001/* 002 * Copyright (C) 2016 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.Optional; 021import java.util.Set; 022import org.checkerframework.checker.nullness.qual.Nullable; 023 024/** 025 * An interface for <a 026 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data, 027 * whose edges have associated non-unique values. 028 * 029 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes. 030 * 031 * <p>There are three primary interfaces provided to represent graphs. In order of increasing 032 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally 033 * prefer the simplest interface that satisfies your use case. See the <a 034 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type"> 035 * "Choosing the right graph type"</a> section of the Guava User Guide for more details. 036 * 037 * <h3>Capabilities</h3> 038 * 039 * <p>{@code ValueGraph} supports the following use cases (<a 040 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of 041 * terms</a>): 042 * 043 * <ul> 044 * <li>directed graphs 045 * <li>undirected graphs 046 * <li>graphs that do/don't allow self-loops 047 * <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered 048 * <li>graphs whose edges have associated values 049 * </ul> 050 * 051 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges, 052 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use 053 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of 054 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your 055 * interpretation of the edge value as its multiplicity.) 056 * 057 * <h3>Building a {@code ValueGraph}</h3> 058 * 059 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To 060 * create an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link 061 * ValueGraphBuilder} class: 062 * 063 * <pre>{@code 064 * MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build(); 065 * }</pre> 066 * 067 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a 068 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If 069 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on 070 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link 071 * ImmutableValueGraph}. 072 * 073 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link 074 * ImmutableValueGraph#copyOf(ValueGraph)}: 075 * 076 * <pre>{@code 077 * ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph); 078 * }</pre> 079 * 080 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph} 081 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe. 082 * 083 * <p>The Guava User Guide has <a 084 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more 085 * information on (and examples of) building graphs</a>. 086 * 087 * <h3>Additional documentation</h3> 088 * 089 * <p>See the Guava User Guide for the {@code common.graph} package (<a 090 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for 091 * additional documentation, including: 092 * 093 * <ul> 094 * <li><a 095 * href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence"> 096 * {@code equals()}, {@code hashCode()}, and graph equivalence</a> 097 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization"> 098 * Synchronization policy</a> 099 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes 100 * for implementors</a> 101 * </ul> 102 * 103 * @author James Sexton 104 * @author Joshua O'Madadhain 105 * @param <N> Node parameter type 106 * @param <V> Value parameter type 107 * @since 20.0 108 */ 109@Beta 110public interface ValueGraph<N, V> extends BaseGraph<N> { 111 // 112 // ValueGraph-level accessors 113 // 114 115 /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ 116 @Override 117 Set<N> nodes(); 118 119 /** Returns all edges in this graph. */ 120 @Override 121 Set<EndpointPair<N>> edges(); 122 123 /** 124 * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an 125 * edge connecting node A to node B if this {@link ValueGraph} has an edge connecting A to B. 126 */ 127 Graph<N> asGraph(); 128 129 // 130 // ValueGraph properties 131 // 132 133 /** 134 * Returns true if the edges in this graph are directed. Directed edges connect a {@link 135 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 136 * undirected edges connect a pair of nodes to each other. 137 */ 138 @Override 139 boolean isDirected(); 140 141 /** 142 * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting 143 * to add a self-loop to a graph that does not allow them will throw an {@link 144 * IllegalArgumentException}. 145 */ 146 @Override 147 boolean allowsSelfLoops(); 148 149 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 150 @Override 151 ElementOrder<N> nodeOrder(); 152 153 // 154 // Element-level accessors 155 // 156 157 /** 158 * Returns the nodes which have an incident edge in common with {@code node} in this graph. 159 * 160 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 161 * 162 * @throws IllegalArgumentException if {@code node} is not an element of this graph 163 */ 164 @Override 165 Set<N> adjacentNodes(N node); 166 167 /** 168 * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing 169 * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 170 * 171 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 172 * 173 * @throws IllegalArgumentException if {@code node} is not an element of this graph 174 */ 175 @Override 176 Set<N> predecessors(N node); 177 178 /** 179 * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing 180 * {@code node}'s outgoing edges in the direction (if any) of the edge. 181 * 182 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 183 * 184 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 185 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 186 * 187 * @throws IllegalArgumentException if {@code node} is not an element of this graph 188 */ 189 @Override 190 Set<N> successors(N node); 191 192 /** 193 * Returns the edges in this graph whose endpoints include {@code node}. 