Creating Instances of class within its definition

Hi,
I am a newbie to Java and am doing a course on Java. In one my assignments it asks us to develop the following method for a Class called CraftStock:
"Define a new helper method void increaseStockCapacity() that creates a
new expanded array as follows. It should create a temporary instance of
CraftStock by using its single-argument constructor ? the value of the argument n is determined by the current size of the original craft stock array plus the
extraCapacity constant. All items in the original array must be copied from the
original array to the temporary instance of CraftStock. The array reference in the
original array instance of CraftStock must then be reassigned to the new
temporary array instance."
My question is - is this possible - can one create an instance of a class from within its definition? I can't find anyhting on this from the books.
Any help would be appreciated.
Thanks
Dev

Isn't there a caveat to all this?
If one creates an instance of a class within its own
definition, doesn't one face the potential danger of
attempting to access some part of the object which
has not yet been constructed?nope. why would it? we're creating a new instance of the class, from scratch. we're not accessing any part of the object, we're creating a new one. the fact that this is being done within the definition of the class is irrelevant. by the time anything comes to call on a static method of the class, the class is initialized already. remember, static methods don't have any dependency on a particular instance
sorry, that doesn't read very well, does it! what I mean is that the class itself is not the same thing as an instance of that class
Message was edited by:
georgemc

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Create instance of class

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How to get instance of Class with its type parameters

Hi,
Have any of you folks been dealing with generics long enough to show me how this should be written? Or point me to the answer (I have searched as well as I could).
I boiled down my situation to the included sample code, which is long only because of the inserted comments. And while boiling I accidentally came across a surprise solution (a bug?), but would obviously prefer a smoother solution.
My Questions (referred to in the code comments):
#1. Is there a way to get my parameterized type (classarg) without resorting to using the bogus (proto) object?
#2. Can anyone understand why the "C" and "D" attempts are different? (All I did was use an intermediate variable????) Is this a bug?
Thanks so much for any input.
/Mel
class GenericWeird
 /* a generic class -- just an example */
 static class CompoundObject<T1,T2>
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 public static void main(String[] args)
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 TypedThing<CompoundObject<String,String>> stringstringTypedThing = null;
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 stringTypedThing = new TypedThing<String>(String.class);
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Thanks so much, Bruce. (Oh my goodness, how would I have ever come up with that on my own?)
So in summary
This doesn't compile:
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 Class coclass = (Class)CompoundObject.class;
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Coldfusion create instance of class from data element when cosuming a webservice

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Thanks for the explanation and example. At first, I didn't understand what you were getting at, but after reading "Using Top-Level Containers" and "How to Use Root Panes" java tutorials it made much more sense. Unfortunately, the books I've read up to this point, did not cover those topics at all. The books simply stated that the first step in creating a Swing gui was to extend the JFrame, or JApplet, or etc.
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Trying to access methods from a .class file by creating instance of class

Hey all,
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If you could help at all, I would greatly appreciate it.
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No, I don't know the package name....Is there a way
to import the Input.class by itself without importing
the entire packge?
I tried extending the driver program too...It didn't
work either...
Message was edited by:
BoxMan56How do you know you have a class called Input.class ?
You got a jar file which contains it ? or just a simple .class file ?
You have to set the classpath in either case.
But for the former, you should also need to explicitly telling which package containing the class file you looking for (i.e. Input.class)
e.g. java.util.Vector which is a class called Vector inside java.util package.
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Thanks for the explanation and example. At first, I didn't understand what you were getting at, but after reading "Using Top-Level Containers" and "How to Use Root Panes" java tutorials it made much more sense. Unfortunately, the books I've read up to this point, did not cover those topics at all. The books simply stated that the first step in creating a Swing gui was to extend the JFrame, or JApplet, or etc.
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TestUserInterface.java:5: cannot find symbol
symbol: class UserInterface
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Both the classes are in the same named packaged. Below is my code:
package projects.web;
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import javax.swing.*;
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public class TestUserInterface{
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Why cannot create instance to an abstract class?

