Re: Creation of collection objects in a loop
Lew wrote:
Hendrik Maryns wrote:
List<Record> records = new ArrayList<Record>();
for (from beginning to the end of text file) {
Record record = new Record( gatherData );
records.add(record);
}
Daniel Moyne wrote:
ok Hendrik you propose a List to store my instances but at the first
iteration you do :
Record record = new Record(data1);
at the second iteration you do :
Record record = new Record(data2);
then with this what happens to the first instanciated object record as
you keep using the same name "record" ?
The first record is pointed to from within the List. The variable
'record' now points to a new instance.
Review the Java tutorial on the Sun Java site.
Remember that a variable is not an object. A (reference) variable is
a pointer to an object. You can simply point it to a different object
at any time.
This is a vitally important point to assimilate if you are going to
use Java successfully. It influences things like garbage collection
(GC) and whether you get memory leaks (actually packratting, but Java
folks refer to that as "leaking" anyway).
A variable is not an object, it is a pointer to an object. It can
point to different objects at different times. That is why the
attempt to dereference a variable that points to null results in what
is called a NullPointerException.
--
Lew
'record' is actually updated within the loop ; I thought the same instance
was rewritten over ! ; I have tried this piece of code with 2 records with
a map (Hendrik proposed a list that works as well) where I store all
instances :
CODE-----------
/* java imported classes */
import java.util.Map;
import java.util.HashMap;
import java.util.TreeMap;
public class record_map {
public static void main(String args[]) {
my_start();
}
public static void my_start() {
Map<Integer,Record> maprecord = new HashMap<Integer,Record>();
int array_x[]={3,7};
int array_y[]={4,8};
int array_r[]={5,9};
for (int i=0;i<2;i++) {
Record record = new Record(array_x[i],array_y[i],array_r[i]);
maprecord.put(i,record);
}
Record record0=maprecord.get(0);
System.out.println(record0.x);
System.out.println(record0.y);
System.out.println(record0.r);
Record record1=maprecord.get(1);
System.out.println(record1.x);
System.out.println(record1.y);
System.out.println(record1.r);
}
static class Record {
Record (int x, int y, int r) {
this.x=x;
this.y=y;
this.r=r;
}
int x;
int y;
int r;
}
}
CODE-----------
With the key I have kind of order index linked to the record.
I consider this topics closed ; thanks to everybody.