Create GraphML XML files that can be used in the yEd Graph Editor

import com.tinkerpop.blueprints.pgm.Vertex;
import com.tinkerpop.blueprints.pgm.impls.tg.TinkerGraph;
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
 
public class Main {
 
  public static void main(String[] args) throws Exception {
    OutputStream out = new FileOutputStream(new File("./test.graphml"));
    TinkerGraph graph = new TinkerGraph();
 
    Vertex source = graph.addVertex("1");
    Vertex target = graph.addVertex("2");
    graph.addEdge("3", source, target, "connection");
 
    YedFileWriter writer = new YedFileWriter(graph);
    writer.outputGraph(out);
  }
}

import com.tinkerpop.blueprints.pgm.Edge;
import com.tinkerpop.blueprints.pgm.Graph;
import com.tinkerpop.blueprints.pgm.Vertex;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.util.Iterator;
 
/**
 * YedFileWriter writes a Graph for the yEd Graph Editor to a GraphML OutputStream.
 * 
 * @author Benny Neugebauer (https://www.bennyn.de)
 */
public class YedFileWriter {
 
  private Graph graph = null;
  private String xml = null;
 
  public YedFileWriter(Graph graph) {
    this.graph = graph;
  }
 
  private String getGraphMLHeader() {
    String header = "<?xml version=\"1.0\" ?>";
    header += "\r\n<graphml\r\n  xmlns=\"http://graphml.graphdrawing.org/xmlns\"\r\n  xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"\r\n  xmlns:y=\"http://www.yworks.com/xml/graphml\"\r\n  xmlns:yed=\"http://www.yworks.com/xml/yed/3\"\r\n  xsi:schemaLocation=\"http://graphml.graphdrawing.org/xmlns\r\n  http://www.yworks.com/xml/schema/graphml/1.1/ygraphml.xsd\"\r\n>";
    header += "\r\n  <key for=\"node\" id=\"d5\" attr.name=\"description\" attr.type=\"string\" />";
    header += "\r\n  <key for=\"node\" id=\"d6\" yfiles.type=\"nodegraphics\"/>";
    header += "\r\n  <graph id=\"G\" edgedefault=\"directed\">";
    return header;
  }
 
  private String getGraphMLFooter() {
    String footer = "\r\n  </graph>\r\n</graphml>";
    return footer;
  }
 
  private String getNode(String id) {
    String node = "\r\n    <node id=\"" + id + "\">";
    node += "\r\n      <data key=\"d5\"/>";
    node += "\r\n      <data key=\"d6\">";
    node += "\r\n        <y:ShapeNode>";
    node += "\r\n          <y:NodeLabel>" + id + "</y:NodeLabel>";
    node += "\r\n          <y:Shape type=\"rectangle\"/>";
    node += "\r\n        </y:ShapeNode>";
    node += "\r\n      </data>";
    node += "\r\n    </node>";
    return node;
  }
 
  private String getEdge(Edge edge) {
    Vertex source = edge.getOutVertex();
    Vertex target = edge.getInVertex();
    String edgeId = (String) edge.getId();
    String sourceId = (String) source.getId();
    String targetId = (String) target.getId();
    String label = edge.getLabel();
 
    String edgeXml = "\r\n    <edge id=\"" + edgeId + "\" source=\"" + sourceId + "\" target=\"" + targetId + "\" label=\"" + label + "\">";
    edgeXml += "\r\n    </edge>";
 
    return edgeXml;
  }
 
  private void createGraphXml() {
    xml = getGraphMLHeader();
 
    // Create nodes
    Iterable<Vertex> vertices = graph.getVertices();
    Iterator<Vertex> verticesIterator = vertices.iterator();
    while (verticesIterator.hasNext()) {
      Vertex vertex = verticesIterator.next();
      String id = (String) vertex.getId();
      String node = getNode(id);
      xml += node;
    }
    // Create edges
    Iterable<Edge> edges = graph.getEdges();
    Iterator<Edge> edgesIterator = edges.iterator();
    while (edgesIterator.hasNext()) {
      Edge edge = edgesIterator.next();
      String edgeXml = getEdge(edge);
      xml += edgeXml;
    }
 
    xml += getGraphMLFooter();
  }
 
  public void outputGraph(final OutputStream out) throws IOException {
    createGraphXml();
    System.out.println(xml);
    try (BufferedWriter br = new BufferedWriter(new OutputStreamWriter(out))) {
      br.write(xml);
      br.flush();
    }
    out.flush();
    out.close();
  }
}