Dear Developers....
abstract class can create be its own constructor but why they didn't create object

We cann't create an object for abstract class
because they have abstract methods which depends on
its subclass for its implementation. if it is
possible to create the object for abstract class
means then you can call a method which has no
implementation(abstract methods) which is a illegal
one thats what the ruleWhat about classes with no abstract members?
public abstract class A {}No danger of accessing anything illegal here.
The rule is far more simple. By declaring a class as abstract, the creator defines that no instances of this class may exist. Period. If you could create instances, the class wouldn't be abstract anymore, per definition. It's like asking why a green light doesn't shine red. If it would, it wouldn't be green anymore.

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hi,
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ok ive changed it now to "eccs.model.behaviours" and it seems to work:) many thanks!!!
by the following you mean that instead of using the method "doesFileExist(fileName)" i just catch the exception and throw it to something like the System.out.println() ?
Why don't you simply try to call Class.forName() and catch the exception if it doesn't exist? Because as soon as you package up your class files in a jar file (which you should) your approach won't work at all.i dont think il be creating a JAR file as i want the user to be able to create his/her own classes and add them to the directory to be used in the application. is this the correct way to do this??
again many thanks for ye're help:)

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Is there a way to create an instance of a class given it's
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don't want to have to keep adding conditions as the system grows.
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Thanks for the heads up Ed!
One way to be ableto get around your issue (not that this
exists in AS 3.0) is to be able to "register" a class so the
compiler/linker can link it into the swf and you get compile time
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This is the way Delphi allows for this. The C# complier is
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One way to get around this in AS 3.0 (I think) would be to
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No instances can be created from abstract classes

Dear SAPGurus,
I have developed a oData Service in my backend system & able to register the service in the gateway hub system. When i try to execute the metadata of the service from gateway system i am able to see the metadata with list of entity types availble in the data model.
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Do you have . in the CLASSPATH when compiling your servlet?
ron <[email protected]> wrote:
I have created and compiled the following classes within
C:\bea\wlserver6.1\config\mydomain\applications\DefaultWebApp\WEB-INF\classes
1)LoginServlet
2)LoginManager
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but I get the following errors:
Cannot resolve symbol: LoginManager lm = new LoginManager();
^
Cannot resolve symbol: LoginManager lm = new LoginManager();
^
4)I have the .java and .class files located within the same directory, so I don't
see what's wrong here??? HELP!
LoginManager looks like:
public class LoginManager {
public boolean authenticateUser(String username, String password){
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//simple code for string matching
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LoginServlet looks like:
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class LoginServlet extends HttpServlet {
 // Constructors
 // Variables
 public static final String CONTENT_TYPE = "text/html";
 public boolean LOGIN_STATUS;
 // Methods
 public void service (HttpServletRequest req, HttpServletResponse res)
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 String password = "";
 LoginManager lm = new LoginManager();
 username = req.getParameter("username");
 password = req.getParameter("password");
 LOGIN_STATUS = lm.authenticateUser(username, password);
 res.setContentType(CONTENT_TYPE);
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 out.println("<html><head><title>Hello World</title></head>" +
 "<body>Hello! Your login status is " + LOGIN_STATUS +
 "</body>" +
 "</html>");
 public void init (ServletConfig config) throws ServletException {
 super.init(config);
Dimitri

Create a Exception Class

hi all ,,
can any one tell me how to create a exception class in ABAP OOPS concept ??
if possible can give link also for document

