_LENGTH               100
SYMBOL_RESOLUTION                     YES
PREALLOCATE_IOWORKER                  NO
CACHE_IN_SHARED_MEMORY                NO
INDEX_LEAF_CACHING                    2
NO_SYNC_TO_DISK_WANTED                NO
SPINLOCK_LOOP_COUNT                   30000
SPINLOCK_BACKOFF_BASE                 1
SPINLOCK_BACKOFF_FACTOR               2
SPINLOCK_BACKOFF_MAXIMUM              64
ROW_LOCKS_PER_TRANSACTION             50
USEUNICODECOLUMNCOMPRESSION           NO
about send you the data from tables, i dont have permission to do that, since all data is in a production system, the customer dont give me the rights to send any information. sorry about that.
best regards
Clóvis

Concrete classes implement abstract class and implements the interface

I have one query..
In java collection framework, concrete classes extend the abstract classes and implement the interface. What is the reason behind extending the class and implementing the interface when the abstract class actually claims to implement that interface?
For example :
Class Vector extends AbstractList and implements List ,....
But the abstract class AbstractList implements List.. So the class Vector need not explicitly implement interface List.
So, what is the reason behind this explicit definition...?
If anybody knows please let me know..
Thanx
Rajendra.

Why do you post this question again? You already asked this once in another thread and it has been extensively debated in that thread: http://forum.java.sun.com/thread.jsp?forum=31&thread=347682

Concrete class name in StackTraceElement

Does anyone know how to get information about concrete classes
on the java stack trace? It is customary to get stack traces that
list abstract classes or interfaces, but in the example below
what was the concrete class implementing HttpServlet or extending
the abstract class xxxCommandServlet that caused the exception?
Java's StackTraceElement doesn't have a reference to the object
that is calling a method that is put on the stack. Surely that
information is on the stack frame since they must be passing
around a "this" pointer as one of the parameters, but how to get it?
Wouldn't it be nice to have an additional field in StackTraceElement,
callingClass. I wonder if it is planned since it would help in
debugging.
public final class StackTraceElement implements java.io.Serializable {
// Initialized by VM
private String declaringClass;
private String callingClass; // NEW FIELD??
private String methodName;
private String fileName;
private int lineNumber;
Thanks very much,
Dmitri
Example stack trace
at com.xxx.servlet.xxxCommandServlet.doGet(xxxCommandServlet.java:66)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:265)
at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubImpl.java:200)
at weblogic.servlet.internal.WebAppServletContext.invokeServlet(WebAppServletContext.java:2495)
at weblogic.servlet.internal.ServletRequestImpl.execute(ServletRequestImpl.java:2204)
at weblogic.kernel.ExecuteThread.execute(ExecuteThread.java:139)
at weblogic.kernel.ExecuteThread.run(ExecuteThread.java:120)

No, com.xxx.servlet.xxxCommandServlet is an abstract
class. Nonetheless, as the stack trace indicated, there is a method named doGet, and the exception occurred at line 66 in xxxCommandServlet.java. So you're saying you can't directly instantiate xxxCommandServlet, this is true, but whatever extends that abstract class inherited the doGet method, of course. It shouldn't matter what the actual class was; the error wasn't in the actual class, but in the xxxCommandServlet (or resulting from something else within the stack trace).

Abstract/ concrete class questions/problems

I am new to java and working on a abstract problem. I'm getting several errors. Here is the code I have so far for the abstract class. I commented out the super and it compiles but I'm not sure if it correct. I'm suppose to create a abstract base class Animal. Single constructor requires String to indicate type of animal which then is stored in an instance variable. I also have to add a few methods (describe(), move(), etc).
public abstract class Animal
     public Animal(String type)
          //super(type);
     public abstract String describe();
     public abstract String sound();
     public abstract String sleep();
     public abstract String move();
}

