Graph (Guava: Google Core Libraries for Java 20.0-rc1 API)

Type Parameters:

N - Node parameter type

All Known Subinterfaces:

MutableGraph<N>, MutableValueGraph<N,V>, ValueGraph<N,V>

All Known Implementing Classes:

AbstractGraph, AbstractValueGraph, ImmutableGraph, ImmutableValueGraph
```
@Beta
public interface Graph<N>
```
An interface for graph data structures. A graph is composed of a set of nodes (sometimes called vertices) and a set of edges connecting pairs of nodes. Graphs are useful for modeling many kinds of relations. If the relation to be modeled is symmetric (such as "distance between cities"), that can be represented with an undirected graph, where an edge that connects node U to node V also connects node V to node U. If the relation to be modeled is asymmetric (such as "employees managed"), that can be represented with a directed graph, where edges are strictly one-way.
There are three main 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.
To choose the right interface, answer these questions:
1. Do you have data (objects) that you wish to associate with edges?
  Yes: Go to question 2. No: Use Graph.
2. Are the objects you wish to associate with edges unique within the scope of a graph? That is, no two objects would be equal to each other. A common example where this would not be the case is with weighted graphs.
  Yes: Go to question 3. No: Use ValueGraph.
3. Do you need to be able to query the graph for an edge associated with a particular object? For example, do you need to query what nodes an edge associated with a particular object connects, or whether an edge associated with that object exists in the graph?
  Yes: Use Network. No: Go to question 4.
4. Do you need explicit support for parallel edges? For example, do you need to remove one edge connecting a pair of nodes while leaving other edges connecting those same nodes intact?
  Yes: Use Network. No: Use ValueGraph.
Although MutableValueGraph and MutableNetwork both require users to provide objects to associate with edges when adding them, the differentiating factor is that in ValueGraphs, these objects can be any arbitrary data. Like the values in a Map, they do not have to be unique, and can be mutated while in the graph. In a Network, these objects serve as keys into the data structure. Like the keys in a Map, they must be unique, and cannot be mutated in a way that affects their equals/hashcode or the data structure will become corrupted.
In all three interfaces, nodes have all the same requirements as keys in a Map.
The Graph interface does not support parallel edges(), and forbids implementations or extensions with parallel edges. It is possible to encode a notion of edge multiplicity into the values of a ValueGraph (e.g. with an integer or a list of values), but this will not be reflected in methods such as degree(Object). For that functionality, see Network.
All mutation methods live on the subinterface MutableGraph. 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 prefer the non-mutating Graph interface.
We provide an efficient implementation of this interface via GraphBuilder. When using the implementation provided, all collection-returning methods provide live, unmodifiable views of the graph. In other words, you cannot add an element to the collection, but if an element is added to the Graph that would affect the collection, the collection will be updated automatically. This also means that you cannot mutate a Graph in a way that would affect a collection while iterating over that collection. For example, you cannot remove either foo or any successors of foo from the graph while iterating over successors(foo) (unless you first make a copy of the successors), just as you could not remove keys from a Map while iterating over its Map.keySet(). Behavior in such a case is undefined, and may result in ConcurrentModificationException.
Example of use:
```
 MutableGraph managementGraph = GraphBuilder.directed().build();
 managementGraph.putEdge("Big Boss", "Middle Manager Jack");
 managementGraph.putEdge("Big Boss", "Middle Manager Jill");
 managementGraph.putEdge("Middle Manager Jack", "Joe");
 managementGraph.putEdge("Middle Manager Jack", "Schmoe");
 managementGraph.putEdge("Middle Manager Jill", "Jane");
 managementGraph.putEdge("Middle Manager Jill", "Doe");
 for (String employee : managementGraph.nodes()) {
   Set reports = managementGraph.successors(employee);
   if (!reports.isEmpty()) {
     System.out.format("%s has the following direct reports: %s%n", employee, reports);
   }
 }
 
```
Since:

20.0

Author:

James Sexton, Joshua O'Madadhain

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method and Description
`Set<N>`	`adjacentNodes(Object node)` Returns the nodes which have an incident edge in common with `node` in this graph.
`boolean`	`allowsSelfLoops()` Returns true if this graph allows self-loops (edges that connect a node to itself).
`int`	`degree(Object node)` Returns the count of `node`'s incident edges, counting self-loops twice (equivalently, the number of times an edge touches `node`).
`Set<EndpointPair<N>>`	`edges()` Returns all edges in this graph.
`boolean`	`equals(Object object)` For the default `Graph` implementations, returns true iff `this == object` (reference equality).
`int`	`hashCode()` For the default `Graph` implementations, returns `System.identityHashCode(this)`.
`int`	`inDegree(Object node)` Returns the count of `node`'s incoming edges (equal to `predecessors(node).size()`) in a directed graph.
`boolean`	`isDirected()` Returns true if the edges in this graph are directed.
`ElementOrder<N>`	`nodeOrder()` Returns the order of iteration for the elements of `nodes()`.
`Set<N>`	`nodes()` Returns all nodes in this graph, in the order specified by `nodeOrder()`.
`int`	`outDegree(Object node)` Returns the count of `node`'s outgoing edges (equal to `successors(node).size()`) in a directed graph.
`Set<N>`	`predecessors(Object 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.
`Set<N>`	`successors(Object 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.

- Method Detail
  - nodes
```
Set<N> nodes()
```
    Returns all nodes in this graph, in the order specified by nodeOrder().
  - edges
```
Set<EndpointPair<N>> edges()
```
    Returns all edges in this graph.
  - isDirected
```
boolean isDirected()
```
    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.
  - allowsSelfLoops
```
boolean allowsSelfLoops()
```
    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 UnsupportedOperationException.
  - nodeOrder
```
ElementOrder<N> nodeOrder()
```
    Returns the order of iteration for the elements of nodes().
  - adjacentNodes
```
Set<N> adjacentNodes(Object node)
```
    Returns the nodes which have an incident edge in common with node in this graph.
    
    Throws:
    
    IllegalArgumentException - if node is not an element of this graph
  - predecessors
```
Set<N> predecessors(Object 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:
    
    IllegalArgumentException - if node is not an element of this graph
  - successors
```
Set<N> successors(Object 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).
    
    Throws:
    
    IllegalArgumentException - if node is not an element of this graph
  - degree
```
int degree(Object 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 adjacentNodes(node).size() + (1 if node has an incident self-loop, 0 otherwise).
    If the count is greater than Integer.MAX_VALUE, returns Integer.MAX_VALUE.
    
    Throws:
    
    IllegalArgumentException - if node is not an element of this graph
  - inDegree
```
int inDegree(Object 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:
    
    IllegalArgumentException - if node is not an element of this graph
  - outDegree
```
int outDegree(Object 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:
    
    IllegalArgumentException - if node is not an element of this graph
  - equals
```
boolean equals(@Nullable
               Object object)
```
    For the default Graph implementations, returns true iff this == object (reference equality). External implementations are free to define this method as they see fit, as long as they satisfy the Object.equals(Object) contract.
    To compare two Graphs based on their contents rather than their references, see Graphs.equivalent(Graph, Graph).
    
    Overrides:
    
    equals in class Object
  - hashCode
```
int hashCode()
```
    For the default Graph implementations, returns System.identityHashCode(this). External implementations are free to define this method as they see fit, as long as they satisfy the Object.hashCode() contract.
    
    Overrides:
    
    hashCode in class Object

Interface Graph<N>

Method Summary

Method Detail

nodes

edges

isDirected

allowsSelfLoops

nodeOrder

adjacentNodes

predecessors

successors

degree

inDegree

outDegree

equals

hashCode