HI Jayakumar,
Please go thru this document
Exceptions are represented by objects that are instances of exception classes. Defining an exception is, therefore, the same as creating an exception class.
All exception classes must inherit from the common superclass CX_ROOT and one of its subordinate classes:
CX_STATIC_CHECK
CX_DYNAMIC_CHECK
CX_NO_CHECK
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Used to specify the exception of a class more precisely by using several exception texts. Is evaluated in the GET_TEXT method.
PREVIOUS
If an exception is mapped to another exception, a reference to the original exception can be defined in this attribute via the EXPORTING addition of the RAISE EXCEPTION statement and by means of the constructor IMPORTING PARAMETER with the same name. This can result in a chain of exception objects. In the event of a runtime error, the exception texts of all the exceptions in the chain are output. Mapping an exception to another exception is only beneficial if the context in which the original exception occurred is important for characterizing the error situation.
KERNEL_ERRID
The name of the associated runtime error is stored in this attribute if the exception was raised by the runtime environment, for example, COMPUTE_INT_ZERODIVIDE with a division by zero. If the exception is not handled, precisely this runtime error occurs.
Parameters cannot be transferred to the constructor for this attribute. If the exception is raised with RAISE EXCEPTION, the attribute is set to initial.
Global Exception Classes
Global exception classes are defined and managed in the Class Builder. If the correct naming convention (prefix CX_) and the class type Exception Class is chosen when a new class is created, the Class Builder automatically becomes the Exception Builder.
The Exception Builder offers precisely the functionality required to define exception classes and generates independently-defined components that must not be changed. When classes are created, the category of the exception must be specified, in other words, whether it is to inherit from CX_STATIC_CHECK, CX_DYNAMIC_CHECK, or CX_NOCHECK.
Tab Pages of the Exception Builder
The Exception Builder has the tab pages Properties, Attributes, Methods, and Texts.
The properties do not normally need to be changed.
Except for the four inherited attributes mentioned above, further public attributes can be generated by the Exception Builder. The contents of these attributes specify the exception more clearly and manage the exception texts.
All of the methods are inherited from CX_ROOT. New methods cannot be added. The instance constructor is generated automatically. It ensures that, when an exception is raised, the attributes have the right values. It also transfers the text of the superclass for an exception class whose exception text is not specified explicitly.
The instance constructor is generated on the basis of the attributes, which are set up on the basis of the exception texts. Changing the attributes in superclasses can, therefore, invalidate the constructors of subordinate classes. The constructors of subordinate classes can be regenerated under Utilities ® CleanUp ® Constructor.
Texts are a special feature of exception classes and the Exception Builder. For further information, refer to Exception Texts.
Local Exception Classes
Local exception classes can be defined for specific exceptions that only occur within one single ABAP program. The condition for a local exception class is that it inherits from one of the three classes CX_STATIC_CHECK, CX_DYNAMIC_CHECK, or CX_NO_CHECK, or from their subordinate classes. An individual constructor and individual attributes can be created. Individual methods should not be created, however, and the methods of superclasses should not be redefined.
Examples of Local Exception Classes
report DEMO_LOCAL_EXCEPTION_1.
class CX_LOCAL_EXCEPTION definition
                        inheriting from CX_STATIC_CHECK.
endclass.
start-of-selection.
    try.
      raise exception type CX_LOCAL_EXCEPTION.
    catch CX_LOCAL_EXCEPTION.
      message 'Local Exception!' type 'I'.
endtry.
This example shows a minimal local exception class, which is simply the local representation of one of the three direct subordinate classes of CX_ROOT. It can be used in the program.
report DEMO_LOCAL_EXCEPTION_2.
class CX_LOCAL_EXCEPTION definition
                        inheriting from CX_STATIC_CHECK.
public section.
    data LOCAL_TEXT type STRING.
    methods CONSTRUCTOR importing TEXT type STRING.
endclass.
class CX_LOCAL_EXCEPTION implementation.
method CONSTRUCTOR.
    SUPER->CONSTRUCTOR( ).
    LOCAL_TEXT = TEXT.
endmethod.
endclass.
data OREF type ref to CX_LOCAL_EXCEPTION.
start-of-selection.
    try.
      raise exception type CX_LOCAL_EXCEPTION
                      exporting TEXT = `Local Exception`.
    catch CX_LOCAL_EXCEPTION into OREF.
      message OREF->LOCAL_TEXT type 'I'.
endtry.
In this example, the exception class from the previous example is extended to include an individual attribute and constructor. The IMPORTING parameter of the constructor must be supplied when the exception is raised (it is required here). The attribute can be evaluated in the handler of the exception.
report DEMO_LOCAL_EXCEPTION_3.
class CX_LOCAL_EXCEPTION definition
      inheriting from CX_SY_ARITHMETIC_ERROR.
public section.
    methods CONSTRUCTOR importing SITUATION type STRING.
endclass.
class CX_LOCAL_EXCEPTION implementation.
method CONSTRUCTOR.
    SUPER->CONSTRUCTOR( OPERATION = SITUATION ).
endmethod.
endclass.
data OREF type ref to CX_LOCAL_EXCEPTION.
data TEXT type STRING.
start-of-selection.
    try.
      raise exception type CX_LOCAL_EXCEPTION
            exporting SITUATION = `START-OF-SELECTION`.
    catch CX_LOCAL_EXCEPTION into OREF.
      TEXT = OREF->GET_TEXT( ).
      message TEXT type 'I'.
endtry.
In this example, an exception class is derived from one of the predefined exception classes for error situations in the runtime environment. An individual constructor is defined with an individual IMPORTING parameter that supplies the superclass constructor with this parameter. When the exception is handled, the exception text, as defined in the superclass, is read with GET_TEXT.

I want the Definitaion fo Class and its Components in ABAP . With Example

Gowri

Classes
Classes are templates for objects. Conversely, you can say that the type of
an object is the same as its class. A class is an abstract description of an object. You could say that it is a set of instructions for building an object. The attributes
of objects are defined by the components of the class, which describe the
state and behavior of objects.

Local and Global Classes
Classes in ABAP Objects can be declared either globally or locally. You define global classes and interfaces in the Class Builder (Transaction SE24) in the
ABAP Workbench. They are stored centrally in class pools in the class library
in the R/3 Repository. All of the ABAP programs in an R/3 System can access the global classes. Local classes are defined within an ABAP program. Local classes and interfaces can only be used in the program in which they are defined. When you use a class in an ABAP program, the system first searches for a local class with the specified name. If it does not find one, it then looks for a global class. Apart from the visibility question, there is no difference between using a global class and using a local class.
There is, however, a significant difference in the way that local and global classes are designed. If you are defining a local class that is only used in a single program, it is usually sufficient to define the outwardly visible components so that it fits into that program. Global classes, on the other hand, must be able to be used anywhere. This means that certain restrictions apply when you define the interface of a global class, since the system must be able to guarantee that any program using an object of a global class can recognize the data type of each interface parameter.
The following sections describe how to define local classes and interfaces in an ABAP program. For information about how to define local classes and interfaces, refer to the Class Builder section of the ABAP Workbench Tools documentation.
Defining Local Classes
Local classes consist of ABAP source code, enclosed in the ABAP statements CLASS ... ENDCLASS. A complete class definition consists of a declaration part and, if required, an implementation part. The declaration part of a class <class> is a statement block:
CLASS <class> DEFINITION.
ENDCLASS.
It contains the declaration for all components (attributes, methods, events) of the class. When you define local classes, the declaration part belongs to the global program data. You should therefore place it at the beginning of the program.
If you declare methods in the declaration part of a class, you must also write an implementation part for it. This consists of a further statement block:
CLASS <class> IMPLEMENTATION.
ENDCLASS.
The implementation part of a class contains the implementation of all methods of the class. The implementation part of a local class is a processing block. Subsequent coding that is not itself part of a processing block is therefore not accessible.
Structure of a Class
The following statements define the structure of a class:
A class contains components
Each component is assigned to a visibility section
Classes implement methods
The following sections describe the structure of classes in more detail.
Class Components
The components of a class make up its contents. All components are declared in the declaration part of the class. The components define the attributes of the objects in a class. When you define the class, each component is assigned to one of the three visibility sections, which define the external interface of the class. All of the components of a class are visible within the class. All components are in the same namespace. This means that all components of the class must have names that are unique within the class.
There are two kinds of components in a class - those that exist separately for each object in the class, and those that exist only once for the whole class, regardless of the number of instances. Instance-specific components are known as instance components. Components that are not instance-specific are called static components.
In ABAP Objects, classes can define the following components. Since all components that you can declare in classes can also be declared in interfaces, the following descriptions apply equally to interfaces.
Attributes
Attributes are internal data fields within a class that can have any ABAP data type. The state of an object is determined by the contents of its attributes. One kind of attribute is the reference variable. Reference variables allow you to create and address objects. Reference variables can be defined in classes, allowing you to access objects from within a class.
Instance Attributes
The contents of instance attributes define the instance-specific state of an object. You declare them using the DATA statement.
