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.Set; 021import org.checkerframework.checker.nullness.qual.Nullable; 022 023/** 024 * An interface for <a 025 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data, 026 * whose edges have associated non-unique values. 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 ValueGraph} 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 * <li>graphs whose edges have associated values 048 * </ul> 049 * 050 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges, 051 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use 052 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of 053 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your 054 * interpretation of the edge value as its multiplicity.) 055 * 056 * <h3>Building a {@code ValueGraph}</h3> 057 * 058 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To 059 * create an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link 060 * ValueGraphBuilder} class: 061 * 062 * <pre>{@code 063 * MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build(); 064 * }</pre> 065 * 066 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a 067 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If 068 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on 069 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link 070 * ImmutableValueGraph}. 071 * 072 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link 073 * ImmutableValueGraph#copyOf(ValueGraph)}: 074 * 075 * <pre>{@code 076 * ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph); 077 * }</pre> 078 * 079 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph} 080 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe. 081 * 082 * <p>The Guava User Guide has <a 083 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more 084 * information on (and examples of) building graphs</a>. 085 * 086 * <h3>Additional documentation</h3> 087 * 088 * <p>See the Guava User Guide for the {@code common.graph} package (<a 089 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for 090 * additional documentation, including: 091 * 092 * <ul> 093 * <li><a 094 * href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence"> 095 * {@code equals()}, {@code hashCode()}, and graph equivalence</a> 096 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization"> 097 * Synchronization policy</a> 098 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes 099 * for implementors</a> 100 * </ul> 101 * 102 * @author James Sexton 103 * @author Joshua O'Madadhain 104 * @param <N> Node parameter type 105 * @param <V> Value parameter type 106 * @since 20.0 107 */ 108@Beta 109public interface ValueGraph<N, V> extends BaseGraph<N> { 110 // 111 // ValueGraph-level accessors 112 // 113 114 /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ 115 @Override 116 Set<N> nodes(); 117 118 /** Returns all edges in this graph. */ 119 @Override 120 Set<EndpointPair<N>> edges(); 121 122 /** 123 * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an 124 * edge connecting node A to node B if this {@link ValueGraph} has an edge connecting A to B. 125 */ 126 Graph<N> asGraph(); 127 128 // 129 // ValueGraph properties 130 // 131 132 /** 133 * Returns true if the edges in this graph are directed. Directed edges connect a {@link 134 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 135 * undirected edges connect a pair of nodes to each other. 136 */ 137 @Override 138 boolean isDirected(); 139 140 /** 141 * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting 142 * to add a self-loop to a graph that does not allow them will throw an {@link 143 * IllegalArgumentException}. 144 */ 145 @Override 146 boolean allowsSelfLoops(); 147 148 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 149 @Override 150 ElementOrder<N> nodeOrder(); 151 152 /** 153 * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of 154 * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link 155 * #successors(Object)} and {@link #incidentEdges(Object)}. 156 * 157 * @since 29.0 158 */ 159 @Override 160 ElementOrder<N> incidentEdgeOrder(); 161 162 // 163 // Element-level accessors 164 // 165 166 /** 167 * Returns a live view of the nodes which have an incident edge in common with {@code node} in 168 * this graph. 169 * 170 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 171 * 172 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 173 * {@code view} returned by this method will be invalidated, and will throw {@code 174 * IllegalStateException} if it is accessed in any way, with the following exceptions: 175 * 176 * <ul> 177 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 178 * expression involving {@code view} will throw) 179 * <li>{@code hashCode()} does not throw 180 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 181 * behavior is undefined 182 * </ul> 183 * 184 * @throws IllegalArgumentException if {@code node} is not an element of this graph 185 */ 186 @Override 187 Set<N> adjacentNodes(N node); 188 189 /** 190 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 191 * traversing {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 192 * 193 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 194 * 195 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 196 * returned by this method will be invalidated, and will throw {@code IllegalStateException} if it 197 * is accessed in any way. 198 * 199 * @throws IllegalArgumentException if {@code node} is not an element of this graph 200 */ 201 @Override 202 Set<N> predecessors(N node); 203 204 /** 205 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 206 * traversing {@code node}'s outgoing edges in the direction (if any) of the edge. 207 * 208 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 209 * 210 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 211 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 212 * 213 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 214 * {@code view} returned by this method will be invalidated, and will throw {@code 215 * IllegalStateException} if it is accessed in any way, with the following exceptions: 216 * 217 * <ul> 218 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 219 * expression involving {@code view} will throw) 220 * <li>{@code hashCode()} does not throw 221 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 222 * behavior is undefined 223 * </ul> 224 * 225 * @throws IllegalArgumentException if {@code node} is not an element of this graph 226 */ 227 @Override 228 Set<N> successors(N node); 229 230 /** 231 * Returns a live view of the edges in this graph whose endpoints include {@code node}. 