async "map" over 2d array list

gistfile1.java

import java.util.ArrayList;

// next step is to change type from A to B,
// map :: (a -> b) -> [a] -> [b]

public class Main {

    public static void main(String[] args) {

        PartitionedArrayList<Integer> xs = new PartitionedArrayList<Integer>(3);
        // 3 is passed as a param, this means that no sub array will contain more than 3 Integers

        xs.insert(1);
        xs.insert(2);
        xs.insert(3);
        xs.insert(4);
        xs.insert(5);
        xs.insert(6);
        xs.insert(7);

        System.out.println(xs);
        // [1,2,3,],[4,5,6,],[7,],

        xs.map(new Proc<Integer>() {
            @Override
            public Integer call(Integer x) {
                return x + 10;
            }});

        System.out.println(xs);
        // [11,12,13,],[14,15,16,],[17,],

        xs.map(new Proc<Integer>() {
            @Override
            public Integer call(Integer x) {
                return x * 10;
            }});

        System.out.println(xs);
        // [110,120,130,],[140,150,160,],[170,],
    }
}



class Proc<E> {
    // just a simple wrapper
    public E call(E object) {
        return null;
    }
}


class PartitionedArrayList<E> {
    
    final int partitionSize;
    volatile ArrayList<ArrayList<E>> rows = new ArrayList<ArrayList<E>>();
    volatile int rowIndex = 0; // how many sub arrays do we have?
    volatile int columnIndex = 0; // what is our position in the last array

    public PartitionedArrayList(int partitionSize) {
        this.partitionSize = partitionSize;
        rows.add(new ArrayList<E>());
    }

    @Override
    public String toString() {
        String str = "";
        for (ArrayList<E> row : rows) {
            str += "[";
            for (E item : row) {
                str += item.toString() + ",";
            }
            str += "],";
        }
        return str;
    }

    public synchronized void insert(E item) {
        if (columnIndex == partitionSize) {
            columnIndex = 0 ;
            rowIndex ++ ;
            rows.add(new ArrayList<E>()) ;
        }
        rows.get(rowIndex).add(item);
        columnIndex++;
    }


    public void map(Proc<E> proc) {
        ArrayList<Thread> threads = new ArrayList<>();
        for (ArrayList<E> row : rows) {
            Thread t = new Thread(new Runnable() {
                @Override
                public void run() {
                    int col_index = 0;
                    for (E item : row) {
                        // System.out.println("proc " + proc + " called by " + this);
                        // System.out.println("about to update " + item + " at position " + col_index);
                        row.set(col_index, proc.call(item));
                        col_index++;
                    }
                }
            });
            t.start();
            threads.add(t);
        }
        for (Thread t : threads) {
            try {
                t.join();
            } catch (InterruptedException e) {
            }
        }
    }


}


/*
[1,2,3,],[4,5,6,],[7,],
proc Main$1@59ca08c called by PartitionedArrayList$1@b5b5e4f
proc Main$1@59ca08c called by PartitionedArrayList$1@13646692
about to update 4
about to update 1
proc Main$1@59ca08c called by PartitionedArrayList$1@13646692
proc Main$1@59ca08c called by PartitionedArrayList$1@b5b5e4f
proc Main$1@59ca08c called by PartitionedArrayList$1@59815c2e
about to update 5
about to update 7
about to update 2
proc Main$1@59ca08c called by PartitionedArrayList$1@13646692
proc Main$1@59ca08c called by PartitionedArrayList$1@b5b5e4f
about to update 3
about to update 6
[11,12,13,],[14,15,16,],[17,],
proc Main$2@6e5070e8 called by PartitionedArrayList$1@60979352
about to update 11
proc Main$2@6e5070e8 called by PartitionedArrayList$1@20f1cd53
proc Main$2@6e5070e8 called by PartitionedArrayList$1@60979352
proc Main$2@6e5070e8 called by PartitionedArrayList$1@6c5bb88c
about to update 17
about to update 14
about to update 12
proc Main$2@6e5070e8 called by PartitionedArrayList$1@20f1cd53
proc Main$2@6e5070e8 called by PartitionedArrayList$1@60979352
about to update 15
about to update 13
proc Main$2@6e5070e8 called by PartitionedArrayList$1@20f1cd53
about to update 16
[110,120,130,],[140,150,160,],[170,],

Process finished with exit code 0
*/