how to create multidim arraylist and access it

[Lester D Jones]

Could anyone provide an example of how to create a two dimension arraylist and how to get its elements?

thanks

ldj

[Jeff Albertson]

import java.util.*;
 
public class Test {
    //Java
    static void f(int XMAX, int YMAX) {
        //creation
        List matrix = new ArrayList();
        for(int i=0; i<XMAX; ++i) {
            List col = new ArrayList();
            for (int j=0; j<YMAX; ++j)
                col.add("(" + i + "," + j + ")");
            matrix.add(col);
        }
 
        //access
        String  s = (String) ((List)matrix.get(XMAX/2)).get(YMAX/2);
        System.out.println(s);
    }
 
    //Java 1.5
    static void g(int XMAX, int YMAX) {
        //creation
        List<List<String>> matrix = new ArrayList<List<String>>();
        for(int i=0; i<XMAX; ++i) {
            List<String> col = new ArrayList<String>();
            for (int j=0; j<YMAX; ++j)
                col.add("(" + i + "," + j + ")");
            matrix.add(col);
        }
 
        //access
        String  s = matrix.get(XMAX/2).get(YMAX/2);
        System.out.println(s);
    }
 
    public static void main(String[] args) {
        f(10,10);
        g(10,10);
    }
}

[Joel McNary]

Keep in mind that Lists (and arrays) are one-dimensional in Java. You can simulate higher-dimensional constructs by nesting one-dimsenional ones.

So, a two-dimensional array is a one-dimensional array of one-dimensional arrays:

String[][] anArrayOfStringArrays;

Likewise, a two-dimensional list is a List of Lists:

//Without Generics
java.util.List aListOfStringLists;
 
//With Generics
java.util.List<java.util.List<String>> aListOfStringLists;

(note that the use of generics makes the visualization easier)

Access of them is how you would expect. To get the second string of the third array, you first get the third array and then get the second String:

String[] aStringArray = anArrayOfStringArrays[2];
String aString = aStringArray[1];
//Or...
String aString = anArrayOfStringArrays[2][1];

Lists (without Generics) are a little more complicated, but Generics cleans them up:

//Without Generics:
java.util.List aStringList = (java.util.List)aListOfStringLists.get(2);
String aString = (String)aStringList.get(1);
 
//With Generics
String aString = aListOfStringLists.get(2).get(1);

Hope that this helps.

[Lester D Jones]

Using Jeff's example as a base I want to create a matrix with the parameters of a request object. Here is the snipet:

List<List<String>> matrix = new ArrayList<List<String>>();
while(reqParams.hasMoreElements()) {
List<String> paramNameCol = new ArrayList<String>();
List<String> paramCol = new ArrayList<String>();
paramName = (String)reqParams.nextElement();
paramNameCol.add(paramName);
paramCol.add(request.getParameter(paramName));
matrix.add(paramNameCol);
matrix.add(paramCol);
}
System.out.println(matrix.get(0).get(1));

I would expect to print the parameter but instead the paramenter is in matrix.get(1).get(0). It seems to me that I am adding a row instead of a column. What am I doing wrong?

thanks to all for the help

ldj

[Campbell Ritchie]

albert kao,
Your post was moved to a new topic.