Static Attributes
The contents of static attributes define the state of the class that is valid for all instances of the class. Static attributes exist once for each class. You declare them using the CLASS-DATA statement. They are accessible for the entire runtime of the class.
All of the objects in a class can access its static attributes. If you change a static attribute in an object, the change is visible in all other objects in the class.
Methods
Methods are internal procedures in a class that define the behavior of an object. They can access all of the attributes of a class. This allows them to change the data content of an object. They also have a parameter interface, with which users can supply them with values when calling them, and receive values back from them The private attributes of a class can only be changed by methods in the same class.
The definition and parameter interface of a method is similar to that of function modules. You define a method <met> in the definition part of a class and implement it in the implementation part using the following processing block:
METHOD <meth>.
ENDMETHOD.
You can declare local data types and objects in methods in the same way as in other ABAP procedures (subroutines and function modules). You call methods using the CALL METHOD statement.
Instance Methods
You declare instance methods using the METHODS statement. They can access all of the attributes of a class, and can trigger all of the events of the class.
Static Methods
You declare static methods using the CLASS-METHODS statement. They can only access static attributes and trigger static events.
Special Methods
As well as normal methods, which you call using CALL METHOD, there are two special methods called CONSTRUCTOR and CLASS_CONSTRUCTOR, which are automatically called when you create an object (CONSTRUCTOR) or when you first access the components of a class (CLASS_CONSTRUCTOR).
Events
Objects or classes can use events to trigger event handler methods in other objects or classes. In a normal method call, one method can be called by any number of users. When an event is triggered, any number of event handler methods can be called. The link between the trigger and the handler is not established until runtime. In a normal method call, the calling program determines the methods that it wants to call. These methods must exist. With events, the handler determines the events to which it wants to react. There does not have to be a handler method registered for every event.
The events of a class can be triggered in the methods of the same class using the RAISE EVENT statement. You can declare a method of the same or a different class as an event handler method for the event <evt> of class <class> using the addition FOR EVENT <evt> OF <class>.
Events have a similar parameter interface to methods, but only have output parameters. These parameters are passed by the trigger (RAISE EVENT statement) to the event handler method, which receives them as input parameters.
The link between trigger and handler is established dynamically in a program using the SET HANDLER statement. The trigger and handlers can be objects or classes, depending on whether you have instance or static events and event handler methods. When an event is triggered, the corresponding event handler methods are executed in all registered handling classes.
Instance Events
You declare instance events using the EVENTS statement. An instance event can only be triggered in an instance method.
Static Events
You declare static events using the CLASS-EVENTS statement. All methods (instance and static methods) can trigger static events. Static events are the only type of event that can be triggered in a static method.
See also Triggering and Handling Events.
Types
You can define your own ABAP data types within a class using the TYPES statement. Types are not instance-specific, and exist once only for all of the objects in a class.
Constants
Constants are special static attributes. You set their values when you declare them, and they can then no longer be changed. You declare them using the CONSTANTS statement. Constants are not instance-specific, and exist once only for all of the objects in a class.
Visibility Sections
You can divide the declaration part of a class into up to three visibility areas:
CLASS <class> DEFINITION.
PUBLIC SECTION.
PROTECTED SECTION.
PRIVATE SECTION.
ENDCLASS.
These areas define the external visibility of the class components, that is, the interface between the class and its users. Each component of a class must be assigned to one of the visibility sections.
Public Section
All of the components declared in the public section are accessible to all users of the class, and to the methods of the class and any classes that inherit from it. The public components of the class form the interface between the class and its users.

