How to create and initialize dynamic variables...

[krishna81m]

How to create dynamic variables as follows

for(...)
{
Run This( "int var" + i = i;);
}

to be able to create and initialize var1 = 1; var2 = 2;

I was able to do this in a couple of scientific programming languages without any problem..

Help please..

[BigDaddyLH]

Easily done with a Map.

[chaarmann]

You should do it with arrays/maps instead of using a couple of loose variables.

Defining "int[] vars" is better than using "var1, var2, var3 etc."

But if you still want, despite all experience:
you can define "public static Integer var1, var2 etc" as member of a class and then use "reflection" to modify the values of this class in a loop.

Class c = Class.forName("myClass");
for (i=1; ....)
{
  Field f = c.getField("var" + i);
  f.setInt(this, i);
}

(code not yet tested, but I hope you get the idea.)

[BigDaddyLH]

You should do it with arrays/maps instead of using a couple of loose variables.

Defining "int[] vars" is better than using "var1, var2, var3 etc."

But if you still want, despite all experience:
you can define "public static Integer var1, var2 etc" as member of a class and then use "reflection" to modify the values of this class in a loop.

Class c = Class.forName("myClass");
for (i=1; ....)
{
  Field f = c.getField("var" + i);
  f.setInt(this, i);
}

(code not yet tested, but I hope you get the idea.)

The idea I get is that this would be a spectacularly bad idea!

[krishna81m]

   1. Class c = Class.forName("myClass");
   2. for (i=1; ....)
   3. {
   4. Field f = c.getField("var" + i);
   5. f.setInt(this, i);
   6. }
 

This is exactly what I thought of initially. The problem with this code really is that you need to know the field names, type of variables that you are setting aprior as in f.setInt(this,i)
and I have to initialize different number and type of fields.

So I thought I could create a string equivalent

List<someObjectName> myList;
myList.add(someObjectName(var1, var2, var3,....))


Let me first explain my actual problem in detail first:

I have a dynamically created query that is run on the database and we have the result set columns which can be used to initialize a known object. These objects (type not known) with constructors have to be populated dynamically. Essentially a list of these objects must be passed to some table in a jsp for display purposes. The jsp page dynamic tag libraries only accepts bean/objects.

As an awkward solution to this, we tried a genericBean object with 10 string fields in it and hence the object has a constructor with 10 fields, a maximum number of fields we might have to display in the jsp table. As different queries return different number of columns (converted to string equivalents), the remaining fields in the constructor are set as empty strings. At the jsp page, we control how many columns we want to display in the jsp from this list of objects.

Is there a solution to this problem?

[krishna81m]

package org.struts.ets.utility;
 
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.ArrayList;
import java.util.List;
 
import org.struts.bean.GenericDAOBean;
 
public class DynamicObjectCreation
{
    public static void main(String... args)
    {
        Constructor[] ctors = GenericDAOBean.class.getDeclaredConstructors();
        Constructor ctor = null;
        for (int i = 0; i < ctors.length; i++)
        {
            ctor = ctors[i];
            if (ctor.getGenericParameterTypes().length == 2)
                break;
        }
 
        try
        {
            ctor.setAccessible(true);
 
            // This is what is created dynamically and passed to the
            // action class
            Object myObj1 = ctor.newInstance("A", "B");
            Object myObj2 = ctor.newInstance("C", "D");
            List<Object> myObjList = new ArrayList<Object>();
            myObjList.add(myObj1);
            myObjList.add(myObj2);
 
            // the action class casts into the object type it needs
            GenericDAOBean myGenObj = (GenericDAOBean) myObjList.get(0); // myObj1
            // Field f = c.getClass().getDeclaredField("var1");
            // f.setAccessible(true);
            // System.out.println("output: " + f.get(c).toString());
            System.out.println("output: " + myGenObj.getVar1());
 
            // passing a list of these generic objects
            List<GenericDAOBean> myGenObjList = new ArrayList<GenericDAOBean>();
            myGenObjList = (myGenObjList)myObjList;
 
