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 javax.annotation.Nullable;
021
022/**
023 * An interface for <a href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-
024 * structured data, whose edges have associated non-unique values.
025 *
026 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
027 *
028 * <p>There are three main interfaces provided to represent graphs. In order of increasing
029 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
030 * prefer the simplest interface that satisfies your use case. See the <a
031 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
032 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
033 *
034 * <h3>Capabilities</h3>
035 *
036 * <p>{@code ValueGraph} supports the following use cases (<a
037 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
038 * terms</a>):
039 *
040 * <ul>
041 *   <li>directed graphs
042 *   <li>undirected graphs
043 *   <li>graphs that do/don't allow self-loops
044 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
045 *   <li>graphs whose edges have associated values
046 * </ul>
047 *
048 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges,
049 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use
050 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of
051 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your
052 * interpretation of the edge value as its multiplicity.)
053 *
054 * <h3>Building a {@code ValueGraph}</h3>
055 *
056 * <p>The implementation classes that `common.graph` provides are not public, by design. To create
057 * an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link
058 * ValueGraphBuilder} class:
059 *
060 * <pre>{@code
061 *   MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build();
062 * }</pre>
063 *
064 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a
065 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If
066 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on
067 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link
068 * ImmutableValueGraph}.
069 *
070 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link
071 * ImmutableValueGraph#copyOf(ValueGraph)}:
072 *
073 * <pre>{@code
074 *   ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph);
075 * }</pre>
076 *
077 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph}
078 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe.
079 *
080 * <p>The Guava User Guide has <a
081 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
082 * information on (and examples of) building graphs</a>.
083 *
084 * <h3>Additional documentation</h3>
085 *
086 * <p>See the Guava User Guide for the {@code common.graph} package (<a
087 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
088 * additional documentation, including:
089 *
090 * <ul>
091 *   <li><a
092 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
093 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
094 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
095 *       Synchronization policy</a>
096 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
097 *       for implementors</a>
098 * </ul>
099 *
100 * @author James Sexton
101 * @author Joshua O'Madadhain
102 * @param <N> Node parameter type
103 * @param <V> Value parameter type
104 * @since 20.0
105 */
106@Beta
107public interface ValueGraph<N, V> extends Graph<N> {
108
109  /**
110   * If there is an edge connecting {@code nodeU} to {@code nodeV}, returns the non-null value
111   * associated with that edge.
112   *
113   * <p>In an undirected graph, this is equal to {@code edgeValue(nodeV, nodeU)}.
114   *
115   * @throws IllegalArgumentException if there is no edge connecting {@code nodeU} to {@code nodeV}.
116   */
117  V edgeValue(Object nodeU, Object nodeV);
118
119  /**
120   * If there is an edge connecting {@code nodeU} to {@code nodeV}, returns the non-null value
121   * associated with that edge; otherwise, returns {@code defaultValue}.
122   *
123   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
124   * defaultValue)}.
125   */
126  V edgeValueOrDefault(Object nodeU, Object nodeV, @Nullable V defaultValue);
127
128  //
129  // ValueGraph identity
130  //
131
132  /**
133   * For the default {@link ValueGraph} implementations, returns true if {@code this == object}
134   * (reference equality). External implementations are free to define this method as they see fit,
135   * as long as they satisfy the {@link Object#equals(Object)} contract.
136   *
137   * <p>To compare two {@link ValueGraph}s based on their contents rather than their references, see
138   * {@link Graphs#equivalent(ValueGraph, ValueGraph)}.
139   */
140  @Override
141  boolean equals(@Nullable Object object);
142
143  /**
144   * For the default {@link ValueGraph} implementations, returns {@code
145   * System.identityHashCode(this)}. External implementations are free to define this method as they
146   * see fit, as long as they satisfy the {@link Object#hashCode()} contract.
147   */
148  @Override
149  int hashCode();
150}