Review

class RAMDirectory {
  /** Creates a new, empty file in the directory with the given name. Returns a stream writing this file. */
  public IndexOutput createOutput(String name) throws IOException {
    ensureOpen();
    RAMFile file = new RAMFile(this);
    synchronized (this) {
      RAMFile existing = (RAMFile)fileMap.get(name);
      if (existing!=null) {
        sizeInBytes -= existing.sizeInBytes;
        existing.directory = null;
      }
      fileMap.put(name, file);
    }
    return new RAMOutputStream(file);
  }

}