001/*
002 * Copyright (C) 2014 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.graph;
018
019import com.google.common.annotations.Beta;
020import java.util.Set;
021import org.checkerframework.checker.nullness.compatqual.NullableDecl;
022
023/**
024 * An interface for <a
025 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data,
026 * whose edges are anonymous entities with no identity or information of their own.
027 *
028 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
029 *
030 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
031 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
032 * prefer the simplest interface that satisfies your use case. See the <a
033 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
034 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
035 *
036 * <h3>Capabilities</h3>
037 *
038 * <p>{@code Graph} supports the following use cases (<a
039 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
040 * terms</a>):
041 *
042 * <ul>
043 *   <li>directed graphs
044 *   <li>undirected graphs
045 *   <li>graphs that do/don't allow self-loops
046 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
047 * </ul>
048 *
049 * <p>{@code Graph} explicitly does not support parallel edges, and forbids implementations or
050 * extensions with parallel edges. If you need parallel edges, use {@link Network}.
051 *
052 * <h3>Building a {@code Graph}</h3>
053 *
054 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
055 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link
056 * GraphBuilder} class:
057 *
058 * <pre>{@code
059 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
060 * }</pre>
061 *
062 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
063 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not
064 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph),
065 * you should use the non-mutating {@link Graph} interface, or an {@link ImmutableGraph}.
066 *
067 * <p>You can create an immutable copy of an existing {@code Graph} using {@link
068 * ImmutableGraph#copyOf(Graph)}:
069 *
070 * <pre>{@code
071 * ImmutableGraph<Integer> immutableGraph = ImmutableGraph.copyOf(graph);
072 * }</pre>
073 *
074 * <p>Instances of {@link ImmutableGraph} do not implement {@link MutableGraph} (obviously!) and are
075 * contractually guaranteed to be unmodifiable and thread-safe.
076 *
077 * <p>The Guava User Guide has <a
078 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
079 * information on (and examples of) building graphs</a>.
080 *
081 * <h3>Additional documentation</h3>
082 *
083 * <p>See the Guava User Guide for the {@code common.graph} package (<a
084 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
085 * additional documentation, including:
086 *
087 * <ul>
088 *   <li><a
089 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
090 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
091 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
092 *       Synchronization policy</a>
093 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
094 *       for implementors</a>
095 * </ul>
096 *
097 * @author James Sexton
098 * @author Joshua O'Madadhain
099 * @param <N> Node parameter type
100 * @since 20.0
101 */
102@Beta
103public interface Graph<N> extends BaseGraph<N> {
104  //
105  // Graph-level accessors
106  //
107
108  /** {@inheritDoc} */
109  @Override
110  Set<N> nodes();
111
112  /** {@inheritDoc} */
113  @Override
114  Set<EndpointPair<N>> edges();
115
116  //
117  // Graph properties
118  //
119
120  /** {@inheritDoc} */
121  @Override
122  boolean isDirected();
123
124  /** {@inheritDoc} */
125  @Override
126  boolean allowsSelfLoops();
127
128  /** {@inheritDoc} */
129  @Override
130  ElementOrder<N> nodeOrder();
131
132  //
133  // Element-level accessors
134  //
135
136  /** {@inheritDoc} */
137  @Override
138  Set<N> adjacentNodes(N node);
139
140  /** {@inheritDoc} */
141  @Override
142  Set<N> predecessors(N node);
143
144  /** {@inheritDoc} */
145  @Override
146  Set<N> successors(N node);
147
148  /** {@inheritDoc} */
149  @Override
150  int degree(N node);
151
152  /** {@inheritDoc} */
153  @Override
154  int inDegree(N node);
155
156  /** {@inheritDoc} */
157  @Override
158  int outDegree(N node);
159
160  /** {@inheritDoc} */
161  @Override
162  boolean hasEdgeConnecting(N nodeU, N nodeV);
163
164  //
165  // Graph identity
166  //
167
168  /**
169   * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the
170   * same structural relationships as those in this graph.
171   *
172   * <p>Thus, two graphs A and B are equal if <b>all</b> of the following are true:
173   *
174   * <ul>
175   *   <li>A and B have equal {@link #isDirected() directedness}.
176   *   <li>A and B have equal {@link #nodes() node sets}.
177   *   <li>A and B have equal {@link #edges() edge sets}.
178   * </ul>
179   *
180   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
181   * For example, two graphs may be considered equal even if one allows self-loops and the other
182   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
183   * in which they are iterated over, are irrelevant.
184   *
185   * <p>A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}.
186   */
187  @Override
188  boolean equals(@NullableDecl Object object);
189
190  /**
191   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of
192   * the set returned by {@link #edges()}.
193   *
194   * <p>A reference implementation of this is provided by {@link AbstractGraph#hashCode()}.
195   */
196  @Override
197  int hashCode();
198}