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Java Lists/Arrays

Posted on 2007-03-27
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Last Modified: 2010-03-31
How do I assign the values of a list to an array?

For example, I want to do something like:

for (int i=o; i<list.size(); i++) {
arr[i] = list.get(i);
}

The reason I want to do this is because I can't seem to be able to access the list object from another class... But I can access arrays..

Answer to either of the problems would be great!

Thanks

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Question by:vrameen
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16 Comments
 
LVL 86

Expert Comment

by:CEHJ
ID: 18801835
>>I can't seem to be able to access the list object from another class... But I can access arrays..

Then you need to put the list in the same scope as the arrays
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LVL 86

Expert Comment

by:CEHJ
ID: 18801845
If it's in the same package, make sure the List is an instance member
0
 

Author Comment

by:vrameen
ID: 18801899
@CEHJ,

Thanks for the help... I already put:

public ArrayList list = new ArrayList();

Outside of any methods etc..
Any variables I have their can be accessed, but not this one object..

Furthermore.. any ideas how I can do something like:

for (int i=o; i<list.size(); i++) {
arr[i] = list.get(i);
}

The above code just throws error about incompatible types (list is object, slots[i][j] is a string)..


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LVL 10

Expert Comment

by:ADSLMark
ID: 18801903
Check your methods/attributes for private/public. It would be smart to have a get method for the list object, like:

public LinkedList getList()
{
    return this.list; //no matter if list is private or public, you access the attribute if you are in your own class
}

Another solution is to make the declaration of list public and also outside the method.
So you might have sth like:
class MyClass
{
  public void someCalculation()
  {
      LinkedList list = new LinkedList();
      .....
  }
}
but, now you cannot access list. You should have declared list outside the method like:

class MyClass
{
  private LinkedList list;
  public void someCalculation()
  {
      this.list = new LinkedList();
      .....
  }
}

while your at it, you might want to instantiate the list attribute in the constructor and add the "getter" i proposed in the beginning. The code would look sth like:

class MyClass
{
  private LinkedList list;
  public MyClass()
  {
      this.list = new LinkedList();
  }
  public void someCalculation()
  {
      .....
  }
  public LinkedList getList()
  {
      return this.list;
  }
}

Mark
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 18801957
>>
Thanks for the help... I already put:

public ArrayList list = new ArrayList();

Outside of any methods etc..
>>

That can't be the case or you'd be able to access it
>>Furthermore.. any ideas how I can do something like

'arr' should be declared as Object[] or you need to cast
0
 

Author Comment

by:vrameen
ID: 18802102
@CEHJ

Did you mean something like this?:

   public void popArr() {
       
        for(int i=0; i<listsize; i++) {
           
            mods[i] = (String) list.get(i);
        }
       
       
        for (int j=0; j<mods.length; j++) {
           
            System.out.println(mods[j]);
        }
    }

I get nullpointer exception doing the above
0
 

Author Comment

by:vrameen
ID: 18802112
Also, I really have declared the list outside of any methods..

Extract from code:

import java.sql.*;
import javax.sql.*;
import javax.swing.*;
import java.util.*;

class dbManage {
   
    final static String path = "jdbc:mysql";
    final static String host = "localhost";
    final static String port = "3306";
    final static String db = "tt";
    final static String login = "root";
    final static String user = "root";
    final static String password = "rameen";
    final static String conStr = "jdbc:mysql://localhost:3306/tt";
    public static String[] arr = new String[getModAmt()];
    private static int mod;
    public static String[] mods;
    //public static String[] arr;
    public static int listsize;
   
   public ArrayList list = new ArrayList();
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 18802172
>>mods[i] = (String) list.get(i);

You can't access element i if the List doesn't have it. Nor can you assign to mods[i] if it doesn't exist
0
 

Author Comment

by:vrameen
ID: 18802258
@CEHJ

In the constructor, I've already set it to populate the list.
0
 
LVL 30

Expert Comment

by:Mayank S
ID: 18802259
Maybe I'm missing something.

Why can't you use the toArray () method?

arr = list.toArray () ;

?
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 18802381
What is 'mods' and where is it initialized?
0
 

Author Comment

by:vrameen
ID: 18802449
in the code i posted, you can see mods is initialized the same way outside the method..

mayan, i'll give it a go!
0
 
LVL 30

Accepted Solution

by:
Mayank S earned 500 total points
ID: 18802482
What is the data type of the array? If it is String, you can do:

arr = ( String[] ) list.toArray ( new String[0] ) ;
0
 
LVL 86

Expert Comment

by:CEHJ
ID: 18802539
>>you can see mods is initialized the same way outside the method..

It isn't initialized at all - it's just declared

>>public static String[] mods;
0
 

Author Comment

by:vrameen
ID: 18802596
@mayankeagle

Thanks, that worked fine..

0
 

Author Comment

by:vrameen
ID: 18802600
@CEHJ

Apologies, my mistake.. I guess it wasn't initialized..
0

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