Find a relation between nodes

Connection.java

package graph;

import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Consumer;

public class Connection {

  public static void main(String[] args) {
    Connection c = new Connection();
    Set<Node> nodes = c.findConnections(c.createInput());
    c.printRelations(nodes);
  }

  private void printRelations(Set<Node> nodes) {
    List<Node> relations = new ArrayList<>();
    for (Node node : nodes) {
      if (!node.isVisited) {
        if (relations.isEmpty()) {
          // start new relation.
          preOrder(node, relations);
        } else {
          // print relation and clear it.
          System.out.println("Relation: " + relations);
          relations.clear();
          preOrder(node, relations);
        }
      }
    }
    if (!relations.isEmpty()) {
      System.out.println("Relation: " + relations);
      relations.clear();
    }
  }

  private void preOrder(Node node, List<Node> relations) {
    if (relations != null && node != null && !node.isVisited) {
      node.isVisited = true;
      relations.add(node);
      for (Node adjNode : node.getAdjacentNodes()) {
        preOrder(adjNode, relations);
      }
    }
  }

  private Set<Node> findConnections(JsonArray json) {
    final Set<Node> result = new HashSet<>();
    for (JsonElement aJson : json) {
      JsonObject row = (JsonObject) aJson;
      row.entrySet().forEach(new Consumer<Map.Entry<String, JsonElement>>() {
        @Override
        public void accept(Map.Entry<String, JsonElement> rowEntry) {
          final Node node = getNodeInstance(rowEntry.getKey(), result);
          result.add(node);

          for (JsonElement adjElement : rowEntry.getValue().getAsJsonArray()) {
            Node adjNode = getNodeInstance(adjElement.getAsString(), result);
            node.addAdjacentNode(adjNode);
            result.add(adjNode);
          }
        }
      });
    }
    return result;
  }

  private JsonArray createInput() {
    JsonArray array = new JsonArray();
    array.add(createNode("A", createAdjacentNodes("B", "C")));
    array.add(createNode("B", createAdjacentNodes("D")));
    array.add(createNode("C", createAdjacentNodes("D")));
    array.add(createNode("D", createAdjacentNodes()));
    array.add(createNode("E", createAdjacentNodes("F", "H")));
    array.add(createNode("F", createAdjacentNodes()));
    array.add(createNode("G", createAdjacentNodes()));
    array.add(createNode("H", createAdjacentNodes()));
    System.out.println(array);
    return array;
  }

  private JsonObject createNode(String item, JsonArray adjacentNodes) {
    JsonObject node = new JsonObject();
    node.add(item, adjacentNodes);
    return node;
  }

  private JsonArray createAdjacentNodes(String... item) {
    JsonArray array = new JsonArray();
    for (String i : item) {
      array.add(i);
    }
    return array;
  }

  private Node getNodeInstance(String data, Set<Node> list) {
    if (list.isEmpty()) {
      return new Node(data);
    }

    for (Node tmp : list) {
      if (tmp.data.equals(data)) {
        return tmp;
      }
    }
    return new Node(data);
  }
}