code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.graphs.datastructures;
  
  import java.util.LinkedHashSet;
  import java.util.Objects;
  import java.util.Set;
  
  /**
   * A generic node of a graph.
   * @param <T> the element type of the node
   * @version 2.0.0
   * @since 1.2
   */
  public final class Node<T> {
  
      private final Graph<T> graph;
      private final T content;
      private final Set<Node<T>> neighbours;
  
      /**
       * Constructor.
       * @param content the content of the node
       * @param graph   the graph the node will be a part of
       */
      Node(final T content, final Graph<T> graph) {
          this.content = content;
          this.graph = graph;
          this.neighbours = new LinkedHashSet<>();
      }
  
      /**
       * Adds the given node to the neighbours of this node. Both nodes must be in the same graph.
       * @param o the given node
       */
      void connectTo(final Node<T> o) {
          if (!this.graph.equals(o.graph)) {
              throw new IllegalArgumentException("Cannot connect to nodes of two different graphs.");
          }
          if (this.equals(o)) {
              return;
          }
          this.neighbours.add(o);
      }
  
      /**
       * Removes the given node from the neighbours of this node.
       * @param o the given node
       */
      void disconnectFrom(final Node<T> o) {
          this.neighbours.remove(o);
      }
  
      /**
       * Returns the content of the node.
       * @return the content of the node
       */
      public T content() {
          return this.content;
      }
  
      /**
       * Returns the neighbours of the node.
       * @return the neighbours of the node
       */
      public Set<Node<T>> neighbours() {
          return new LinkedHashSet<>(this.neighbours);
      }
  
      /**
       * Returns the graph to which the node belongs.
       * @return the node's graph
       */
      public Graph<T> graph() {
          return this.graph;
      }
  
      @Override
      public int hashCode() {
          int result = this.graph.hashCode();
          result = 31 * result + (this.content != null ? this.content.hashCode() : 0);
          return result;
      }
  
      @Override
      public boolean equals(final Object o) {
          if (this == o) {
              return true;
          }
          if (o == null || getClass() != o.getClass()) {
              return false;
          }
          final Node<?> node = (Node<?>) o;
          return this.graph.equals(node.graph) && Objects.equals(this.content, node.content);
      }
  
      @Override
      public String toString() {
          final StringBuilder sb = new StringBuilder();
          sb.append("Node{content=").append(this.content).append(", neighbours:");
          for (final Node<T> neighbour : this.neighbours) {
              sb.append(neighbour.content()).append(",");
          }
          sb.deleteCharAt(sb.length() - 1);
          sb.append("}");
          return sb.toString();
      }
  }
  