How do I create an instance of a class only known at runtime?

Hi, thanks for reading - my problem is this:
I am trying to build a program which searches through a folder and locates all *.class files present and forms an array of these classes. These classes will all be children of a parent class, template.class, so a common class interface will be involved.
I have done my background research and have been pointed in the general direction of the .lang package, the reflection package, beans and so forth but frankly I have not a clue. I'm not even sure what this kind of operation is called...
Thanks for your time.

String classname = "Abc.class";
Class class = Class.forName(classname); // catch ClassNotFoundException
Object object = class.newInstance(); // catch InstantiationExceptiion
MyIntrface myInterface = (MyInterface)object; // catch ClassCastException

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How can you create an instance of a class using ClassLoader given only

the class name as a String. I have the code below in the try block.
Class myTest = this.getClass().getClassLoader().loadClass("Testclass");
Object obj = myTest.newInstance();
String className = obj.getClass().getName();I don't want to typecast the class like
Testclass obj = (TestClass) myTest.newInstance();I want to be able to create the classs at runtime and then get the methods in the class and execute those methods. Can it be done without having to code the typecasting in before compile time?

I read on the web of people creating objects from interfacesDoesn't sound like the thing to do... Theoretically you could create dummy classes on the fly that implement some interface, but that's not very useful.
Sounds like you are trying to load classes and execute them via interfaces. Like this:
     Class clazz = Class.forName("java.util.LinkedList");
     Collection c = (Collection) clazz.newInstance();
     c.add("hello");
     c.add("world");
     System.out.println(Arrays.toString(c.toArray()));LinkedList is the class, Collection is the interface which LinkedList implements. You can then call the methods declared in the interface. The interface gets "compiled in" into the program and the classes that implement it get loaded whenever and from wherever you load them.
You could also use reflection to call methods without an interface, but that is type-unsafe, inelegant, hackish, and just plain ugly.

How to create an instance of a class?

how do you create sn instance of a class, and how do you call a method from another class?

You may need to read thru the information provided on this page to understand how to create objects: http://java.sun.com/docs/books/tutorial/java/data/objectcreation.html
I'd also suggest you read the tutorial available at: http://java.sun.com/docs/books/tutorial/java/index.html
Regarding how you call a method belonging to another class you could do it in the foll. ways depending on whether the method is static or not - a static method may be called using the class name followed by a dot and the static method name while a non-static method would require you to create an instance of the class and then use that instance name followed by a dot and the method name. All said and done i'd still suggest you read thru the complete Java programming tutorial to get a good grounding on all these concepts and fundamentals of the language if you are looking to master the technology.
Thanks
John Morrison

Creating multiple instances of a class in LabVIEW object-oriented programming

How do you create multiple instances of a class in a loop? Or am I thinking about this all wrong?
For instance, I read in a file containing this information:
Person Name #1
Person Age #1
Hobby #1
Hobby #2
Hobby #3
Person Name #2
Person Age #2
Hobby #1
Hobby #2
Hobby #3
Person Name #3
Person Age #3
Hobby #1
Hobby #2
Hobby #3
If I define a Person class with name, age, and an array of strings (for the hobbies), how can I create several new Person instances in a loop while reading through the text file?
FYI, new to LabVIEW OOP but familiar with Java OOP. Thank you!

First of all, let's get your terminology correct. You are not creating multiple instances of a class. You are creating Objects of the class.
Use autoindexing to create an array of your class type.
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Unofficial Forum Rules and Guidelines

Creating an instance of a class with no default constructor

Hello gurus,
I wrote my own serialization and RMI protocol for both C++ and Java that follows closely what the default Java version does. I'm trying to recreate an object on the Java side that was sent over the wire. The first step is to create an instance of the class. How do I create an instance of a class that has no constructor (i.e. the only instances are static, created by the class itself and returned by static methods) or one that has no default constructor (like Integer)? The Java serialization seems to support it but the reflection API doesn't seem to have any support for this (i.e. Class::newInstance() and Constructor::newInstance()). It seems that through the standard API you can only create an object via one of its constructors. There must be a "hidden" method somewhere that allows the Java serialization to create an object without calling a constructor - where is it?
Dominique

There must be a "hidden" method
somewhere that allows the Java serialization to create
an object without calling a constructor - where is
it?You are correct, the way in which the Serialization creates Objects is "hidden" deep within the runtime.
If it were not hidden, you would be able to find it, and use it to violate the integrity of the VM.

