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 * @throws IllegalArgumentException if {@code node} is not an element of this graph 161 */ 162 @Override 163 Set<N> adjacentNodes(N node); 164 165 /** 166 * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing 167 * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 168 * 169 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 170 * 171 * @throws IllegalArgumentException if {@code node} is not an element of this graph 172 */ 173 @Override 174 Set<N> predecessors(N node); 175 176 /** 177 * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing 178 * {@code node}'s outgoing edges in the direction (if any) of the edge. 179 * 180 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 181 * 182 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 183 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 184 * 185 * @throws IllegalArgumentException if {@code node} is not an element of this graph 186 */ 187 @Override 188 Set<N> successors(N node); 189 190 /** 191 * Returns the edges in this graph whose endpoints include {@code node}. 192 * 193 * @throws IllegalArgumentException if {@code node} is not an element of this graph 194 * @since 24.0 195 */ 196 @Override 197 Set<EndpointPair<N>> incidentEdges(N node); 198 199 /** 200 * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, 201 * the number of times an edge touches {@code node}). 202 * 203 * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. 204 * 205 * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of 206 * self-loops incident to {@code node}). 207 * 208 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 209 * 210 * @throws IllegalArgumentException if {@code node} is not an element of this graph 211 */ 212 @Override 213 int degree(N node); 214 215 /** 216 * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) 217 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 218 * 219 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 220 * 221 * @throws IllegalArgumentException if {@code node} is not an element of this graph 222 */ 223 @Override 224 int inDegree(N node); 225 226 /** 227 * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) 228 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 229 * 230 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 231 * 232 * @throws IllegalArgumentException if {@code node} is not an element of this graph 233 */ 234 @Override 235 int outDegree(N node); 236 237 /** 238 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 239 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. 240 * 241 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 242 * 243 * @since 23.0 244 */ 245 @Override 246 boolean hasEdgeConnecting(N nodeU, N nodeV); 247 248 /** 249 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 250 * any, specified by {@code endpoints}). This is equivalent to {@code 251 * edges().contains(endpoints)}. 252 * 253 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 254 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 255 * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally 256 * throw if the object cannot be present in the collection), and the desire to have this method's 257 * behavior be compatible with {@code edges().contains(endpoints)}. 258 * 259 * @since 27.1 260 */ 261 @Override 262 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 263 264 /** 265 * Returns the value of the edge that connects {@code nodeU} to {@code nodeV} (in the order, if 266 * any, specified by {@code endpoints}), if one is present; otherwise, returns {@code 267 * Optional.empty()}. 268 * 269 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 270 * graph 271 * @since 23.0 (since 20.0 with return type {@code V}) 272 */ 273 Optional<V> edgeValue(N nodeU, N nodeV); 274 275 /** 276 * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified 277 * by {@code endpoints}), if one is present; otherwise, returns {@code Optional.empty()}. 278 * 279 * <p>If this graph is directed, the endpoints must be ordered. 280 * 281 * @throws IllegalArgumentException if either endpoint is not an element of this graph 282 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 283 * @since 27.1 284 */ 285 Optional<V> edgeValue(EndpointPair<N> endpoints); 286 287 /** 288 * Returns the value of the edge that connects {@code nodeU} to {@code nodeV}, if one is present; 289 * otherwise, returns {@code defaultValue}. 290 * 291 * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU, 292 * defaultValue)}. 293 * 294 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 295 * graph 296 */ 297 @Nullable 298 V edgeValueOrDefault(N nodeU, N nodeV, @Nullable V defaultValue); 299 300 /** 301 * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified 302 * by {@code endpoints}), if one is present; otherwise, returns {@code defaultValue}. 303 * 304 * <p>If this graph is directed, the endpoints must be ordered. 305 * 306 * @throws IllegalArgumentException if either endpoint is not an element of this graph 307 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 308 * @since 27.1 309 */ 310 @Nullable 311 V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue); 312 313 // 314 // ValueGraph identity 315 // 316 317 /** 318 * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and 319 * the same structural relationships as those in this graph. 320 * 321 * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true: 322 * 323 * <ul> 324 * <li>A and B have equal {@link #isDirected() directedness}. 325 * <li>A and B have equal {@link #nodes() node sets}. 326 * <li>A and B have equal {@link #edges() edge sets}. 327 * <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B. 328 * </ul> 329 * 330 * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 331 * For example, two graphs may be considered equal even if one allows self-loops and the other 332 * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order 333 * in which they are iterated over, are irrelevant. 334 * 335 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}. 336 */ 337 @Override 338 boolean equals(@Nullable Object object); 339 340 /** 341 * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a 342 * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object) 343 * edge value}. 344 * 345 * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}. 346 */ 347 @Override 348 int hashCode(); 349}