How to use an iterator to replace and count

Question

I'm having trouble figuring out how an iterator can be used to replace an element and count the number of elements that were replaced. This is the code I have so far:

public class example {
    public static void main (String args[]){
        //create an ArrayList
        ArrayList<String> list = new ArrayList<String>();


        //add elements to the array list
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("B");
        list.add("B");

        //use iterator to display original list
        Iterator iter = list.iterator();
        while (iter.hasNext()){
            Object element = iter.next();
            System.out.println (element + " ");
            }

        // call replace         
        String b = "B";
        String x = "X";
        ArrayList<String> myList = new ArrayList<String>();
        replace (b, x, myList);
    }

    public static <E> int replace(E match, E replacement, ArrayList<E> myList) {

        //throw exceptions if null
            if (match == null){
                throw new IllegalArgumentException ("match cannot be null");
            }
            if (replacement == null){
                throw new IllegalArgumentException ("replacement cannot be null");
            }
            if (myList == null){
                throw new IllegalArgumentException ("myList cannot be null");
            }
            //return 0 if myList is empty
            boolean emptylist = myList.isEmpty();
            if (emptylist = true){
                System.out.println("0");
            }


    }

I've used the iterator to print out the elements in the list, but now I have to use the iterator to replace and return the number of replacements. In this case, I want to replace the "B"'s with "X"'s and count the number of "X"'s. I'm assuming I want to put the iterator in the generic method, but I don't really know what direction to head in...

Answer 1

Try using the ListIterator class, documented here: http://docs.oracle.com/javase/6/docs/api/java/util/ListIterator.html

The documentation for the set method describes how to modify list elements.

Answer 2

You iterate over the list, and when you find a matching element, you set it with its replacement:

public static <E> int replace(E match, E replacement, ArrayList<E> myList) {

    //throw exceptions if null
    if (match == null){
        throw new IllegalArgumentException ("match cannot be null");
    }
    if (replacement == null){
        throw new IllegalArgumentException ("replacement cannot be null");
    }
    if (myList == null){
        throw new IllegalArgumentException ("myList cannot be null");
    }

    int counter = 0;
    ListIterator<E> iter = myList.listIterator();
    while (iter.hasNext()) {
        E val = iter.next();
        if (val.equals(match)) {
            iter.set(replacement);
            ++counter;
        }
    }
    return counter;
}

Answer 3

You don't need iterator to just replace the element, you need it when you add or remove element when you are looping over it. You can do the replace with just simple for loop.

public class example {
  public static void main (String[] args) throws java.lang.Exception
    {
         ArrayList<String> list = new ArrayList<String>();

        //add elements to the array list
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("B");
        list.add("B");

        String b = "B";
        String x = "X";

       System.out.print(replace (b, x, list));            

    }

    public static <E> int replace(E match, E replacement, ArrayList<E> myList) {
        //throw exceptions if null
            if (match == null || replacement == null || myList == null ){
                throw new IllegalArgumentException ("match cannot be null");
            }

            int i=0;
            int replaceCount=0;            
            for(E str : myList){
                if(str.equals(match)){
                    myList.set(i,replacement);
                    replaceCount++;
                }
                i++;
            }
        return replaceCount;
        }
}