Attributes.java

public class Attributes {
  private final Map<String, Set<String>> attributes;

  public Attributes() {
    this.attributes = new HashMap<>();
  }

  public boolean attributeExists(String attributeName) {
    return attributes.containsKey(attributeName);
  }

  public Set<String> values(String attributeName) {
    return attributes.get(attributeName);
  }
  
  public String getSingleValue(String attributeName) {
    return values(attributeName).iterator().next();
  }

  public Attributes addAttribute(String attributeName, Collection<String> values) {
    this.attributes.put(attributeName, new HashSet<>(values));
    return this;
  }
}