Re: Newbie Question - ArrayLists and methods
Taria wrote:
Hello all (again),
My problem here is that I'm trying to build a list of ArrayLists that
hold data and I want to add the newly derived data into a table where
it's dependent on the first row. A short version of my program to
illustrate what I mean:
import java.util.*;
public class MyProg2 {
public static void main(String[] args) {
List table = new ArrayList ();
List<List<Integer>> table = new ArrayList<List<Integer>>();
List <Integer> data = new ArrayList <Integer>();
data.add(1);
data.add(3);
data.add(4);
table.add(data);
System.out.println ("table(0) = " + table.get(0));
ArrayList <Integer> newNode = new ArrayList <Integer>();
newNode = createNode((ArrayList)table.get(0),0);
.... so you can change this line to:
newNode = createNode(table.get(0),0);
table.add(newNode);
System.out.println ("Added in a new row and table is now:");
System.out.println ("table(0) = " + table.get(0));
System.out.println ("table(1) = " + table.get(1));
} //end main driver
public static ArrayList createNode(ArrayList items,int lParen){
public static List<Integer> createNode(List<Integer> items, int lParen){
int a = 0; int b=0; int c=0;
if (items.size() >= 2){
a = Integer.valueOf(items.get(lParen).toString());
b = Integer.valueOf(items.get(rParen).toString());
typo? I see no `rParen' defined anywhere.
c = a + b;
items.remove(lParen);
items.remove(lParen);
items.add(lParen,c);
}
return items;
}
}
Here is my rough estimate of what the second method should look like:
public static List<Integer> createNode(List<Integer> items,
int leftIndex, int rightIndex) {
// Create a copy so we can modify without changing...
items = new ArrayList<Integer>(items);
if (items.size() >= 2){
int sum = items.get(leftIndex)+items.get(rightIndex);
// Remove from the right first -- otherwise we're off by one.
items.remove(rightIndex);
items.remove(leftIndex);
items.add(leftIndex,sum);
}
return items;
}
(I'm unable to get rid of all the unchecked msgs because putting the
<Integer> tag sometimes made the program uncompilable. Use --nowarn
when you compile this program. :)
Have you read up on generics?
Iin this code, row 0 of table is changed in the method while it
creates row 1, but I don't understand why and don't know how to keep
it from changing. From this behavior, it's leading me to believe
ArrayLists are passed by value or is this the way of ArrayLists? I
thought parameters were passed by reference? What am I missing here?
Row 0 is changing because the parameters are being passed by reference.
You send in the list of items, and it simultaneously affects both the
to-be-returned value and the input (rows 1 and 0, respectively). To keep
it from changing, you want to create a copy of the row, which is what
the first line of my method does.
--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth