java cross join list

java-Cross join list

<p>Бывает так, что необходимо сделать cross join данных из List<T>, основываясь на подсказке со <a href="http://stackoverflow.com/questions/38568550/cross-join-two-lists-java">stackoverflow</a>, специально для такого случая я написал пару классов:</p>
[code:]LANG[:code]import java.lang.reflect.Array;
import java.util.List;
import java.util.Objects;
import java.util.logging.Logger;
import java.util.stream.Collectors;

/**
 * Делаем cross join списков
 * <p>
 * @author ezhov_da
 */
public class CrossJoinList<T>
{
    private static final Logger LOG = Logger.getLogger(CrossJoinList.class.getName());
    private static CrossJoinList listCrossJoin;

    public static CrossJoinList getInstance()
    {
        if (Objects.isNull(listCrossJoin))
        {
            listCrossJoin = new CrossJoinList();
        }
        return listCrossJoin;
    }

    public List<ObjectCrossJoin<T>> crossJoinTwoList(Class<T> clazz, List<T> listOne, List<T> listTwo)
    {
        return listOne
                .stream()
                .flatMap(t
                        -> listTwo
                        .stream()
                        .map(t1 ->
                                {
                                    T[] array = (T[]) Array.newInstance(clazz, 2);
                                    array[0] = t;
                                    array[1] = t1;
                                    return new ObjectCrossJoin<T>(array);
                        }
                        )
                ).collect(Collectors.toList());
    }


    List<ObjectCrossJoin<T>> objectCrossJoins;

    public synchronized List<ObjectCrossJoin<T>> executeCrossJoin(Class<T> clazz, List<T>... listsForJoin)
    {
        switch (listsForJoin.length)
        {
            case 0:
            case 1:
                throw new IllegalArgumentException("Not use for join one list. Min two list.");
            case 2:
                return crossJoinTwoList(clazz, listsForJoin[0], listsForJoin[1]);
            default:
                //сделано для того, чтоб cross join проводился в последовательности следования массива
                List<T> listOne = listsForJoin[listsForJoin.length - 2];
                List<T> listTwo = listsForJoin[listsForJoin.length - 1];
                objectCrossJoins = crossJoinTwoList(clazz, listOne, listTwo);
                for (int i = listsForJoin.length - 3; i >= 0; i--)
                {
                    objectCrossJoins = listsForJoin[i]
                            .stream()
                            .flatMap(t
                                    -> objectCrossJoins
                                    .stream()
                                    .map((ObjectCrossJoin<T> t1) ->
                                            {
                                                T[] array = t1.getArrayCrossJoin();
                                                final T[] arrayResult = (T[]) Array.newInstance(clazz, array.length + 1);
                                                arrayResult[0] = t;
                                                for (int i1 = 1; i1 < arrayResult.length; i1++)
                                                {
                                                    arrayResult[i1] = array[i1 - 1];
                                                }
                                                ObjectCrossJoin<T> objectCrossJoin = new ObjectCrossJoin<>(arrayResult);
                                                return objectCrossJoin;
                                    })
                            ).collect(Collectors.toList());
                }
                return objectCrossJoins;
        }
    }
}



import java.util.logging.Logger;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Объект для замножения
 * <p>
 * @author ezhov_da
 */
public class ObjectCrossJoin<T>
{
    private static final Logger LOG = Logger.getLogger(ObjectCrossJoin.class.getName());
    private T[] arrayCrossJoin;

    public ObjectCrossJoin(T... arrayCrossJoin)
    {
        this.arrayCrossJoin = arrayCrossJoin;
    }

    public T[] getArrayCrossJoin()
    {
        return arrayCrossJoin;
    }

    @Override
    public String toString()
    {
        return Stream.of(arrayCrossJoin).map(t -> t.toString()).collect(Collectors.joining(","));
    }


}

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