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.LinkedHashMap;
  import java.util.LinkedHashSet;
  import java.util.Map;
  import java.util.Set;
  
  /**
   * A generic graph.  Two graphs are only equal if they really are the same object.
   * @param <T> the element type of the graph's nodes
   * @version 2.0.0
   * @since 1.2
   */
  public final class Graph<T> {
  
      private final String name;
      private final Map<T, Node<T>> nodes;
  
      /**
       * Constructor for a new graph.
       */
      public Graph() {
          this("");
      }
  
      /**
       * Constructor for a new graph.
       * @param name the name of the graph
       */
      public Graph(final String name) {
          this.nodes = new LinkedHashMap<>();
          this.name = name;
      }
  
      /**
       * Returns the node with a given content. If such a node does not exist, it is created and added to the graph.
       * @param content the given node content
       * @return the node with the given content
       */
      public Node<T> node(final T content) {
          final Node<T> search = this.nodes.get(content);
          if (search != null) {
              return search;
          }
          final Node<T> n = new Node<>(content, this);
          this.nodes.put(content, n);
          return n;
      }
  
      /**
       * Returns all nodes of the graph.
       * @return a set containing all nodes of the graph
       */
      public Set<Node<T>> nodes() {
          return new LinkedHashSet<>(this.nodes.values());
      }
  
      /**
       * Adds an edge between two given nodes, which must both belong to this graph. (Does nothing if the nodes are already connected)
       * @param o the first given node
       * @param t the second given node
       */
      public void connect(final Node<T> o, final Node<T> t) {
          if (!o.equals(t)) {
              o.connectTo(t);
              t.connectTo(o);
          }
      }
  
      /**
       * Removes the edge between two given nodes. (Does nothing if the nodes are not connected)
       * @param o the first given node
       * @param t the second given node
       */
      public void disconnect(final Node<T> o, final Node<T> t) {
          if (!o.equals(t)) {
              o.disconnectFrom(t);
              t.disconnectFrom(o);
          }
      }
  
      /**
       * Returns the name of the graph.
       * @return the name of the graph
       */
      public String name() {
          return this.name;
      }
  
      @Override
      public String toString() {
          return "Graph{" +
                  "name='" + this.name + '\'' +
                  ", nodes=" + this.nodes.values() +
                  '}';
      }
  }
  