How to create an instance of a class which is stored in a String?

I've class name stored in a String Object.
now i've to create an instance of that class name.
How?

This is very dangerous ground because you give up compile-time safety, but you can get a Class object using Class.forName(String). Then you can use methods of the class Class to operate on it (including creating an instance).

Re: How do you create and use "common" type classes?

Hi,
You have 2 potential solutions in your case :
1- Sub-class TextNullable class of Framework and add your methods in the
sub-class.
This is the way Domain class work. Only Nullable classes are sub-classable.
This is usefull for Data Dictionary.
The code will be located in any partition that uses or references the supplier
plan.
2- Put your add on code on a specific class and instanciate it in your user
classes (client or server).
You could also use interface for a better conception if needed. The code will
also be in any partition that uses or references the supplier plan where your
add on class is located.
If you don't want that code to be on each partition, you could use libraries :
configure as library the utility plan where is your add-on class.
You can find an example of the second case (using a QuickSort class,
GenericArray add-on) with the "QuickSort & List" sample on my personal site
http://perso.club-internet.fr/dnguyen/
Hope this helps,
Daniel Nguyen
Freelance Forte Consultant
http://perso.club-internet.fr/dnguyen/
Robinson, Richard a écrit:
I'm relatively new to forte and I'd like to know how can you handle utility
type classes that you want to use through out your application? Ideally
what I want is a static class with static methods.
Let's say that I have a StringUtil class that has a bunch of methods for
manipulating strings.
My problem is that we have code that runs on the client and code that runs
on the server. Both areas could use the StringUtil class, but from what I
understand, I have to create StringUtil in a plan and then create a server
object of type StringUtil. The server object will eventually get assigned
to a partition. That's not good since I really want the server object to
physically reside at the server end and at the client end. (Actually, I
don't want a server object, I just want to invoke a static method of a
static class).
Any clues on how to solve this problem would be appreciated.
Also, what is the url at Sage-it that has a summary of all emails that have
been posted to [email protected]? Perhaps this question has been
answered previously.
Thanks in advance
To unsubscribe, email '[email protected]' with
'unsubscribe forte-users' as the body of the message.
Searchable thread archive <URL:http://pinehurst.sageit.com/listarchive/>-
To unsubscribe, email '[email protected]' with
'unsubscribe forte-users' as the body of the message.
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Hi Richard,
Your question about "utility classes" brings up a number of issues, all of
which are important to long-term success with Forte.
There is no such thing as a static method (method that is associated with a
class but without an implicit object reference - this/self/me "pointer") in
TOOL, nor is there such thing as a global method (method not associated
with a class at all). This is in contrast to C++, which has both, and
Java, which has static methods, but not global classes. Frequently, Forte
developers will write code like this:
result, num : double;
// get initial value for num....
tmpDoubleData : DoubleData = new;
tmpDoubleData.DoubleValue = num;
result = tmpDoubleData.Sqrt().DoubleValue;
tmpDoubleData = NIL; // send a hint to the garbage collector
in places where a C++ programmer would write:
double result, num;
// get initial value for num....
result = Math::Sqrt(num);
or a Java programmer would write:
double result, num;
// get initial value for num....
result = Math.sqrt(num);
The result of this is that you end up allocating an extra object now and
then. In practice, this is not a big deal memory-wise. If you have a
server that is getting a lot of hits, or if you are doing some intense
processing, then you could pre-allocate and reuse the data object. Note
that optimization has its own issues, so you should start by allocating
only when you need the object.
If you are looking for a StringUtil class, then you will want to use an
instance of TextData or TextNullable. If you are looking to add methods,
you could subclass from TextNullable, and add methods. Note that you will
still have to instantiate an object and call methods on that object.
The next issue you raise is where the object resides. As long as you do
not have an anchored object, you will always have a copy of an object on a
partition. If you do not pass the object in a call to another partition,
the object never leaves. If you pass the object to another partition, then
the other partition will have its own copy of the object. This means that
the client and the server will have their own copies, which is the effect
you are looking for.
Some developers new to Forte will try to get around the lack of global
methods in TOOL by creating a user-visible service object and then calling
methods on it. If you have a general utility, like string handling, this
is a bad idea, since a service object can reside only on a single
partition.
Summary:
* You may find everything you want in TextData.
* Unless you anchor the object, the instance will reside where you
intuitively expect it.
* To patch over the lack of static methods in TOOL, simply allocate an
instance when required.
Feel free to email me if you have more questions on this.
At the bottom of each message that goes through the mailing list server,
the address for the list archive is printed:
http://pinehurst.sageit.com/listarchive/.
Good Luck,
CSB
-----Original Message-----
From: Robinson, Richard
Sent: Tuesday, March 02, 1999 5:44 PM
To: '[email protected]'
Subject: How do you create and use "common" type classes?
I'm relatively new to forte and I'd like to know how can you handle utility
type classes that you want to use through out your application? Ideally
what I want is a static class with static methods.
Let's say that I have a StringUtil class that has a bunch of methods for
manipulating strings.
My problem is that we have code that runs on the client and code that runs
on the server. Both areas could use the StringUtil class, but from what I
understand, I have to create StringUtil in a plan and then create a server
object of type StringUtil. The server object will eventually get assigned
to a partition. That's not good since I really want the server object to
physically reside at the server end and at the client end. (Actually, I
don't want a server object, I just want to invoke a static method of a
static class).
Any clues on how to solve this problem would be appreciated.
Also, what is the url at Sage-it that has a summary of all emails that have
been posted to [email protected]? Perhaps this question has been
answered previously.
Thanks in advance

