Create pairs/combinations of items

Pairs.java

public class Pairs {

    /**
     * Create a list of pairs of the given items.
     * e.g. ["A", "B", "C", "D"] will return ["A:B", "A:C", "A:D", "B:C", "B:D", "C:D"]
     * @param items the items to create the pair
     * @param <T> type of the items
     * @return a list of pairs
     */
    public static <T> List<Pair<T, T>> of(final List<T> items) {
        final List<Pair<T, T>> pairs = new ArrayList<>();
        items.forEach((T left) -> items.stream().filter(right -> !left.equals(right) && !pairs.contains(new Pair<>(right, left)))
                .forEach(right -> pairs.add(new Pair<>(left, right))));
        return pairs;
    }

    /**
     * Create a group of pairs of the given items, group by the pair's left.
     * e.g. ["A", "B", "C", "D"] will return [A=>[A:B,A:C,A:D], B=>[B:C,B:D], [C=>C:D]]
     * @param items the items to create the pair and grouped
     * @return the grouped pairs
     */
    public static <T> Map<String, List<Pair<T, T>>> groupLeft(final List<T> items) {
        return of(items).stream().collect( groupingBy(Pair::leftString) );
    }

}