Creating an array of objects of a class known ONLY at RUNTIME

I have a method in a class which expects an array of objects of particular type (say A) as its only argument. I 'invoke' this method using reflection. I have the instances of the class A stored in a Vector. When I do 'toArray' on this vector, what I get back is an array of Objects and not an array of objects of A. I assign this array to the zeroth element of another Object array and pass that array as a second argument to the 'invoke' method. Of course, it throws an Exception saying - argument types mismatch. Because I am sending an array of Objects to a method expecting an array of objects of A. My problem is I don't know A until runtime. So I have to handle whatever I am getting back after doing 'toArray' on a vector as a Generic Object. I know there's another version of 'toArray' in the Vector class. But it's API documentation didn't help a lot.
Can anybody please help me solve this problem? Any sort of hints/code snippet would be of great help.
Does the above description makes any sense? If not, please let me know. I would try to illustrate it with a code sample.
Thanks a lot in advance.

Sorry, for the small typo. It is -
clazz[] arr = (clazz [])java.lang.reflect.Array.newInstance(clazz, n);
instead
Thanks for the reply. I could do this. But I am
getting a handle to the method to be 'invoke'd by
looking at the types of the parameters its
expecting(getParameterTypes). So I can't change it to
accept an array of Generic Objects.
I looked at the java.lang.reflect.Array class to do
this. This is what I am trying to do-
String variableName is an input coming into my
program.
n is the size of the array I'm trying to create.
Class clazz = Class.forName(variableName);
clazz[] arr = (clazz
[])java.lang.reflect.newInstance(clazz, n);
According to Reflection API, it should work, right?
But compiler yells at me saying 'class clazz not
found'.
Any suggestions/hints/help?
Thanks, again.

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How to cast an object to a class known only at runtime

I have a HashMap with key as a class name and value as the instance of that class. When I do a 'get' on it with the class name as the key, what I get back is a generic java.lang.Object. I want to cast this generic object to the class to which it belongs. But I don't know the class at compile time. It would be available only at runtime. And I need to invoke methods on the instance of this specifc class.
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Thanks a lot in advance.

Thanks all for the prompt replies. I am using
reflection and so a generic object is fine, I guess.
But a general question (curiosity) specific to your
comment - extraordinarily generic...
I understand there's definitely some overhead of
reflection. So is it advisable to go for interface
instead of reflection?
Thanks.Arguments for interfaces rather than reflection...
Major benefit at run-time:
Invoking a method using reflection takes more than 20 times as long without using a JIT compiler (using the -Xint option of Java 1.3.0 on WinNT)... Unable to tell with the JIT compiler, since the method used for testing was simple enough to inline - which resulted in an unrealistic 100x speed difference.
The above tests do not include the overhead of exception handling, nor for locating the method to be executed - ie, in both the simple method invocation and reflective method invocations, the exact method was known at compile time and so there is no "locative" logic in these timings.
Major benefit at compile-time:
Compile-time type safety! If you are looking for a method doSomething, and you typo it to doSoemthing, if you are using direct method invocation this will be identified at compile time. If you are using reflection, it will compile successfully and throw a MethodNotFoundException at run time.
Similarly, direct method invocation offers compile-time checking of arguments, result types and expected exceptions, which all need to be dealt with at runtime if using reflection.
Personal and professional recommendation:
If there is any common theme to the objects you're storing in your hashtable, wrap that into an interface that defines a clear way to access expected functionality. This leaves implementations of the interface (The objects you will store in the hashtable) to map the interface methods to the required functionality implementation, in whatever manner they deem appropriate (Hopefully efficiently :-)
If this is feasible, you will find it will produce a result that performs better and is more maintainable. If the interface is designed well, it should also be just as extensible as if you used reflection.

