Coding in the small with Google Collections: Sets.union, intersection and difference

Part 16 in a Series.

The traditional approach to unions is to first create a new Set, and then to addAll using each component set. You can use a similar approach to do differences and intersections.

Before:

  private static final ImmutableSet<String> LEGAL_PARAMETERS;
  static {
    Set<String> tmp = new HashSet<String>();
    tmp.addAll(REQUIRED_PARAMETERS);
    tmp.addAll(OPTIONAL_PARAMETERS);
    LEGAL_PARAMETERS = ImmutableSet.copyOf(tmp);
  }

  public void login(Map<String, String> params) {
    if (!LEGAL_PARAMETERS.containsAll(params.keySet())) {
      Set<String> unrecognized = new HashSet<String>(params.keySet());
      unrecognized.removeAll(LEGAL_PARAMETERS);
      throw new IllegalArgumentException("Unrecognized parameters: "
          + unrecognized);
    }

    if (!params.keySet().containsAll(REQUIRED_PARAMETERS)) {
      Set<String> missing = new HashSet<String>(REQUIRED_PARAMETERS);
      missing.removeAll(params.keySet());
      throw new IllegalArgumentException("Missing parameters: " + missing);
    }

    ...
  }

Google Collections has methods that do set arithmetic in a single line.

After:

  private static final ImmutableSet<String> LEGAL_PARAMETERS
      = Sets.union(REQUIRED_PARAMETERS, OPTIONAL_PARAMETERS).immutableCopy();

  public void login(Map<String, String> requestParameters) {
    if (!LEGAL_PARAMETERS.containsAll(requestParameters.keySet())) {
      throw new IllegalArgumentException("Unrecognized parameters: "
          + Sets.difference(requestParameters.keySet(), LEGAL_PARAMETERS));
    }

    if (!requestParameters.keySet().containsAll(REQUIRED_PARAMETERS)) {
      throw new IllegalArgumentException("Missing parameters: "
          + Sets.difference(REQUIRED_PARAMETERS, requestParameters.keySet()));
    }

    ...
  }

Unlike the traditional approach, these methods don't do any copies! Instead, they return views that delegate to the provided sets. In the occasional case when the copy is worthwhile, there's a handy method immutableCopy to give you one.

# posted by Jesse Wilson on Monday, August 11, 2008