thanks for the replies. I modified my code but I have a few more errors I can't figure out. Can you browse the code and help point me in the right direction.
Here are the errors I get
cannot find symbol
symbol : constructor Cat(java.lang.String,java.lang.String)
location: class Cat
cannot find symbol
symbol : constructor Robin(java.lang.String)
location: class Robin
abstract base class:
public abstract class Animal
     String type;
     public Animal(String type)
       this.type = type;
     public abstract String describe();
     public abstract String sound();
     public abstract String sleep();
     public abstract String move();
}concrete class
public class Cat extends Animal
     private String name;
     protected String breed;
     public Cat()
       super("Cat");
     public String describe()
          return new String(",a breed of Cat called");
     public String sound()
          return new String("Meow");
     public String sleep()
          return new String("Kitty is having purfect dreams!");
     public String move()
          return new String("This little Kitty moves fast!");
}another abstract class
public abstract class Bird extends Animal
     protected String breed;
     public Bird()
       super("Bird");
     public abstract String move();
}here is the abstract test program itself
public class AbstractTest
     public static void main(String[] args)
          Cat cat = new Cat("Kitty", "Angora");
          Robin bird = new Robin("Rockin");
          System.out.println("Form the cat:   ");
          System.out.print("This is:     "); cat.describe();
          System.out.print("Sound:       "); cat.sound();
          System.out.print("Sleeping:    "); cat.sleep();
          System.out.print("Moving:      "); cat.move();
          System.out.println("\n");
          System.out.println("For the robin:     "); bird.describe();
          System.out.print("This is:             "); bird.sound();
          System.out.print("Sound:               "); bird.sleep();
          System.out.print("Moving:              "); bird.move();
          System.out.print("\n");
          System.out.println("nEnd of program.");
}

Working out the Concrete classes of an Interface

Hi all I have an interface e.g. Animal with 3 concrete classes:
Dog, Cat and Bird.
Is there a way of working out that the Animial interace contains these 3 concrete classes in anyway?
Thanks,

notforgoogle wrote:
Is there a way of working out that the Animial interace contains these 3 concrete classes in anyway?Not really, no.
First of all, it doesn't "contain" them. The relationship between class and interface is one-way.
You could whip up something that examines every class on your classpath, or a portion of it, and cheks whether it implements that interface. However, this will not work in general, since classes can be loaded from anywhere, and you may not always have the right or ability to list the classes present there, only to request specfic classes by name.

IRunner interface depends on concrete class

I am interested in implementing the org.flexunit.runner.IRunner interface. However, there is a reference to the concrete class AsyncTestToken:
function run( notifier:IRunNotifier, previousToken:AsyncTestToken ):void;
Can previousToken be typed to an interface instead?
Robert

Looking through ParentRunner... the run() signature is named slightly differently:
public function run( notifier:IRunNotifier, parentToken:AsyncTestToken ):void
vs. IRunner:
function run( notifier:IRunNotifier, previousToken:AsyncTestToken ):void
Which is the more accurate description: "parent" or "previous"?
Robert

Concrete Class

Hi
Could someone tell me what the difference is between an abstract class, interface class and a concerte class. Cant figure it out.
Thank you
David

an interface has deferred methods that must be used by the same name by the implenting class(es)
an abstract class CAN have deferred (abstract) methods that must be implemented by the extending class
a concrete class is 'normal' class with no deferred methods

Type param in interface referring to concrete class

I want to declare a method in an interface that takes a type parameter that represents the concrete implementing subclass. I can live with something less than that, though I would like to avoid casting as much as possible...
Here's my attempt at a simplified (though contrived) example:
public interface Collectable {
    Collection<Collectable> collectMe();
public class StampCollection implements Collectable<Stamp> {
    // Super-specialized collection for holding stamps
public class Stamp implements Collectable {
    public Collection<Stamp> collectMe() {
        return new StampCollection(this);
}This doesn't work because StampCollection is deemed to be incompatible with Collection<Collectable>. I'm assuming this is because the type parameter for the interface that StampCollection implements is Stamp, which is more specific than Collectable. Is that right? What are my options for fixing this (with minimal casting).

