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.errorprone.annotations.DoNotMock; 020import java.util.Collection; 021import java.util.Set; 022import org.jspecify.annotations.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 are anonymous entities with no identity or information of their own. 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 * <h2>Capabilities</h2> 038 * 039 * <p>{@code Graph} 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 * </ul> 049 * 050 * <p>{@code Graph} explicitly does not support parallel edges, and forbids implementations or 051 * extensions with parallel edges. If you need parallel edges, use {@link Network}. 052 * 053 * <h2>Building a {@code Graph}</h2> 054 * 055 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To 056 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link 057 * GraphBuilder} class: 058 * 059 * {@snippet : 060 * MutableGraph<Integer> graph = GraphBuilder.undirected().build(); 061 * } 062 * 063 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype 064 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not 065 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph), 066 * you should use the non-mutating {@link Graph} interface, or an {@link ImmutableGraph}. 067 * 068 * <p>You can create an immutable copy of an existing {@code Graph} using {@link 069 * ImmutableGraph#copyOf(Graph)}: 070 * 071 * {@snippet : 072 * ImmutableGraph<Integer> immutableGraph = ImmutableGraph.copyOf(graph); 073 * } 074 * 075 * <p>Instances of {@link ImmutableGraph} do not implement {@link MutableGraph} (obviously!) and are 076 * contractually guaranteed to be unmodifiable and thread-safe. 077 * 078 * <p>The Guava User Guide has <a 079 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more 080 * information on (and examples of) building graphs</a>. 081 * 082 * <h2>Additional documentation</h2> 083 * 084 * <p>See the Guava User Guide for the {@code common.graph} package (<a 085 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for 086 * additional documentation, including: 087 * 088 * <ul> 089 * <li><a 090 * href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence"> 091 * {@code equals()}, {@code hashCode()}, and graph equivalence</a> 092 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization"> 093 * Synchronization policy</a> 094 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes 095 * for implementors</a> 096 * </ul> 097 * 098 * @author James Sexton 099 * @author Joshua O'Madadhain 100 * @param <N> Node parameter type 101 * @since 20.0 102 */ 103@DoNotMock("Use GraphBuilder to create a real instance") 104public interface Graph<N> extends BaseGraph<N> { 105 // 106 // Graph-level accessors 107 // 108 109 /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ 110 @Override 111 Set<N> nodes(); 112 113 /** Returns all edges in this graph. */ 114 @Override 115 Set<EndpointPair<N>> edges(); 116 117 // 118 // Graph properties 119 // 120 121 /** 122 * Returns true if the edges in this graph are directed. Directed edges connect a {@link 123 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 124 * undirected edges connect a pair of nodes to each other. 125 */ 126 @Override 127 boolean isDirected(); 128 129 /** 130 * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting 131 * to add a self-loop to a graph that does not allow them will throw an {@link 132 * IllegalArgumentException}. 133 */ 134 @Override 135 boolean allowsSelfLoops(); 136 137 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 138 @Override 139 ElementOrder<N> nodeOrder(); 140 141 /** 142 * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of 143 * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link 144 * #successors(Object)} and {@link #incidentEdges(Object)}. 145 * 146 * @since 29.0 147 */ 148 @Override 149 ElementOrder<N> incidentEdgeOrder(); 150 151 // 152 // Element-level accessors 153 // 154 155 /** 156 * Returns a live view of the nodes which have an incident edge in common with {@code node} in 157 * this graph. 158 * 159 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 160 * 161 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 162 * {@code view} returned by this method will be invalidated, and will throw {@code 163 * IllegalStateException} if it is accessed in any way, with the following exceptions: 164 * 165 * <ul> 166 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 167 * expression involving {@code view} will throw) 168 * <li>{@code hashCode()} does not throw 169 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 170 * behavior is undefined 171 * </ul> 172 * 173 * @throws IllegalArgumentException if {@code node} is not an element of this graph 174 */ 175 @Override 176 Set<N> adjacentNodes(N node); 177 178 /** 179 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 180 * traversing {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 181 * 182 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 183 * 184 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 185 * {@code view} returned by this method will be invalidated, and will throw {@code 186 * IllegalStateException} if it is accessed in any way, with the following exceptions: 187 * 188 * <ul> 189 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 190 * expression involving {@code view} will throw) 191 * <li>{@code hashCode()} does not throw 