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 com.google.errorprone.annotations.DoNotMock; 021import java.util.Collection; 022import java.util.Set; 023import org.jspecify.annotations.Nullable; 024 025/** 026 * An interface for <a 027 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data, 028 * whose edges are anonymous entities with no identity or information of their own. 029 * 030 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes. 031 * 032 * <p>There are three primary interfaces provided to represent graphs. In order of increasing 033 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally 034 * prefer the simplest interface that satisfies your use case. See the <a 035 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type"> 036 * "Choosing the right graph type"</a> section of the Guava User Guide for more details. 037 * 038 * <h3>Capabilities</h3> 039 * 040 * <p>{@code Graph} supports the following use cases (<a 041 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of 042 * terms</a>): 043 * 044 * <ul> 045 * <li>directed graphs 046 * <li>undirected graphs 047 * <li>graphs that do/don't allow self-loops 048 * <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered 049 * </ul> 050 * 051 * <p>{@code Graph} explicitly does not support parallel edges, and forbids implementations or 052 * extensions with parallel edges. If you need parallel edges, use {@link Network}. 053 * 054 * <h3>Building a {@code Graph}</h3> 055 * 056 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To 057 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link 058 * GraphBuilder} class: 059 * 060 * <pre>{@code 061 * MutableGraph<Integer> graph = GraphBuilder.undirected().build(); 062 * }</pre> 063 * 064 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype 065 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not 066 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph), 067 * you should use the non-mutating {@link Graph} interface, or an {@link ImmutableGraph}. 068 * 069 * <p>You can create an immutable copy of an existing {@code Graph} using {@link 070 * ImmutableGraph#copyOf(Graph)}: 071 * 072 * <pre>{@code 073 * ImmutableGraph<Integer> immutableGraph = ImmutableGraph.copyOf(graph); 074 * }</pre> 075 * 076 * <p>Instances of {@link ImmutableGraph} do not implement {@link MutableGraph} (obviously!) and are 077 * contractually guaranteed to be unmodifiable and thread-safe. 078 * 079 * <p>The Guava User Guide has <a 080 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more 081 * information on (and examples of) building graphs</a>. 082 * 083 * <h3>Additional documentation</h3> 084 * 085 * <p>See the Guava User Guide for the {@code common.graph} package (<a 086 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for 087 * additional documentation, including: 088 * 089 * <ul> 090 * <li><a 091 * href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence"> 092 * {@code equals()}, {@code hashCode()}, and graph equivalence</a> 093 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization"> 094 * Synchronization policy</a> 095 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes 096 * for implementors</a> 097 * </ul> 098 * 099 * @author James Sexton 100 * @author Joshua O'Madadhain 101 * @param <N> Node parameter type 102 * @since 20.0 103 */ 104@Beta 105@DoNotMock("Use GraphBuilder to create a real instance") 106public interface Graph<N> extends BaseGraph<N> { 107 // 108 // Graph-level accessors 109 // 110 111 /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ 112 @Override 113 Set<N> nodes(); 114 115 /** Returns all edges in this graph. */ 116 @Override 117 Set<EndpointPair<N>> edges(); 118 119 // 120 // Graph properties 121 // 122 123 /** 124 * Returns true if the edges in this graph are directed. Directed edges connect a {@link 125 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 126 * undirected edges connect a pair of nodes to each other. 127 */ 128 @Override 129 boolean isDirected(); 130 131 /** 132 * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting 133 * to add a self-loop to a graph that does not allow them will throw an {@link 134 * IllegalArgumentException}. 135 */ 136 @Override 137 boolean allowsSelfLoops(); 138 139 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 140 @Override 141 ElementOrder<N> nodeOrder(); 142 143 /** 144 * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of 145 * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link 146 * #successors(Object)} and {@link #incidentEdges(Object)}. 147 * 148 * @since 29.0 149 */ 150 @Override 151 ElementOrder<N> incidentEdgeOrder(); 152 153 // 154 // Element-level accessors 155 // 156 157 /** 158 * Returns a live view of the nodes which have an incident edge in common with {@code node} in 159 * this graph. 