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 javax.annotation.CheckForNull; 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") 106@ElementTypesAreNonnullByDefault 107public interface Graph<N> extends BaseGraph<N> { 108 // 109 // Graph-level accessors 110 // 111 112 /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ 113 @Override 114 Set<N> nodes(); 115 116 /** Returns all edges in this graph. */ 117 @Override 118 Set<EndpointPair<N>> edges(); 119 120 // 121 // Graph properties 122 // 123 124 /** 125 * Returns true if the edges in this graph are directed. Directed edges connect a {@link 126 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 127 * undirected edges connect a pair of nodes to each other. 128 */ 129 @Override 130 boolean isDirected(); 131 132 /** 133 * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting 134 * to add a self-loop to a graph that does not allow them will throw an {@link 135 * IllegalArgumentException}. 136 */ 137 @Override 138 boolean allowsSelfLoops(); 139 140 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 141 @Override 142 ElementOrder<N> nodeOrder(); 143 144 /** 145 * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of 146 * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link 147 * #successors(Object)} and {@link #incidentEdges(Object)}. 148 * 149 * @since 29.0 150 */ 151 @Override 152 ElementOrder<N> incidentEdgeOrder(); 153 154 // 155 // Element-level accessors 156 // 157 158 /** 159 * Returns a live view of the nodes which have an incident edge in common with {@code node} in 160 * this graph. 161 * 162 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 163 * 164 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 165 * {@code view} returned by this method will be invalidated, and will throw {@code 166 * IllegalStateException} if it is accessed in any way, with the following exceptions: 167 * 168 * <ul> 169 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 170 * expression involving {@code view} will throw) 171 * <li>{@code hashCode()} does not throw 172 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 173 * behavior is undefined 174 * </ul> 175 * 176 * @throws IllegalArgumentException if {@code node} is not an element of this graph 177 */ 178 @Override 179 Set<N> adjacentNodes(N node); 180 181 /** 182 * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by 183 * traversing {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 184 * 185 * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. 186 * 187 * <p>If {@code node} is removed from the graph after this method is called, the {@code Set} 188 * {@code view} returned by this method will be invalidated, and will throw {@code 189 * IllegalStateException} if it is accessed in any way, with the following exceptions: 190 * 191 * <ul> 192 * <li>{@code view.equals(view)} evaluates to {@code true} (but any other {@code equals()} 193 * expression involving {@code view} will throw) 194 * <li>{@code hashCode()} does not throw 195 * <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s 196 * behavior is undefined 197 * </ul> 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 // Graph identity 320 // 321 322 /** 323 * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the 324 * same structural relationships as those in this graph. 325 * 326 * <p>Thus, two graphs A and B are equal if <b>all</b> of the following are true: 327 * 328 * <ul> 329 * <li>A and B have equal {@link #isDirected() directedness}. 330 * <li>A and B have equal {@link #nodes() node sets}. 331 * <li>A and B have equal {@link #edges() edge sets}. 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 AbstractGraph#equals(Object)}. 340 */ 341 @Override 342 boolean equals(@CheckForNull 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 346 * the set returned by {@link #edges()}. 347 * 348 * <p>A reference implementation of this is provided by {@link AbstractGraph#hashCode()}. 349 */ 350 @Override 351 int hashCode(); 352}