https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2014 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.graph;

import com.google.common.annotations.Beta;
import java.util.Set;
import org.checkerframework.checker.nullness.qual.Nullable;


An interface for graph-structured data,
whose edges are anonymous entities with no identity or information of their own.

A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.

There are three primary interfaces provided to represent graphs. In order of increasing complexity they are: Graph, ValueGraph, and Network. You should generally prefer the simplest interface that satisfies your use case. See the 
"Choosing the right graph type" section of the Guava User Guide for more details.

Capabilities


Graph supports the following use cases (definitions of
terms):

    
  
  • directed graphs
  
  • undirected graphs
  
  • graphs that do/don't allow self-loops
  
  • graphs whose nodes/edges are insertion-ordered, sorted, or unordered



Graph explicitly does not support parallel edges, and forbids implementations or extensions with parallel edges. If you need parallel edges, use Network. 
Building a Graph


The implementation classes that common.graph provides are not public, by design. To create an instance of one of the built-in implementations of Graph, use the GraphBuilder class: 

MutableGraph<Integer> graph = GraphBuilder.undirected().build();



GraphBuilder.build() returns an instance of MutableGraph, which is a subtype of Graph that provides methods for adding and removing nodes and edges. If you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph), you should use the non-mutating Graph interface, or an ImmutableGraph. 
You can create an immutable copy of an existing Graph using ImmutableGraph.copyOf(Graph<Object>): 

ImmutableGraph<Integer> immutableGraph = ImmutableGraph.copyOf(graph);



Instances of ImmutableGraph do not implement MutableGraph (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe. 
The Guava User Guide has more
information on (and examples of) building graphs.

Additional documentation


See the Guava User Guide for the common.graph package ("Graphs Explained") for
additional documentation, including:



Author:James Sexton, Joshua O'Madadhain
Type parameters:
  • <N> – Node parameter type
Since:20.0
/**
 * An interface for <a
 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data,
 * whose edges are anonymous entities with no identity or information of their own.
 *
 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
 *
 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
 * prefer the simplest interface that satisfies your use case. See the <a
 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
 *
 * <h3>Capabilities</h3>
 *
 * <p>{@code Graph} supports the following use cases (<a
 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
 * terms</a>):
 *
 * <ul>
 *   <li>directed graphs
 *   <li>undirected graphs
 *   <li>graphs that do/don't allow self-loops
 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
 * </ul>
 *
 * <p>{@code Graph} explicitly does not support parallel edges, and forbids implementations or
 * extensions with parallel edges. If you need parallel edges, use {@link Network}.
 *
 * <h3>Building a {@code Graph}</h3>
 *
 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link
 * GraphBuilder} class:
 *
 * <pre>{@code
 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }</pre>
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not
 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph),
 * you should use the non-mutating {@link Graph} interface, or an {@link ImmutableGraph}.
 *
 * <p>You can create an immutable copy of an existing {@code Graph} using {@link
 * ImmutableGraph#copyOf(Graph)}:
 *
 * <pre>{@code
 * ImmutableGraph<Integer> immutableGraph = ImmutableGraph.copyOf(graph);
 * }</pre>
 *
 * <p>Instances of {@link ImmutableGraph} do not implement {@link MutableGraph} (obviously!) and are
 * contractually guaranteed to be unmodifiable and thread-safe.
 *
 * <p>The Guava User Guide has <a
 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
 * information on (and examples of) building graphs</a>.
 *
 * <h3>Additional documentation</h3>
 *
 * <p>See the Guava User Guide for the {@code common.graph} package (<a
 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
 * additional documentation, including:
 *
 * <ul>
 *   <li><a
 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
 *       Synchronization policy</a>
 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
 *       for implementors</a>
 * </ul>
 *
 * @author James Sexton
 * @author Joshua O'Madadhain
 * @param <N> Node parameter type
 * @since 20.0
 */
@Beta
public interface Graph<N> extends BaseGraph<N> {
  //
  // Graph-level accessors
  //

  
Returns all nodes in this graph, in the order specified by nodeOrder(). 
/** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
  @Override
  Set<N> nodes();

  
Returns all edges in this graph. 
/** Returns all edges in this graph. */
  @Override
  Set<EndpointPair<N>> edges();

  //
  // Graph properties
  //

  
Returns true if the edges in this graph are directed. Directed edges connect a source node to a target node, while undirected edges connect a pair of nodes to each other. 
/**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  @Override
  boolean isDirected();

  
Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting to add a self-loop to a graph that does not allow them will throw an IllegalArgumentException. 
/**
   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
   * to add a self-loop to a graph that does not allow them will throw an {@link
   * IllegalArgumentException}.
   */
  @Override
  boolean allowsSelfLoops();

  
Returns the order of iteration for the elements of nodes(). 
/** Returns the order of iteration for the elements of {@link #nodes()}. */
  @Override
  ElementOrder<N> nodeOrder();

  //
  // Element-level accessors
  //

  
Returns the nodes which have an incident edge in common with node in this graph. 
Throws:
/**
   * Returns the nodes which have an incident edge in common with {@code node} in this graph.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  Set<N> adjacentNodes(N node);

  
Returns all nodes in this graph adjacent to node which can be reached by traversing node's incoming edges against the direction (if any) of the edge.