            // production code should handle these exceptions more gracefully
        } catch (InstantiationException x)
        {
            x.printStackTrace();
        } catch (InvocationTargetException x)
        {
            x.printStackTrace();
        } catch (IllegalAccessException x)
        {
            x.printStackTrace();
        } 
    }
}

See that this is where I have the problem,

List<GenericDAOBean> myGenObjList = new ArrayList<GenericDAOBean>();
// or 
// List<GenericDAOBean> myGenObjList = null;
 
myGenObjList = (myGenObjList)myObjList;

It says myGenObjList cannot be resolved to a type

[BigDaddyLH]

myGenObjList = (myGenObjList)myObjList;

For casting, what goes in the parentheses must be a type, not a variable.

[chaarmann]

...
The problem with this code really is that you need to know the field names, type of variables that you are setting aprior as in f.setInt(this,i)
and I have to initialize different number and type of fields.
...

There is no need to "know" the field names in advance. You can just enumerate them.
For example:

Class c = Class.forName("myClass");
Field[] f = c.getFields();
for (int i=0; i < f.length; i++)
{
 ...

Just look in the JDK description for all methods of class "Class".
You can get all information about the class there, also skope, field types, methods, method parameters etc.

But if you only want to pass data, then you can just declare the method parameter as "Object" and pass it. Then you can test with "instanceOf" which type it is. But that also means you must do some inboxing/outboxing for primitive types. Then for example you cannot store an integer as "int", but must convert it to "Integer" before passing.

Second suggestion: if you already have converted all table column values to "String", then you can use "Properties" to store them in a manner of key=variableName, value=variableValue. No need to hardcode a fixed size array as what you described in your example.
If you have more than one record(row) returned from database, you can use "Vector" of "Properties" and pass/return that as argument/returnValue to your functions.

[djamshed]

Here is a very hard-coded alternative to your problem:
1) Create an interface with value setter

     public interface RowWithColumnValues {
             void setColumnValue(int columnId, String value);
    }
 

2) Hard-code part: create classes that implement the interface above with exactly 1"escalating" variable and extending the "previous" variable's class:

 public class RowWithColumn1 implements RowWithColumnValues {
 
      public String column1;
 
     public void setColumnValue(int columnId, String value)
     {
           if(columnId == 1)
           column1= value;
     }
}
 
 
public class RowWithColumn2 extends RowWithColumn1 {
 
      public String column2;
 
      public void setColumnValue(int columnId, String value)
      {
            if(columnId == 2)
                 column2 = value;
            else
                  super.setColumnValue(columnId, value);
      }
}
 
 
public class RowWithColumn3 extends RowWithColumn2{
 
      public String column3;
 
      public void setColumnValue(int columnId, String value)
      {
            if(columnId == 3)
                  column3 = value;
            else
                  super.setColumnValue(columnId, value);
      }
}
 
 
//etc, yes, you have to make as much classes as your possible maximum number of variables
 

3) Create a Factory that will return you the proper class with variables

public RowWithColumnValues getRowObject(int numberOfColumns)
{
         RowWithNodes rowObject = null;
         switch(numberOfColumns)
         {
                  case 1: rowObject= new RowWithColumn1(); break;
                  case 2: rowObject= new RowWithColumn2(); break;
                  case 3: rowObject= new RowWithColumn3(); break;
                  //....
                  case 100: rowObject= new RowWithColumn100(); break;
                  default: rowObject= new RowWithColumn100(); break;
         }
 
         return rowObject;
}
 

Now, you can use this as:

RowWithColumnValues row= new RowWithColumnValues().getRowObject(numOfColumns);
// set column values
for(int i = 1; i < numOfColumns+1; i++)
{
        row.setColumnValue(i, rs.getString(i));
}
rows.add(row); 
 
 

A bit more clearer explanation is given at http://djamshed.blogspot.com/2008/03...r-dynamic.html

[krishna81m]

Thanks Jamshed,

I am looking into it and also going through your blog entry. Will get back to you soon.

-Cheers
Krishna