Creating an arrays of objects from a class

I was wondering does any one know how to create an array of objects from a class?
I am trying to create an array of objects of a class.
class name ---> Class objectArray[100] = new Class;
I cant seem to make a single class but i need to figure out how to create an array of objects.
I can make a normal class with Class object = new Class

There are four lines of code in your for-loop that actually do something:
for(index = 0; index < rooms.length; index++) {
/*1*/ assignWidth.setWidth(Double.parseDouble(in.readLine()));
/*2*/ rooms[index] = assignWidth;
/*3*/ assignLength.setWidth(Double.parseDouble(in.readLine());
/*4*/ rooms[index] = assignLength;
}1.) Sets the width of an object, that has been instantiated outside the loop.
2.) assigns that object to the current position in the array
3.) Sets the width of a second object that has been instantiated outside the loop
4.) assigns that other object to the current position in the array
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When I said "construct a new room-object and assign it to rooms[index]" I meant something like this:
for(index = 0; index < rooms.length; index++) {
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2.) set the width of the newly created object
3.) set the length of the newly created object
4.) assign the newly created object to the current position in the array
-T-
btw. this would do the same:
for(index = 0; index < rooms.length; index++) {
    rooms[index] = new Room();
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}but be sure you understand it. Your teacher most likely wants you to explain it ;)

Formatted .data file, reading in and creating an array of objects from data

Hi there, I have written a text file with the following format;
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2 : Dho : Age 57 : Income 110000
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Cheers!

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Create an array of the necessary length to hold the 3 variables, or, if you don't know the length of the file, create a class to contain the data. Then create an ArrayList to hold an instance of the class..
Use the Scanner class to read and parse the file contents into the appropriate variables.
Add each variable to either the array, or to an instance of the class and add that instance to the arraylist.

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This issue is most likely caused by a McAfee extension that isn't working properly and you will have to disable the extension that causes it.
You can also check if there is an update available from McAfee that fixes this problem.

The following error message came when I tried to open Firefox and install Firefox Home. "can't create mcafee plug-in object: TypeError: Components.classes[cid] is undefined"

The following error message came when I tried to open Firefox and install Firefox Home.
"can't create mcafee plug-in object: TypeError: Components.classes[cid] is undefined"

This issue can be caused by an extension or plugin that isn't working properly.
Start Firefox in [[Safe Mode]] to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
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See:
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How to create an Array of Object

Is this correct:
Object[] anArray = new Object[5];
Thanks.

Your code will create an array of 5 null references to class Object. After that line you could code:anArray[0] = new Integer(5);- or -
Object x = anArray[1];but if you code:anArray[3].DoSomething();and have not initialized anArray[3], you will get a NullPointerException.
Doug

Creating an array of objects sets them all to null?

I am using the following code:
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}and for the 'Player' object:
public class Player {
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Yes you are quite right they are null by default (that is nothing is assigned to the array of pointers you have created) you will have t initialise the array the array
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Wiraj

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Hello!
I have a number of classes, in particular a 'room' class and a 'player' class. I've just made them as simple classes with a couple of fields and a few accessor and mutator methods.
I have a 'start' class, and at the moment it has the heading:
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Cheers.
Hussein.
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You keep posting these basic questions and asking for simple answers. Well, learning Java is not a simple task. You need to start at the beginning of a long road and take many steps, one at a time. May I suggest one or more of the following:
Sun's basic Java tutorial
Sun's New To Java Center. Includes an overview of what Java is, instructions for setting up Java, an intro to programming (that includes links to the above tutorial or to parts of it), quizzes, a list of resources, and info on certification and courses.
http://javaalmanac.com. A couple dozen code examples that supplement The Java Developers Almanac.
jGuru. A general Java resource site. Includes FAQs, forums, courses, more.
JavaRanch. To quote the tagline on their homepage: "a friendly place for Java greenhorns." FAQs, forums (moderated, I believe), sample code, all kinds of goodies for newbies. From what I've heard, they live up to the "friendly" claim.
Bruce Eckel's Thinking in Java (Available online.)
Joshua Bloch's Effective Java
Bert Bates and Kathy Sierra's Head First Java.
James Gosling's The Java Programming Language. Gosling is
the creator of Java. It doesn't get much more authoratative than this.