Creating an instance of a class at runtime?

Does anyone know how to create an instance of a class at RunTime? For example, I want to load classes from a JAR file at RunTime, and then create instances of certain classes of each JAR. I ask this because I do not see how to create an instance of one of those classes the traditional way, SomeClass var = new SomeClass(). I am pretty sure that someone out there has done this and succeeded in doing it. All of the post on this stuff only talk about loading the class from a JAR file, but I have already loaded them using URLClassLoader�s findClass() method. I have also created an instance of the class that findClass() returns using newInstance(), but newInstance() returns an object of type Object. How can I convert this object to an object of type SomeClass? You cannot cast it because the compiler will complain due to the import statement issue. So if you cannot include the import statement because that classpath does not exist, then how the heck would you cast the newInstance() object into a SomeClass object?

You can cast the returned object to the type you need...that is what I do in my applet. The trick is that you must get the instance of the applets class loader (or you get a classCastException). Pay attention to this line below - that's the real key here.
"// Get a reference to the applets own classloader"
protected CBaseQuestionnaireFile m_BaseObject = null;// declared up front
m_BaseObject = loadAndRunClass("com.cpm.dataentry.questionnaire.CQuestionnaireFile");// fully qualified base object
Here is the load and run method:
   CBaseQuestionnaireFile loadAndRunClass(String classname)
      com.cpm.common.base.CBaseQuestionnaireFile questBase = null;
      Class cClass = null;
      try
         // first we open the jar file with the classes we need
         String[] aJarList = new String[10];
         // Questionnaire format file
         File fURL1 = new File(GetMainFrame().m_strQuestionnaireFileName);
         URL url1   = new URL(fURL1.toURL(),"");
         URL urlNew = new URL("jar:" + url1.toExternalForm() + "!/" );
         // Server base class directory
         String strServerDirectory = "http://" + GetMainFrame().m_strHost + "/APPLETS/";
         File fURL2 = new File(strServerDirectory);
         URL url2 = new URL(fURL2.toURL(),"");
         URL urlNew2 = new URL("jar:" + url2.toExternalForm() + "!/" );
         // Local base class directory
         String strLocalDirectory = CSystem.GetBasePath() + "/Research/bin/";
         File fURL3 = new File(strLocalDirectory);
         URL url3 = new URL(fURL3.toURL(),"");
         URL urlNew3 = new URL("jar:" + url3.toExternalForm() + "!/" );
         File fURLBase = new File(CSystem.GetBasePath() + "/Research/bin/base.jar");
         URL urlBase = new URL(fURLBase.toURL(),"");
         URL urlNewBase = new URL("jar:" + urlBase.toExternalForm() + "!/" );
         File fURLControl = new File(CSystem.GetBasePath() + "/Research/bin/controls.jar");
         URL urlControl = new URL(fURLControl.toURL(),"");
         URL urlNewControl = new URL("jar:" + urlControl.toExternalForm() + "!/" );
         File fURLUtil = new File(CSystem.GetBasePath() + "/Research/bin/utlities.jar");
         URL urlUtil= new URL(fURLUtil.toURL(),"");
         URL urlNewUtil = new URL("jar:" + urlUtil.toExternalForm() + "!/" );
         // Determine where to look
         URL[] urlList = null;
         if(GetMainFrame().m_isStandalone == false) {
            // From a browser
            CSystem.PrintDebugMessage("Running as an applet");
            if(GetMainFrame().m_bOnLineMode == true) {
               // On line
               CSystem.PrintDebugMessage("*** On Line Mode ***");
               urlList = new URL[2];
