@Override
  protected Map<StoreKey, StoreKey> convertKeys(Collection<? extends StoreKey> input) throws Exception {
    Map<StoreKey, StoreKey> output = new HashMap<>();
    input.forEach(inKey -> {
        StoreKey replaceMent = invalids.contains(inKey) ? null : new MockId(
            inKey.getID().substring(0, inKey.getID().length() / 2) + Integer.toString(RANDOM.nextInt(1000)));
        output.put(inKey, replaceMent);
    });
    return output;
  }
Add invalid mappings.
@param invalids the keys for which no mapping will be generated during conversion.
ambry-messageformat/src/test/java/com/github/ambry/messageformat/MessageSievingInputStreamTest.java