dynamic 2d array in java using Arraylist

Question

I found some problem in creating dynamic 2d array with arraylist,The original code is tediously long to read so i am giving a simple code here,the problem is same in both the cases:

import java.util.*;

class test{
 public static void main(String args[]){
    Integer test[]=new Integer[3];

    ArrayList<Integer[]> al=new ArrayList<Integer[]>();

    int i,t;

     test[0]=1;
     test[1]=2;
     test[2]=3;


     al.add(test);
     test[0]=4;
     test[1]=5;
     test[2]=6;

  al.add(test);

     test[0]=7;
     test[1]=8;
     test[2]=9;

  al.add(test);



     test[0]=10;
     test[1]=11;
     test[2]=12;
  al.add(test);



     Integer table[][]=new Integer[al.size()][];
     table=al.toArray(table);

     for(i=0;i<=al.size()-1;i++){

     for(t=0;t<3;t++){
       System.out.print(" "+i+" "+t+" ");
       System.out.print(" "+table[i][t]+" ");}
     System.out.println();
 }   

   }
}

Output:

 0 0  10  0 1  11  0 2  12
  1 0  10  1 1  11  1 2  12
 2 0  10  2 1  11  2 2  12
 3 0  10  3 1  11  3 2  12

Output expected is

 0 0   1  0 1  2  0 2  3
 1 0   4  1 1  5  1 2  6
 2 0   7  2 1  8  2 2  9
 3 0  10  3 1  11  3 2  12

I don't under stand why the last element is over writting all other elements.

missingfaktor · Accepted Answer · 2010-08-22 08:35:38Z

9

Initialize a new Integer[] every time you add a new row.

That is, do it like this:

Integer[] test = new Integer[3];
List<Integer[]> al = new ArrayList<Integer[]>();
int i,t;
test[0]=1;
test[1]=2;
test[2]=3;
al.add(test);
test = new Integer[3]; // Note this line
test[0]=4;
test[1]=5;
test[2]=6;
al.add(test);
test = new Integer[3]; // Note this line
test[0]=7;
test[1]=8;
test[2]=9;
al.add(test);
test = new Integer[3]; // Note this line
test[0]=10;
test[1]=11;
test[2]=12;
al.add(test);

Or better yet, do it this way:

List<Integer[]> al = new ArrayList<Integer[]>();
al.add(new Integer[]{1, 2, 3});
al.add(new Integer[]{4, 5, 6});
al.add(new Integer[]{7, 8, 9});
al.add(new Integer[]{10, 11, 12});

edited Aug 22, 2010 at 8:35

answered Aug 22, 2010 at 8:30

missingfaktor

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Comments

meriton · Accepted Answer · 2010-08-22 08:30:47Z

1

Arrays are objects. ArrayList.add(E) adds a reference to the given E object to the list; it doesn't copy the object itself.

So you should do:

al.add(test);
test = new int[3];

Which creates a new array object, so the second row of data is written into a separate array.

answered Aug 22, 2010 at 8:30

meriton

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