               urlList[0] = urlNew;
               urlList[1] = urlNew2;
            else {
               // Off line
               CSystem.PrintDebugMessage("*** Off Line Mode ***");
               urlList = new URL[4];
               urlList[0] = urlNew;
               urlList[1] = urlNewBase;
               urlList[2] = urlNewControl;
               urlList[3] = urlNewUtil;
          else {
             CSystem.PrintDebugMessage("*** Stand Alone Mode ***");
             urlList = new URL[1];
             urlList[0] = urlNew;
CSystem.PrintDebugMessage("Question file/path: " + GetMainFrame().m_strQuestionnaireFileName);
        // Set the wait cursor
        GetMainFrame().SetWaitCursor();
        // Get a reference to the applets own classloader
        Class myClass = getClass();
        ClassLoader appletClassLoader = myClass.getClassLoader();
        // Call our multi-jar class loader
        JarClassLoader jarLoader = new JarClassLoader(urlList,appletClassLoader);
CSystem.PrintDebugMessage("CPM Test - passed Jar Loader" + jarLoader.toString());
         // Load the classname from the jarfile
         try
              cClass = jarLoader.loadClass(classname);
         catch(ClassNotFoundException cnfe)
            Object[] optionsConfirm = { "Ok" };
            JOptionPane.showOptionDialog(GetMainFrame(),"Questionnaire file is either damaged or has been tampered with.", "Questionnaire File Error",
            JOptionPane.DEFAULT_OPTION, JOptionPane.WARNING_MESSAGE, null, optionsConfirm, optionsConfirm[0]);
            GetMainFrame().m_strQuestionnaireFileName = "";
            // Clear the wait cursor
            GetMainFrame().ClearWaitCursor();
            return null;
         Class cSuperclass = cClass.getSuperclass();
CSystem.PrintDebugMessage("Test Superclass type is: " + cSuperclass.toString());
         Object o = cClass.newInstance();
CSystem.PrintDebugMessage("Test plain class type is: " + o.toString());
         // Never remove this line of code
         // without it, a crafty user could use our load routine
         // to load/and run some nasty code
         // This test makes SURE that *ONLY* questionnaires get opened
         // and their methods get called
         if(o instanceof com.cpm.common.base.CBaseQuestionnaireFile)
            // Create the object
CSystem.PrintDebugMessage("Test Is instance of CBaseQuestionnaireFile");
CSystem.PrintDebugMessage("CPM Test - Casting to Base Class");
            questBase = (com.cpm.common.base.CBaseQuestionnaireFile)o;
CSystem.PrintDebugMessage("CPM Test - Getting languages from questionnaire");
            GetMainFrame().GetLanguagesFromQuestionnaire(questBase);
CSystem.PrintDebugMessage("CPM Test - Setting locale from Questionnaire selection");
            questBase.SetQuestionnaireLocale(GetMainFrame().m_locale);
CSystem.PrintDebugMessage("CPM Test - Initializing Questionnaire");
            questBase.Initialize();
            questBase.InitializeCards();
          else
            Object[] optionsConfirm = { "Ok" };
            JOptionPane.showOptionDialog(GetMainFrame(),"Questionnaire file is either damaged or has been tampered with.", "Questionnaire File Error",
            JOptionPane.DEFAULT_OPTION, JOptionPane.WARNING_MESSAGE, null, optionsConfirm, optionsConfirm[0]);
            GetMainFrame().m_strQuestionnaireFileName = "";
            // Clear the wait cursor
            GetMainFrame().ClearWaitCursor();
            return null;
      catch (Exception e)
          exDialog.showForThrowable(e.toString(),e);
     // Clear the wait cursor
     GetMainFrame().ClearWaitCursor();
     // Set MouseListener Pointer
//     questBase.SetMouseListenerPointer(GetMainFrame().m_mouseListener);
     return questBase;
   }