Creating an array of objects

Hey,
I basically have two classes and am trying to create an array of one class in the sub class. My code keeps coming up with <identifier> expected on complie:
Main Class
import java.util.ArrayList;
import java.io.*;
* @author David Bainbridge
* @Date 12/02/07
public class Main {
         public ArrayList<City>ListOfCities;
public static void main(String[] arguments) {
     Main.newcity();
     public Main() {
     ListOfCities = new ArrayList<City>(); //this creates an array which stores all instances of the cities.
     public static void newcity() {
          String Name = "Lincoln";
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          Lincoln.displaycity();
}City Class
* The City class represents a City.
* @author David Bainbridge
* @Date 13/02/07
public class City {
     private String Name;
     private String Latitude;
     private String Longitude;
      * Constructor for the City.
     public City(String cityName, String cityLat, String cityLong) {
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          Latitude = cityLat;
          Longitude = cityLong;
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public void displaycity() {
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          System.out.println("Longitude: " + Longitude);
}

Thanks JJCoolB but neither worked!
I have rejigged my code:
import java.util.ArrayList;
import java.io.*;
* @author David Bainbridge
* @Date 12/02/07
public class Main {
         public static ArrayList<City>ListOfCities;
public static void main(String[] arguments) {
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          String Lat = "10";
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}I get two complie errors with it one says:
public static ArrayList<City>ListOfCities; - <identifier> expected
ListOfCities = new ArrayList<City>(); //this creates an array which stores all instances of the cities. - '(' or '[' expected.
Thanks in advance for any help people!

Forwarding the array of object to servlet class

Hi
i am new to this j2ee.So please cooperate with me.
i am having an array of objects in a jsp page.
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i have been trying like this but i guess it is not correct .
<%--
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<%@page import="abc.javabean.EmpJavaBean"%>

<%@page import="javax.servlet.ServletException"%>
<%@page import="javax.servlet.http.HttpServlet"%>
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<%@page import="javax.servlet.RequestDispatcher"%>
<%@page import="java.util.*"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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alert(name);
session.setAttribute("fetchedArrayObject[i]",fetchedArrayObject[i]);
if(thisone==-1)
alert("you must select a radio Button");
return false;
return true;
</script>
</head>
<body bgcolor="#8db6cd">

<%
//int a;
EmpJavaBean fetchedArrayObject[]=new EmpJavaBean[4];
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%>
<h3>ur new no is <%=fetchedArrayObject[0].getEmpNumber()%></h3><br>
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<br></br>
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<br></br>
<table border="1" align="center">
<thead>
<tr>
<th> </th>
<th> <input type="button" value="Emp#" name="searchedPageEmpNumb"> </th>
<th> <input type="button" value="Name" name="searchedPageEmpName"> </th>
<th> <input type="button" value="Priv" name="searchedPageEmpPriv"> </th>
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<th> <input type="button" value="End Date" name="searchedPageEmpEndDate"> </th>
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<tbody>
<tr>
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<td><%=fetchedArrayObject[0].getEmpName()%></td>
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<td><%=fetchedArrayObject[0].getEmpStartdate()%></td>
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</tr>
</tbody>
</table>
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</form>
</body>
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please suggest me the answer.
regards
thanx

I think you need to learn the difference between "java" and "javascript"
calling session.setAttribute from a javascript function will not have the effect that you desire.
If you give all radio buttons the same "name", then request.getParameter("radioGroup") should tell you the value of the selected one.

Create an array of objects

i have to write a program with 3 classes, Student, Coarse, and Main.
Coarse class has 3 instance variables, with the constructor:
public Coarse(String symbol, String name, int credit)Student class has 3 instance variables plus one method, one of these variables is an array of class Coarse, if i write a constructor of Coarse-class in Student, i would limit the number of coarses to one.
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noowa wrote:
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Lists: http://java.sun.com/docs/books/tutorial/collections/implementations/list.html
ArrayList: http://java.sun.com/javase/6/docs/api/java/util/ArrayList.html
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// please helpLast post, thats a promise!!! :-)Still wrong. See corrections in 'for' loop. :)

Creating an array of objects of a class known ONLY at RUNTIME

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