192 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 193 * behavior is undefined 194 * </ul> 195 * 196 * @throws IllegalArgumentException if {@code node} is not an element of this graph 197 */ 198 @Override 199 Set<N> predecessors(N node); 200 201 /** 202 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 203 * traversing {@code node}'s outgoing edges in the direction (if any) of the edge. 204 * 205 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 206 * 207 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 208 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 209 * 210 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 211 * {@code view} returned by this method will be invalidated, and will throw {@code 212 * IllegalStateException} if it is accessed in any way, with the following exceptions: 213 * 214 * <ul> 215 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 216 * expression involving {@code view} will throw) 217 * <li>{@code hashCode()} does not throw 218 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 219 * behavior is undefined 220 * </ul> 221 * 222 * @throws IllegalArgumentException if {@code node} is not an element of this graph 223 */ 224 @Override 225 Set<N> successors(N node); 226 227 /** 228 * Returns a live view of the edges in this graph whose endpoints include {@code node}. 229 * 230 * <p>This is equal to the union of incoming and outgoing edges. 231 * 232 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 233 * {@code view} returned by this method will be invalidated, and will throw {@code 234 * IllegalStateException} if it is accessed in any way, with the following exceptions: 235 * 236 * <ul> 237 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 238 * expression involving {@code view} will throw) 239 * <li>{@code hashCode()} does not throw 240 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 241 * behavior is undefined 242 * </ul> 243 * 244 * @throws IllegalArgumentException if {@code node} is not an element of this graph 245 * @since 24.0 246 */ 247 @Override 248 Set<EndpointPair<N>> incidentEdges(N node); 249 250 /** 251 * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, 252 * the number of times an edge touches {@code node}). 253 * 254 * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. 255 * 256 * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of 257 * self-loops incident to {@code node}). 258 * 259 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 260 * 261 * @throws IllegalArgumentException if {@code node} is not an element of this graph 262 */ 263 @Override 264 int degree(N node); 265 266 /** 267 * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) 268 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 269 * 270 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 271 * 272 * @throws IllegalArgumentException if {@code node} is not an element of this graph 273 */ 274 @Override 275 int inDegree(N node); 276 277 /** 278 * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) 279 * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. 280 * 281 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 282 * 283 * @throws IllegalArgumentException if {@code node} is not an element of this graph 284 */ 285 @Override 286 int outDegree(N node); 287 288 /** 289 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 290 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. 291 * 292 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 293 * 294 * @since 23.0 295 */ 296 @Override 297 boolean hasEdgeConnecting(N nodeU, N nodeV); 298 299 /** 300 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 301 * any, specified by {@code endpoints}). This is equivalent to {@code 302 * edges().contains(endpoints)}. 303 * 304 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 305 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 306 * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally 307 * throw if the object cannot be present in the collection), and the desire to have this method's 308 * behavior be compatible with {@code edges().contains(endpoints)}. 309 * 310 * @since 27.1 311 */ 312 @Override 313 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 314 315 // 316 // Graph identity 317 // 318 319 /** 320 * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the 321 * same structural relationships as those in this graph. 322 * 323 * <p>Thus, two graphs A and B are equal if <b>all</b> of the following are true: 324 * 325 * <ul> 326 * <li>A and B have equal {@link #isDirected() directedness}. 327 * <li>A and B have equal {@link #nodes() node sets}. 328 * <li>A and B have equal {@link #edges() edge sets}. 329 * </ul> 330 * 331 * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 332 * For example, two graphs may be considered equal even if one allows self-loops and the other 333 * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order 334 * in which they are iterated over, are irrelevant. 335 * 336 * <p>A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}. 337 */ 338 @Override 339 boolean equals(@Nullable Object object); 340 341 /** 342 * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of 343 * the set returned by {@link #edges()}. 344 * 345 * <p>A reference implementation of this is provided by {@link AbstractGraph#hashCode()}. 346 */ 347 @Override 348 int hashCode(); 349}