fixed #39

src/main/java/kieker/analysis/util/graph/export/dot/DotElementExporter.java

 package kieker.analysis.util.graph.export.dot;
 
 import java.io.IOException;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.function.Function;
-import java.util.stream.Collectors;
 
 import kieker.analysis.util.graph.Direction;
 import kieker.analysis.util.graph.Edge;
@@ -13,6 +13,8 @@ import kieker.analysis.util.graph.Vertex;
 import kieker.analysis.util.graph.export.AbstractTransformer;
 import kieker.analysis.util.graph.util.dot.DotGraphWriter;
 import kieker.analysis.util.graph.util.dot.attributes.DotClusterAttribute;
+import kieker.analysis.util.graph.util.dot.attributes.DotEdgeAttribute;
+import kieker.analysis.util.graph.util.dot.attributes.DotNodeAttribute;
 
 class DotElementExporter extends AbstractTransformer<Void> {
 
@@ -42,7 +44,7 @@ class DotElementExporter extends AbstractTransformer<Void> {
 
 				dotGraphWriter.addClusterStop();
 			} else {
-				// dotGraphWriter.addNode(vertex.getId().toString(), getAttributes(vertex));
+				dotGraphWriter.addNode(vertex.getId().toString(), getAttributes(vertex));
 			}
 		} catch (IOException e) {
 			handleIOException(e);
@@ -67,19 +69,25 @@ class DotElementExporter extends AbstractTransformer<Void> {
 	}
 
 	protected Map<String, String> getAttributes(final Edge edge) {
-		// System.out.println("Hi!" + configuration.getEdgeAttributes().entrySet().stream().collect(Collectors.toMap(
-		// e -> e.getKey().toString(),
-		// e -> null)));
-		// return null; // TODO temp
-		return configuration.getEdgeAttributes().entrySet().stream().collect(Collectors.toMap(
-				e -> e.getKey().toString(),
-				e -> e.getValue().apply(edge)));
+		final Map<String, String> attributes = new HashMap<>();
+		for (Entry<DotEdgeAttribute, Function<Edge, String>> entry : configuration.getEdgeAttributes().entrySet()) {
+			final String value = entry.getValue().apply(edge);
+			if (value != null) {
+				attributes.put(entry.getKey().toString(), value);
+			}
+		}
+		return attributes;
 	}
 
 	protected Map<String, String> getAttributes(final Vertex vertex) {
-		return configuration.getNodeAttributes().entrySet().stream().collect(Collectors.toMap(
-				e -> e.getKey().toString(),
-				e -> e.getValue().apply(vertex)));
+		final Map<String, String> attributes = new HashMap<>();
+		for (Entry<DotNodeAttribute, Function<Vertex, String>> entry : configuration.getNodeAttributes().entrySet()) {
+			final String value = entry.getValue().apply(vertex);
+			if (value != null) {
+				attributes.put(entry.getKey().toString(), value);
+			}
+		}
+		return attributes;
 	}
 
 	@Override