In an undirected graph, this is equivalent to adjacentNodes(Object). 
Throws:
/**
   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
   *
   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  Set<N> predecessors(N node);

  
Returns all nodes in this graph adjacent to node which can be reached by traversing node's outgoing edges in the direction (if any) of the edge. 
In an undirected graph, this is equivalent to adjacentNodes(Object). 
This is not the same as "all nodes reachable from node by following outgoing edges". For that functionality, see Graphs.reachableNodes(Graph<Object>, Object). 
Throws:
/**
   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
   * {@code node}'s outgoing edges in the direction (if any) of the edge.
   *
   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
   *
   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  Set<N> successors(N node);

  
Returns the edges in this graph whose endpoints include node. 
Throws:
Since:24.0
/**
   * Returns the edges in this graph whose endpoints include {@code node}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   * @since 24.0
   */
  @Override
  Set<EndpointPair<N>> incidentEdges(N node);

  
Returns the count of node's incident edges, counting self-loops twice (equivalently, the number of times an edge touches node). 
For directed graphs, this is equal to inDegree(node) + outDegree(node). 
For undirected graphs, this is equal to incidentEdges(node).size() + (number of self-loops incident to node). 
If the count is greater than Integer.MAX_VALUE, returns Integer.MAX_VALUE. 
Throws:
/**
   * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently,
   * the number of times an edge touches {@code node}).
   *
   * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}.
   *
   * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of
   * self-loops incident to {@code node}).
   *
   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  int degree(N node);

  
Returns the count of node's incoming edges (equal to predecessors(node).size()) in a directed graph. In an undirected graph, returns the degree(Object). 
If the count is greater than Integer.MAX_VALUE, returns Integer.MAX_VALUE. 
Throws:
/**
   * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()})
   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
   *
   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  int inDegree(N node);

  
Returns the count of node's outgoing edges (equal to successors(node).size()) in a directed graph. In an undirected graph, returns the degree(Object). 
If the count is greater than Integer.MAX_VALUE, returns Integer.MAX_VALUE. 
Throws:
/**
   * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()})
   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
   *
   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this graph
   */
  @Override
  int outDegree(N node);

  
Returns true if there is an edge directly connecting nodeU to nodeV. This is equivalent to nodes().contains(nodeU) && successors(nodeU).contains(nodeV). 
In an undirected graph, this is equal to hasEdgeConnecting(nodeV, nodeU). 
Since:23.0
/**
   * Returns true if there is an edge directly connecting {@code nodeU} to {@code nodeV}. This is
   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}.
   *
   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
   *
   * @since 23.0
   */
  @Override
  boolean hasEdgeConnecting(N nodeU, N nodeV);

  //
  // Graph identity
  //

  
Returns true iff object is a Graph that has the same elements and the same structural relationships as those in this graph. 
Thus, two graphs A and B are equal if all of the following are true:



Graph properties besides directedness do not affect equality.
For example, two graphs may be considered equal even if one allows self-loops and the other
doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
in which they are iterated over, are irrelevant.

A reference implementation of this is provided by AbstractGraph.equals(Object). 
/**
   * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the
   * same structural relationships as those in this graph.
   *
   * <p>Thus, two graphs A and B are equal if <b>all</b> of the following are true:
   *
   * <ul>
   *   <li>A and B have equal {@link #isDirected() directedness}.
   *   <li>A and B have equal {@link #nodes() node sets}.
   *   <li>A and B have equal {@link #edges() edge sets}.
   * </ul>
   *
   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
   * For example, two graphs may be considered equal even if one allows self-loops and the other
   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
   * in which they are iterated over, are irrelevant.
   *
   * <p>A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}.
   */
  @Override
  boolean equals(@Nullable Object object);

  
Returns the hash code for this graph. The hash code of a graph is defined as the hash code of the set returned by edges(). 
A reference implementation of this is provided by AbstractGraph.hashCode(). 
/**
   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of
   * the set returned by {@link #edges()}.
   *
   * <p>A reference implementation of this is provided by {@link AbstractGraph#hashCode()}.
   */
  @Override
  int hashCode();
}