Protected Section
All of the components declared in the protected section are accessible to all methods of the class and of classes that inherit from it. Protected components form a special interface between a class and its subclasses. Since inheritance is not active in Release 4.5B, the protected section currently has the same effect as the private section.
Private Section
Components that you declare in the private section are only visible in the methods of the same class. The private components are not part of the external interface of the class.
Encapsulation
The three visibility areas are the basis for one of the important features of object orientation - encapsulation. When you define a class, you should take great care in designing the public components, and try to declare as few public components as possible. The public components of global classes may not be changed once you have released the class.
For example, public attributes are visible externally, and form a part of the interface between an object and its users. If you want to encapsulate the state of an object fully, you cannot declare any public attributes. As well as defining the visibility of an attribute, you can also protect it from changes using the READ-ONLY addition.
"Example :
CLASS C_COUNTER DEFINITION.
PUBLIC SECTION.
 METHODS: SET_COUNTER IMPORTING VALUE(SET_VALUE) TYPE I,
 INCREMENT_COUNTER,
 GET_COUNTER EXPORTING VALUE(GET_VALUE) TYPE I.
PRIVATE SECTION.
 DATA COUNT TYPE I.
ENDCLASS.
CLASS C_COUNTER IMPLEMENTATION.
METHOD SET_COUNTER.
 COUNT = SET_VALUE.
ENDMETHOD.
METHOD INCREMENT_COUNTER.
 ADD 1 TO COUNT.
ENDMETHOD.
METHOD GET_COUNTER.
 GET_VALUE = COUNT.
ENDMETHOD.
ENDCLASS.
The class C_COUNTER contains three public methods - SET_COUNTER, INCREMENT_COUNTER, and GET_COUNTER. Each of these works with the private integer field COUNT. Two of the methods have input and output parameters. These form the data interface of the class. The field COUNT is not outwardly visible.
Reward points if it is usefull...
Girish

AS3: How to create one base class for classes loaded in multiple swfs?

Our application have 3 different modules and all use
fscommand and ExternalInterface alot. Now the problem is we are
combining all those modules, but they all will reside in different
swfs. Is there any way that they extend to same base class and its
static members have only one instance accessible by all
three?

Hi,
You can achieve this via Workshop 9.2 by using the exported Ant script for your project as follows:
-Ensure that all Jars needed to resolve com.bea.p13n.property.EntityPropertyManager (this particular class is found in weblogic92/common/p13n/lib/p13n_ejb.jar) and all referenced classes are added to the Java build path for your WebLogic EJB project (Properties->Java Build Path->Libraries)
-Export the "Workshop Ant Script" for your WebLogic EJB project (File->Export->Workshop Ant Script); this will generate a "build.xml" Ant script under your project root directory.
-Edit this default build script as follows: in the call to the "ejbgen" task within the "build" target, add the attribute: propertyFile="ejbgen.properties"
-Create a file "ejbgen.properties" under the root of your WebLogic EJB project and add the following entry:
YourSessionBeanClassName.remote.baseClass=com.bea.p13n.property.EntityPropertyManager
-Add a "<path refid="java.classpath"/>" within the "<classpath>" for the "java" task element that calls "weblogic.ejbc" (this will ensure that the p13n classes are resolved by EJBC)
Follow the instructions for executing the Ant build script found here: http://edocs.bea.com/workshop/docs92/ws_platform/ideuserguide/conUseCustomAntBuild.html
For more information on the EJBGen property file support, see: http://e-docs.bea.com/wls/docs92/ejb/EJBGen_reference.html

Creating Instances of class within its definition

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