160 * 161 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 162 * 163 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 164 * {@code view} returned by this method will be invalidated, and will throw {@code 165 * IllegalStateException} if it is accessed in any way, with the following exceptions: 166 * 167 * <ul> 168 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 169 * expression involving {@code view} will throw) 170 * <li>{@code hashCode()} does not throw 171 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 172 * behavior is undefined 173 * </ul> 174 * 175 * @throws IllegalArgumentException if {@code node} is not an element of this graph 176 */ 177 @Override 178 Set<N> adjacentNodes(N node); 179 180 /** 181 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 182 * traversing {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 183 * 184 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 185 * 186 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 187 * {@code view} returned by this method will be invalidated, and will throw {@code 188 * IllegalStateException} if it is accessed in any way, with the following exceptions: 189 * 190 * <ul> 191 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 192 * expression involving {@code view} will throw) 193 * <li>{@code hashCode()} does not throw 194 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 195 * behavior is undefined 196 * </ul> 197 * 198 * @throws IllegalArgumentException if {@code node} is not an element of this graph 199 */ 200 @Override 201 Set<N> predecessors(N node); 202 203 /** 204 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 205 * traversing {@code node}'s outgoing edges in the direction (if any) of the edge. 206 * 207 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 208 * 209 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 210 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 211 * 212 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 213 * {@code view} returned by this method will be invalidated, and will throw {@code 214 * IllegalStateException} if it is accessed in any way, with the following exceptions: 215 * 216 * <ul> 217 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 218 * expression involving {@code view} will throw) 219 * <li>{@code hashCode()} does not throw 220 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 221 * behavior is undefined 222 * </ul> 223 * 224 * @throws IllegalArgumentException if {@code node} is not an element of this graph 225 */ 226 @Override 227 Set<N> successors(N node); 228 229 /** 230 * Returns a live view of the edges in this graph whose endpoints include {@code node}. 231 * 232 * <p>This is equal to the union of incoming and outgoing edges. 233 * 234 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 235 * {@code view} returned by this method will be invalidated, and will throw {@code 236 * IllegalStateException} if it is accessed in any way, with the following exceptions: 237 * 238 * <ul> 239 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 240 * expression involving {@code view} will throw) 241 * <li>{@code hashCode()} does not throw 242 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 243 * behavior is undefined 244 * </ul> 245 * 246 * @throws IllegalArgumentException if {@code node} is not an element of this graph 247 * @since 24.0 248 */ 249 @Override 250 Set<EndpointPair<N>> incidentEdges(N node); 251 252 /** 253 * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, 254 * the number of times an edge touches {@code node}). 255 * 256 * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. 257 * 258 * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of 259 * self-loops incident to {@code node}). 260 * 261 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 262 * 263 * @throws IllegalArgumentException if {@code node} is not an element of this graph 264 */ 265 @Override 266 int degree(N node); 267 268 /** 269 * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) 270 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 271 * 272 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 273 * 274 * @throws IllegalArgumentException if {@code node} is not an element of this graph 275 */ 276 @Override 277 int inDegree(N node); 278 279 /** 280 * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) 281 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 282 * 283 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 284 * 285 * @throws IllegalArgumentException if {@code node} is not an element of this graph 286 */ 287 @Override 288 int outDegree(N node); 289 290 /** 291 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 292 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. 293 * 294 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 295 * 296 * @since 23.0 297 */ 298 @Override 299 boolean hasEdgeConnecting(N nodeU, N nodeV); 300 301 /** 302 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 303 * any, specified by {@code endpoints}). This is equivalent to {@code 304 * edges().contains(endpoints)}. 305 * 306 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 307 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 308 * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally 309 * throw if the object cannot be present in the collection), and the desire to have this method's 310 * behavior be compatible with {@code edges().contains(endpoints)}. 311 * 312 * @since 27.1 313 */ 314 @Override 315 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 316 317 // 318 // Graph identity 319 // 320 321 /** 322 * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the 323 * same structural relationships as those in this graph. 324 * 325 * <p>Thus, two 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 * </ul> 332 * 333 * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 334 * For example, two graphs may be considered equal even if one allows self-loops and the other 335 * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order 336 * in which they are iterated over, are irrelevant. 337 * 338 * <p>A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}. 339 */ 340 @Override 341 boolean equals(@Nullable Object object); 342 343 /** 344 * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of 345 * the set returned by {@link #edges()}. 346 * 347 * <p>A reference implementation of this is provided by {@link AbstractGraph#hashCode()}. 348 */ 349 @Override 350 int hashCode(); 351}