232 * 233 * <p>This is equal to the union of incoming and outgoing edges. 234 * 235 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 236 * {@code view} returned by this method will be invalidated, and will throw {@code 237 * IllegalStateException} if it is accessed in any way, with the following exceptions: 238 * 239 * <ul> 240 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 241 * expression involving {@code view} will throw) 242 * <li>{@code hashCode()} does not throw 243 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 244 * behavior is undefined 245 * </ul> 246 * 247 * @throws IllegalArgumentException if {@code node} is not an element of this graph 248 * @since 24.0 249 */ 250 @Override 251 Set<EndpointPair<N>> incidentEdges(N node); 252 253 /** 254 * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, 255 * the number of times an edge touches {@code node}). 256 * 257 * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. 258 * 259 * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of 260 * self-loops incident to {@code node}). 261 * 262 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 263 * 264 * @throws IllegalArgumentException if {@code node} is not an element of this graph 265 */ 266 @Override 267 int degree(N node); 268 269 /** 270 * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) 271 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 272 * 273 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 274 * 275 * @throws IllegalArgumentException if {@code node} is not an element of this graph 276 */ 277 @Override 278 int inDegree(N node); 279 280 /** 281 * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) 282 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 283 * 284 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 285 * 286 * @throws IllegalArgumentException if {@code node} is not an element of this graph 287 */ 288 @Override 289 int outDegree(N node); 290 291 /** 292 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 293 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. 294 * 295 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 296 * 297 * @since 23.0 298 */ 299 @Override 300 boolean hasEdgeConnecting(N nodeU, N nodeV); 301 302 /** 303 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 304 * any, specified by {@code endpoints}). This is equivalent to {@code 305 * edges().contains(endpoints)}. 306 * 307 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 308 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 309 * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally 310 * throw if the object cannot be present in the collection), and the desire to have this method's 311 * behavior be compatible with {@code edges().contains(endpoints)}. 312 * 313 * @since 27.1 314 */ 315 @Override 316 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 317 318 /** 319 * Returns the value of the edge that connects {@code nodeU} to {@code nodeV}, if one is present; 320 * otherwise, returns {@code defaultValue}. 321 * 322 * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU, 323 * defaultValue)}. 324 * 325 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 326 * graph 327 */ 328 @Nullable V edgeValueOrDefault(N nodeU, N nodeV, @Nullable V defaultValue); 329 330 /** 331 * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified 332 * by {@code endpoints}), if one is present; otherwise, returns {@code defaultValue}. 333 * 334 * <p>If this graph is directed, the endpoints must be ordered. 335 * 336 * @throws IllegalArgumentException if either endpoint is not an element of this graph 337 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 338 * @since 27.1 339 */ 340 @Nullable V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue); 341 342 // 343 // ValueGraph identity 344 // 345 346 /** 347 * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and 348 * the same structural relationships as those in this graph. 349 * 350 * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true: 351 * 352 * <ul> 353 * <li>A and B have equal {@link #isDirected() directedness}. 354 * <li>A and B have equal {@link #nodes() node sets}. 355 * <li>A and B have equal {@link #edges() edge sets}. 356 * <li>The {@link #edgeValueOrDefault(N, N, V) value} of a given edge is the same in both A and 357 * B. 358 * </ul> 359 * 360 * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 361 * For example, two graphs may be considered equal even if one allows self-loops and the other 362 * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order 363 * in which they are iterated over, are irrelevant. 364 * 365 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}. 366 */ 367 @Override 368 boolean equals(@Nullable Object object); 369 370 /** 371 * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a 372 * map from each of its {@link #edges() edges} to the associated {@link #edgeValueOrDefault(N, N, 373 * V) edge value}. 374 * 375 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}. 376 */ 377 @Override 378 int hashCode(); 379}