@XmlRootElement(name = "relation")
public class Relation {

    private String type, with;

    // Keep track of which relation should be marked with dashed lines:
    public static String[] dashed = new String[]{"realization", "dependency", "implementation"};

    // Hashmap with the different kinds of symbols:

    public static final Map<String, Relation> SYMBOLS;
    static {
        Map<String, RelationSymbol> out = new HashMap<>();
        out.put("inheritance",new FullArrow());
        SYMBOLS = Collections.unmodifiableMap(out);
    }

  // Get the error:
Error:(33, 46) java: incompatible types: inference variable V has incompatible bounds
    equality constraints: UML.Diagrams.Relation
    lower bounds: UML.RelationDrawings.RelationSymbol