How to remove item in List?

Question

I have two List in nest for loop, When I matched a item in inner, I want to remove it so that performance will up.

List<String[]> brandList = readCsvFile("/tmp/brand.csv");
List<String[]> themeList = readCsvFile("/tmp/theme.csv");

for (String[] brand : brandList) {
    for (String[] theme : themeList) {
        if (brand[0].equals(theme[0])) {
            themeList.remove(theme);
        }
    }
}

I got a java.util.ConcurrentModificationException error. If I changed to CopyOnWriteArrayList, The error as below:

CopyOnWriteArrayList<String[]> themeList = (CopyOnWriteArrayList<String[]>)readCsvFile("/tmp/theme.csv");

java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.util.concurrent.CopyOnWriteArrayList

Now how can I do? omit remove? or anyway else?

I think this is what I need:

List<String[]> brandList = readCsvFile("/tmp/brand.csv");
List<String[]> themeList = readCsvFile("/tmp/theme.csv");

for (String[] brand : brandList) {
    List<String[]> toRemove = new ArrayList<String[]>();

    for (String[] theme : themeList) {
        if (brand[0].equals(theme[0])) {
            toRemove.add(theme);
        }
    }

    for (String[] theme : toRemove) {
        themeList.removeAll(theme);
    }
}

Answer 1

You can't remove items from a Collection while you are iterating over it, which it was a foreach loop in Java essentially does. You have to create a new List<String[]> and collect all elements you wish to remove, then delete them in bulk after you have iterated through the Collection:

List<String[]> brandList = readCsvFile("/tmp/brand.csv");
List<String[]> themeList = readCsvFile("/tmp/theme.csv");
List<String[]> toRemove = new ArrayList<String[]>();

for (String[] brand : brandList) {
    for (String[] theme : themeList) {
        if (brand[0].equals(theme[0])) {
            toRemove.add(theme);
        }
    }
}
themeList.removeAll(theme);

Answer 2

It's not as pretty, but you can do it using iterators:

List<String[]> brandList = readCsvFile("/tmp/brand.csv");
List<String[]> themeList = readCsvFile("/tmp/theme.csv");

for (String[] brand : brandList) {
    Iterator<String[]> themeIterator = themeList.iterator();
    while (themeIterator.hasNext()) {
        String[] theme = themeIterator.next();
        if (brand[0].equals(theme[0])) {
            themeIterator.remove();
            // If you are sure there is only one theme per brand, add a break here
            // break;
        }
    }
}

Depending on the concrete type of List<> themeList is (array list, linked list etc), this may or may not be faster than the copy variant.

Answer 3

If you are using Java 8 features, something like this might serve, and might be faster:

List<String[]> brandList = readCsvFile("/tmp/brand.csv");
List<String[]> themeList = readCsvFile("/tmp/theme.csv");

// Extract unique values of the first column from the brand list
// into a structure suited for fast lookup
Set<String> names = brandList.stream()
        .map(columns -> columns[0])
        .collect(Collectors.toSet())

// Remove all entries from themeList where the value of the
// first column exists in names
themeList.removeIf(columns -> names.contains(columns[0]))