How to iterate through two dimensional ArrayList using iterator?

Question

I would like to iterate through two dimensional ArrayList which includes String objects using iterator. I also would like to iterate in a way that let me choose whether I want to iterate horizontally(row) first or vertically(column) by using a boolean value. How can I implement this in java?

What I've tried so far.

public class IterateThis implements Iterator<String>{
ArrayList<ArrayList<String>> array;
public IterateThis(){
 array = new ArrayList<ArrayList<String>>();
 array.add(new ArrayList<String>());
 array.add(new ArrayList<String>());
 array.add(new ArrayList<String>());
 array.get(0).add("1");
 array.get(0).add("2");
 array.get(0).add("2");
 array.get(1).add("4");
 array.get(1).add("5");
 array.get(1).add("6");
}
Iterator<String> it = array.iterator(); //This gives me an error...why?

I don't know how I can implement the boolean value though.

• m0skit0: I see no boolean value on what you're tried...
• Janny: What type of error you find the there? Please mention the Error here. and "array" is the list of ArrayList<String>. so when you create a Iterator. it should be same type of list.
• m0skit0: "This gives me an error...why?" Because array.iterator() is not an Iterator<String> but an Iterator<List<String>>.
• weidi: since your class implements Iterator, you have to implement the iterate method for class IterateThis, and put the logic of 2-dimensional iteration in that method.

Answer 1

Maybe you need to implement two versions, with a boolean that decides which loop to use:

public void iterate(boolean horizantalFirst){
 if(horizontalFirst){
 for(int i=0; i<array.size(); i++){ // first iterate through the "outer list"
 for(int j=0; j<array.get(i).size(); j++){ // then iterate through all the "inner lists"
 array.get(i).get(j)="1";
 }
 }
 }else{ 
 int j=0; // index to iterate through the "inner lists"
 for(; j<array.get(j).size(); j++){ //dangerous, you need to be sure that there is a j-th element in array
 for(int i=0; i<array.size(); i++){ // iterate here through the outer list, by always working on the j-th element 
 array.get(i).get(j)="1";
 }
 }
 }
}

• GameDroids: sorry, I just realized I didn't use any Iterator
• Leo: I prefer this way, anyway ;-)
• Zip: We had to use Iterator for this task at school. So for loopis no option for me.

Answer 2

Why not try this:

import java.util.ArrayList;
public class Iteration
{
 private ArrayList<ArrayList<String>> array;
 public Iteration()
 {
 array = new ArrayList<>();
 array.add(new ArrayList<String>());
 array.get(0).add("000");
 array.get(0).add("001");
 array.get(0).add("010");
 array.add(new ArrayList<String>());
 array.get(1).add("100");
 array.get(1).add("101");
 array.get(1).add("110");
 array.get(1).add("111");
 iterateRowWise();
 System.out.println("\n\n");
 iterateColumnWise();
 }
 public void iterateRowWise()
 {
 // This uses iterator behind the scene.
 for (ArrayList<String> row : array)
 {
 for (String element : row)
 {
 System.out.print(element + " ");
 }
 System.out.println();
 }
 }
 public void iterateColumnWise()
 {
 int arraySize = array.size();
 int maxColumns = getMaximumListSize();
 for (int c = 0; c < maxColumns; c++)
 {
 for (int r = 0; r < arraySize; r++)
 {
 ArrayList<String> rowList = array.get(r);
 if (c < rowList.size())
 {
 System.out.print(rowList.get(c) + " ");
 }
 }
 System.out.println();
 }
 }
 private int getMaximumListSize()
 {
 int maxListSize = 0;
 for (ArrayList<String> rowList : array)
 {
 if (maxListSize < rowList.size())
 maxListSize = rowList.size();
 }
 return maxListSize;
 }
 public static void main(String[] args)
 {
 new Iteration();
 }
}

The iterateRowWise() method iterates using the iterator, but it does so behind the scene.
The iterateColumnWise() method doesn't use iterator, but its safe to use.

• sakura: rowwise is simple, suggest columnwise!!
• Aman Agnihotri: Added iterateColumnWise() method. ;)

Answer 3

Row-wise iteration is simple as shown in the @Awfully Awesome answer.

Tried a columnwise iteration with assumption that List will always have m cross n elements where m=n

public static void IterateThis() {
 ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
 array.add(new ArrayList<String>());
 array.add(new ArrayList<String>());
 array.get(0).add("1");
 array.get(0).add("2");
 array.get(0).add("2");
 array.get(1).add("4");
 array.get(1).add("5");
 array.get(1).add("6");
 Iterator<ArrayList<String>> it = array.iterator();
 int topLevelIteratorResetCounter = 0;
 int noOfIteratorNextRequired = 1;
 int size = array.size();
 while (it.hasNext()) {
 ArrayList<String> strList = it.next();
 if (noOfIteratorNextRequired > strList.size())
 break;
 Iterator<String> itString = strList.iterator();
 int numtimes = 0;
 String str = null;
 while (numtimes != noOfIteratorNextRequired) {
 str = itString.next();
 numtimes++;
 }
 System.out.println(str);
 numtimes = 0;
 topLevelIteratorResetCounter++;
 if (topLevelIteratorResetCounter == size) { //as column count is equal to column size
 it = array.iterator(); //reset the iterator
 noOfIteratorNextRequired++;
 topLevelIteratorResetCounter = 0;
 }
 }
}

The answer uses Iterator.