You seem to have a typo in your code, first line - that 'i' should be 'j':
for(int j=0;j<n1;j++){
s2=bure.readLine();
st1=new StringTokenizer(s2);
for(int k=0;k<n2;k++){
a[j][k]=Integer.parseInt(st1.nextToken());
}
}
Try this way. Also using st1.hasMoreElements() seems to be useful in case if you are receiving the data from an external source, and is not built by yourself.