Java arraylist find missing fields using streams

Question

I have this below code. Two sets of arraylist. How do I use streams to iterate and compare itemlist_a and itemlist_b to detect that c and d is missing in itemlist_b? Of course I can use the traditional for loop, but is there a different way to achieve using streams or something else?

import java.util.ArrayList;
import java.util.List;

    class item {
        String name;

    public item(String name) {
        this.name = name;
    }
}

public class Test {
    public static void main(String[] args) {
        List<item> itemlist_a = new ArrayList<item>();
        itemlist_a.add(new item("a"));
        itemlist_a.add(new item("b"));

        List<item> itemlist_b = new ArrayList<item>();
        itemlist_b.add(new item("a"));
        itemlist_b.add(new item("b"));
        itemlist_b.add(new item("c"));
        itemlist_b.add(new item("d"));
    }
}

Answer 1

I don't think this is a good application for streams, but you could do it like this:

itemlist_a.stream().filter(e -> !itemlist_b.contains(e)).collect(Collectors.toList());

This will give you a list of all items which are in itemlist_a but not in itemlist_b. However streams are designed for functional programming which means that they should not rely on variables outside the stream. In this case itemlist_b is not in the stream but does influence the outcome of the function. It will work however.

Answer 2

The simplest approach is to override equals and hashCode in Item, then use removeAll on the superset to retain the difference:

//the Item class
@Override
public int hashCode() {
    // TODO
    return Objects.hashCode(name);
}

@Override
public boolean equals(Object obj) {
    // TODO: implement
    return this.name.equals(((item) obj).name);
}

Then you can get the difference:

itemlist_b.removeAll(itemlist_a);

What will be left in itemlist_b after that call will be the difference. If you want itemlist_b to be unchanged, you can do the same thing using a copy

List<Item> diff = new ArrayList<>(itemlist_b);
diff.removeAll(itemlist_a);

And you have the difference in diff .

---

Since you're using lists, you just need to pay attention to your business logic with respect to duplicates. removeAll will remove all matching elements.

Answer 3

Java List API
copy of one list and then remove all the elements common with the other

List<Item> differences = new ArrayList<>(itemlist_b);
differences.removeAll(itemlist_a);

Streams API

List<Item> differences = itemlist_b.stream()
            .filter(element -> !itemlist_a.contains(element))
            .collect(Collectors.toList());

Google Guava

List<Item> differences = new ArrayList<>(Sets.difference(Sets.newHashSet(itemlist_a), Sets.newHashSet(itemlist_b)));

Apache Commons Collections

List<Item> differences = new ArrayList<>((CollectionUtils.removeAll(itemlist_b, itemlist_a)));