Create an instance of a class from class name

Hi,
I have the class name as a String variable. I need to create an instance from this class. How to do it in Java?
Thanks and regards, Marina

Hello
see
[http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)]
and
[http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#newInstance()]
for Classes with paramterless constructor
or
[http://java.sun.com/j2se/1.4.2/docs/api/java/lang/reflect/Constructor.html#newInstance(java.lang.Object[])]
for Constructor with parameters
regards franz

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Create an instance of my class variable

Hello all,
I'm a newbie to iPhone/iPad programming and am having some trouble, I believe the issue is that I'm not creating an instance of my class variable. I've got a class that has a setter and a getter, I know about properties but am using setters and getters for the time being. I've created this class in a View-based application, when I build the program in XCode 3.2.6 everything builds fine. When I step through the code with the debugger there are no errors but the setters are not storing any values and thus the getters are always returning 0 (it's returning an int value). I've produced some of the code involved and I was hoping someone could point out to me where my issue is, and if I'm correct about not instantiating my variable, where I should do that. Thanks so much in advance.
<p>
Selection.h
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Selection.m
#import "Selection.h"
@implementation Selection
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return _choice;
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#import "RockPaperScissorsViewController.h"
@implementation RockPaperScissorsViewController
@synthesize rockButton, paperButton, scissorsButton, label;
//@synthesize humanChoice, computerChoice;
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</p>
So in the above code it's the [humanChoice setChoice:1] that is not working. When I step through that portion with the debugger I get no errors but when I call humanChoice.Choice later on in the code I get a zero.
-NifflerX

It worked, thank you so much. I put
humanChoice = [[Selection alloc]init];
In viewDidLoad and it worked like a charm. Thank you again.
-NifflerX

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??I would be really glad if I could get some code example.....
//HA

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// If the method returns anything back, you can keep a reference to it
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How can I create an instance of "java.util.prefs.WindowsPreferences"?

Hi All,
I was trying to find out a way to access the Windows Registry to add soome keys and and later add a string.
I found WindowsPreferences might be useful in the above mentioned context.
I was trying to create an instance of it, but when I import the class, it says it is invisible.
Can anyone let me know how to instantiate this class.
Note:- Under the same package "java.util.prefs" we have another class "Preferences", Iam able to import this one and use it in my code.
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How to create the instance of a class and to use this object remotely

I have to change a standalone program to do it working on a local net.
the program is prepared for this adjustment, because the only problem for this change came from the use of the database; and, in the application, all the accesses to the database come from only a class that supplies a connection to the database.
In this way I think that I could have (in a local net) a "server application" that has the database embedded inside it.
Furthermore, some client applications (running in different computers of the net) could get access to the database through the connection that comes from an instance of the class that, in the "server application", is made to provide the connection to the database.
I think this could be a good idea...
But I don't have practice with distributed applications and I would ask some suggestion about the way to realize my modification.
(in particular how to get and use, in the "client applications", the instance of the class that give the connection to the database from the "server application").
I would have some help..
thank in advance
regards
tonyMrsangelo.

tonyMrsangelo wrote:
I have to change a standalone program to do it working on a local net.
the program is prepared for this adjustment, because the only problem for this change came from the use of the database; and, in the application, all the accesses to the database come from only a class that supplies a connection to the database.
In this way I think that I could have (in a local net) a "server application" that has the database embedded inside it.
Furthermore, some client applications (running in different computers of the net) could get access to the database through the connection that comes from an instance of the class that, in the "server application", is made to provide the connection to the database.
I think this could be a good idea... Which is why JEE and implementations of that exist.
But I don't have practice with distributed applications and I would ask some suggestion about the way to realize my modification.
(in particular how to get and use, in the "client applications", the instance of the class that give the connection to the database from the "server application").
You can't pass a connection from a server to a client. Nothing will do that.
As suggested you can create a simple RMI server/client set up. Or use a more feature rich (and much more complex) JEE container.
RMI is simple enough for its own tutorial
[http://java.sun.com/docs/books/tutorial/rmi/index.html]
JEE (previously called J2EE) is much more complex and requires books. You can get a brief overlook from the following
[http://java.sun.com/javaee/]

How do I create an instance of a class only known at runtime?

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