Also, I see from some other reading that there seems to be some precendent for doing things this way (having a class implement an interface parameterized by the class itself). For example, on the Angelika Langer Java Generic FAQ linked to from the notice on this forum, she gives an example of
    class Pair<A extends Comparable<A> & Cloneable ,
               B extends Comparable<B> & Cloneable >
      implements Comparable<Pair<A,B>>, Cloneable { ... } Which is a fairly complicated example for our purposes, but does seem to use the same strategy.
Is this a common idiom?
Michael

Reflection: how to get the name of a subclass from parent class?

Suppose I have a parent class P and two subclasses S1, and S2. There's another method which has an argument of type P. Inside this method, I want to inspect the object (of type P) passed in and print its name, such as "S1" or "S2". How do you do that? I tried Class.getSimpleName(), but "P" is returned no matter which subclass objects you have. Thanks:)

That's the same as you said last time, and I'm telling you that's not what happens when I test it:
public class Parent {
public class SubclassOne extends Parent {
public class SubclassTwo extends Parent {
public class TestGetName {
public static void main(String[] argv) {
    showNames(new SubclassOne());
    showNames(new SubclassTwo());
    showNames(new Parent());
private static void showNames(Parent p) {
    System.out.println("Name: " + p.getClass().getName());
    System.out.println("Simple name: " + p.getClass().getSimpleName());
    System.out.println("Canonical name: " + p.getClass().getCanonicalName());
}prints:
Name: SubclassOne
Simple name: SubclassOne
Canonical name: SubclassOne
Name: SubclassTwo
Simple name: SubclassTwo
Canonical name: SubclassTwo
Name: Parent
Simple name: Parent
Canonical name: ParentYou must be doing something else that you're not saying. Either that or you're expressing yourself very poorly. Why not post a simple, self-contained, compilable example of what you claim is happening?

Subclassing a Java Class in JRuby

Hello, everyone. I'm studying JRuby by writing a minimal Swing app in Ruby. It seems to be an effective way of finding Ruby <-> Java integration issues. ;)
I am trying to create a subclass of AbstractAction for a button whose name will be "Quit" and will exit the program. The natural Java way to do this is to have the subclass call its superclass' (AbstractAction's) constructor, as in super("Quit").
This is what I tried:
class QuitAction < javax.swing.AbstractAction
    def initialize
        super.initialize "Quit"
    end
    def actionPerformed(e)
        System::exit 0
    end
endWhen the interpreter read the file, it complained:
frame-sample.rb:78:in `initialize': private method `initialize' called for org.jruby.javasupport.proxy.gen.AbstractAction$Proxy0@911a2d:Java::JavaObject (NoMethodError)How can I call AbstractAction's constructor?
Thanks,
Keith

All -
I found the answer. The syntax to use is:
super 'Quit'So the corrected class is:
class QuitAction < javax.swing.AbstractAction
    def initialize
        super "Quit"
    end
    def actionPerformed(e)
        System::exit 0
    end
end- Keith

Subclassing and inner classes

Hi there,
I have an existing class "A", supplied by a third party, that contains a private inner class "I".
I have subclassed "A" to a new class "B" to add new functionality, and I also want to replace the inner class "I" with new behaviour. The inner class "I" is invoked by methods within "A" that I have not overridden in "B". Replacing "I" appears to be impossible under these circumstances.
My initial experiment was to create a new inner class "I" within "B" with the same name and signature as the "I" defined within "A", so...
Class B extends A {
private class I {}
The code that invokes "I" is still within "A" and so my version of the "I" class is ignored, and the original "I" is invoked instead.
Let's be clear about this, I am not surprised by this behaviour, I'm just wondering if there is any way I can get my "I" invoked by the methods in "A". If that is indeed impossible I will have to write new methods in "B" that override the invoking methods in "A".
I did a search on the forums using the title I gave this topic, and got some fairly close hits, but nothing seemed to quite answer my question.
Hope that all made sense. Thanks for any help.
Regards,
Tim