194 * 195 * <p>This is equal to the union of incoming and outgoing edges. 196 * 197 * @throws IllegalArgumentException if {@code node} is not an element of this graph 198 * @since 24.0 199 */ 200 @Override 201 Set<EndpointPair<N>> incidentEdges(N node); 202 203 /** 204 * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, 205 * the number of times an edge touches {@code node}). 206 * 207 * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. 208 * 209 * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of 210 * self-loops incident to {@code node}). 211 * 212 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 213 * 214 * @throws IllegalArgumentException if {@code node} is not an element of this graph 215 */ 216 @Override 217 int degree(N node); 218 219 /** 220 * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) 221 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 222 * 223 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 224 * 225 * @throws IllegalArgumentException if {@code node} is not an element of this graph 226 */ 227 @Override 228 int inDegree(N node); 229 230 /** 231 * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) 232 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 233 * 234 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 235 * 236 * @throws IllegalArgumentException if {@code node} is not an element of this graph 237 */ 238 @Override 239 int outDegree(N node); 240 241 /** 242 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 243 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. 244 * 245 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 246 * 247 * @since 23.0 248 */ 249 @Override 250 boolean hasEdgeConnecting(N nodeU, N nodeV); 251 252 /** 253 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 254 * any, specified by {@code endpoints}). This is equivalent to {@code 255 * edges().contains(endpoints)}. 256 * 257 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 258 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 259 * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally 260 * throw if the object cannot be present in the collection), and the desire to have this method's 261 * behavior be compatible with {@code edges().contains(endpoints)}. 262 * 263 * @since 27.1 264 */ 265 @Override 266 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 267 268 /** 269 * Returns the value of the edge that connects {@code nodeU} to {@code nodeV} (in the order, if 270 * any, specified by {@code endpoints}), if one is present; otherwise, returns {@code 271 * Optional.empty()}. 272 * 273 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 274 * graph 275 * @since 23.0 (since 20.0 with return type {@code V}) 276 */ 277 Optional<V> edgeValue(N nodeU, N nodeV); 278 279 /** 280 * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified 281 * by {@code endpoints}), if one is present; otherwise, returns {@code Optional.empty()}. 282 * 283 * <p>If this graph is directed, the endpoints must be ordered. 284 * 285 * @throws IllegalArgumentException if either endpoint is not an element of this graph 286 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 287 * @since 27.1 288 */ 289 Optional<V> edgeValue(EndpointPair<N> endpoints); 290 291 /** 292 * Returns the value of the edge that connects {@code nodeU} to {@code nodeV}, if one is present; 293 * otherwise, returns {@code defaultValue}. 294 * 295 * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU, 296 * defaultValue)}. 297 * 298 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 299 * graph 300 */ 301 @Nullable 302 V edgeValueOrDefault(N nodeU, N nodeV, @Nullable V defaultValue); 303 304 /** 305 * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified 306 * by {@code endpoints}), if one is present; otherwise, returns {@code defaultValue}. 307 * 308 * <p>If this graph is directed, the endpoints must be ordered. 309 * 310 * @throws IllegalArgumentException if either endpoint is not an element of this graph 311 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 312 * @since 27.1 313 */ 314 @Nullable 315 V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue); 316 317 // 318 // ValueGraph identity 319 // 320 321 /** 322 * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and 323 * the same structural relationships as those in this graph. 324 * 325 * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true: 326 * 327 * <ul> 328 * <li>A and B have equal {@link #isDirected() directedness}. 329 * <li>A and B have equal {@link #nodes() node sets}. 330 * <li>A and B have equal {@link #edges() edge sets}. 331 * <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B. 332 * </ul> 333 * 334 * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 335 * For example, two graphs may be considered equal even if one allows self-loops and the other 336 * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order 337 * in which they are iterated over, are irrelevant. 338 * 339 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}. 340 */ 341 @Override 342 boolean equals(@Nullable Object object); 343 344 /** 345 * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a 346 * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object) 347 * edge value}. 348 * 349 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}. 350 */ 351 @Override 352 int hashCode(); 353}