Thank you Vasile Braileanu, Ewald, Rick Fencl, and other who try to
guide me.
I have heard about clone(), but never realized the usefulness until this
moment.
I repeat typing the same coding just to emphasize to everybody that the
strQueryResult is not an object that can be added and done. Thanks to
you that have pointed out that I'm not actually adding an object, but
only its reference.
I have modified the coding and it works as expected:
import java.util.ArrayList;
public class TestArrayList1 {
static ArrayList<String[]> arlQueryResult = new ArrayList<String[]>(10);
static String[] strQueryResult = new String[4];
static String[] strQueryResult2 = new String[4];
public static void main(String[] args) {
for (int i = 1; i <= 2; i++){
strQueryResult[0] = "" + i * 4;
strQueryResult[1] = "" + i * 5;
strQueryResult[2] = "" + i * 6;
strQueryResult[3] = "" + i * 7;
arlQueryResult.add(strQueryResult.clone());
}
for (int i = 0; i< arlQueryResult.size(); i++){
for (int j = 0; j<=3; j++){
String str = arlQueryResult.get(i)[j];
System.out.println("Result " + i + j + ": " + str);
}
}
}
}
And the output:
Result 00: 4
Result 01: 5
Result 02: 6
Result 03: 7
Result 10: 8
Result 11: 10
Result 12: 12
Result 13: 14
I have a feeling that calling clone() is more efficient than calling a
method such as addElement(). Please correct me if I'm wrong.
Patrick
On Sat, 2009-01-24 at 20:53 +0200, Vasile Braileanu wrote:
> Hi,
> Ewald gived you a very good advice:
> alocate every time a new array. And emphasized the point that
> ArrayList objects (and all collection classes) are storing references.
> When you create a new array, a new reference is created every time, so
> they aren't repeated. I think Ewald suggested the best practice.
> You give here a very simple example but in practice is usually need of
> a function. Probably in practice you will need a function like this
>
> private static void addElement(String a1, String a2, String a3, String
> a4)
> {
> String[] strArray={a1,a2,a3,a4};
> arlQueryResult.add(strArray);
> }
>
> This way you also get rid of 4 lines in program
> strQueryResult[0] = "a";
> strQueryResult[1] = "b";
> strQueryResult[2] = "c";
> strQueryResult[3] = "d";
>
> For this kind of code I have a simple rule (as many programming books
> suggested and I rediscovered it myself few years ago):
> If you have to run a piece of code you need only once, write it.
> If you have to run a piece of code many times, use fuctions. Many
> times you can reuse the functions and this speeds your work.
>
> Considering you have a program which gives you 4 strings, the function
> above make sense.
>
> You can also make a "shallow copy" which also creates a new reference.
> If you don't want to create a new array you can create a clone (but
> this way you would spend time to create a copy of the array).
> See function addElement below. The "clone" function is in class Object
> which in Java is the root of all classes.
> See here the java documentation here.
> http://java.sun.com/j2se/1.3/docs/api/java/lang/Object.html
>
> import java.util.ArrayList;
>
> public class TestArrayList1 {
> static ArrayList<String[]> arlQueryResult = new
> ArrayList<String[]>(10);
> static String[] strQueryResult = new String[4];
>
> public static void main(String[] args) {
> strQueryResult[0] = "a";
> strQueryResult[1] = "b";
> strQueryResult[2] = "c";
> strQueryResult[3] = "d";
> addElement(strQueryResult);
>
> strQueryResult[0] = "e";
> strQueryResult[1] = "f";
> strQueryResult[2] = "g";
> strQueryResult[3] = "h";
> addElement(strQueryResult);
>
> for (int i = 0; i< arlQueryResult.size(); i++){
> for (int j = 0; j<=3; j++){
> String str = arlQueryResult.get(i)[j];
> System.out.println("Result " + i + j +
> ": " + str);
> }
> }
>
> }
>
> /**
> * add a string element to an ArrayList<String[]>
> * @param element
> */
> private static void addElement(String[] element)
> {
> arlQueryResult.add((String[])element.clone());
> }
> }
>
> Best regards,
> Vasile Braileanu
>
> >
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