When a class is compiled, all the references are resolved. So in the 'A' class file, a call to method someMethod() in inner class I is resolved as
some.package.A$I.someMethod()
Short of replacing the A$I class file, I don't know what you can do. (I think)

Problem with subclass and super class

here is the things i wanted to do
/*Write a method that takes the time as three integer arguments (hours, minutes and seconds),
and returns the number of seconds since the last time it was twelve o'clock.
Write a program that uses this method to calculate the amount of time in seconds between two times,
assuming both are within one twelve hour cycle of a clock.
here is a class to find the last time closes to 120'clock in sec.
import java.io.*;
public class Timer {
     int converter = 60;
     int secinTwelveHour = 43200;
     int converter2 = 12;
public int timerTime (int hour, int min, int sec){
          int totalSec = 0;
          //Finding the time
          if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour <= 11){
               //find last 12 o'Clock
               hour = converter2 + hour;
               //change to sec time
               totalSec = (hour * converter * converter) + (min * converter) + sec;
          }else{
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour >= 12){
               //find last 12 o'Clock in sec
               totalSec = ((hour * converter * converter) + (min * converter) + sec) - secinTwelveHour;
     }else{
          return -1;
}//End of return -1
          }//End of first else statment
     return totalSec;
     }//End of timerTimer
}//End of Program
and here is the super class which uses the class aboved
import java.io.*;
public class FindTime {
public int find2Time (int totalSec1, int totalSec2){
          int timeSec = 0;
          if(Timer.totalSec1 > Timer.totalSec2)
          timeSec = Timer.totalSec1 - Timer.totalSec2;
          else
          timeSec = Timer.totalSec2 - Timer.totalSec1;
     return timeSec;
     }//End of find2Time
public static void main( String [] arg){
     // Construct an instance of the Timer class
          Timer timerClass = new Timer();
          // Make a couple of calls of the method
          int totalSec1 = timerClass.timerTime(12, 3, 45);
          int totalSec2 = timerClass.timerTime(14, 23, 60);
          timeSec1 = find2Time (totalSec1, totalSec2)
          // Now print the values we got back
          System.out.println("Last closes Sec to 12 o'clock" + totalSec1);
          System.out.println("Last closes sec to 12 o'clock" + totalSec2);
          System.out.println("Last closes sec to 12 o'clock" + timeSec);
     }//End of main method
}//End of Program
Now i'm having program with the compliing can anyone help me out like tell me what i'm doing wrong and give me a bit of a code so that i can have a push start
thanks you

Does this do what you want? It is in two seperate classes.
import java.io.*;
public class FindTime {
public static void main( String [] arg){
int timeSec = 0;
// Construct an instance of the Timer class
     Timer timerClass = new Timer();
     // Make a couple of calls of the method
     int totalSec1 = timerClass.timerTime(12, 3, 45);
     int totalSec2 = timerClass.timerTime(14, 23, 60);
     timeSec = java.lang.Math.abs(totalSec1-totalSec2);
     // Now print the values we got back
     System.out.println("Last closes Sec to 12 o'clock " + totalSec1);
     System.out.println("Last closes sec to 12 o'clock " + totalSec2);
     System.out.println("Last closes sec to 12 o'clock " + timeSec);
     }//End of main method
}//End of Program
import java.io.*;
public class Timer {
int converter = 60;
int secinTwelveHour = 43200;
int converter2 = 12;
public int timerTime (int hour, int min, int sec){
     int totalSec = 0;
     //Finding the time
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour <= 11){
     //find last 12 o'Clock
     hour = converter2 + hour;
     //change to sec time
     totalSec = (hour * converter * converter) + (min * converter) + sec;
     } else {
     if (hour > 0 && hour <= 24 && min > 0 && min <=60 && sec > 0 && sec <= 60 && hour >= 12){
     //find last 12 o'Clock in sec
     totalSec = ((hour * converter * converter) + (min * converter) + sec) - secinTwelveHour;
     } else {
          return -1;
     }//End of return -1
}//End of first else statment
return totalSec;
}//End of timerTimer
}//End of Program

Subclass Vs Concrete class

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