Java Stream: How to Sum Value If Element Already Exists

Given the following key…

private class PositionKey {
    String assetId;
    String currencyId;
}

… I need to create a Map<PositionKey, BigDecimal>, where if a given key does not exist, then a new entry shall be added, otherwise, if a given key already exists, then its values shall be increased. Here below is my attempt:

public Map<PositionKey, BigDecimal> getMap(final Long portfolioId) {

    final Map<PositionKey, BigDecimal> map = new HashMap<>();

    // getPositions() returns all the positions associated with the given portfolio
    return getPositions(portfolioId).stream()
        .collect(
            toMap(
                position ->
                    new PositionKey(
                        position.getAssetId(),
                        position.geCurrencyId()),
                position -> position.geValue(),
                (newValue, oldValue) -> oldValue != null ? oldValue.add(newValue) : newValue,
                () -> map));
}

What I dislike in my implementation above, is the fact that I create the Map before streaming the positions. Is there a better way to implement this? Thank you very much.

>Solution :

simple! replace () -> map with either () -> new HashMap<>() or HashMap::new