MockTarget getChild(final String streamTypeName) {
//            if (closed) {
//                throw new RuntimeException("Closed");
//            }

            if (streamTypeName == null) {
                return this;
            }

            return childMap.computeIfAbsent(streamTypeName, k -> {
                final Map<String, ByteArrayOutputStream> typeMap = getOpenOutputStream().get(meta.getId());
                typeMap.put(streamTypeName, new ByteArrayOutputStream());
                return new MockTarget(this, streamTypeName);
            });
        }
<p>
Open a existing stream source.
</p>

@param streamId  The stream id to open a stream source for.
@param anyStatus Used to specify if this method will return stream sources that
                 are logically deleted or locked. If false only unlocked stream
                 sources will be returned, null otherwise.
@return The loaded stream source if it exists (has not been physically
deleted) else null. Also returns null if one exists but is
logically deleted or locked unless <code>anyStatus</code> is
true.
@throws DataException Could be thrown if no volume
stroom-data/stroom-data-store-mock/src/main/java/stroom/data/store/mock/MockStore.java