Java: how to represent graphs?

Question

I\'m implementing some algorithms to teach myself about graphs and how to work with them. What would you recommend is the best way to do that in Java? I was thinking somethi

Answer 1

class Vertex {
    private String name;
    private int score; // for path algos
    private boolean visited; // for path algos
    List<Edge> connections;
}

class Edge {
    private String vertex1Name; // same as Vertex.name
    private String vertex2Name;
    private int length;
}

class Graph {
    private List<Edge> edges;
}

Answer 2

I'd recommend graphviz highly when you get to the point where you want to render your graphs.

And its companions: take a look at Laszlo Szathmary's GraphViz class, along with notugly.xls.

Answer 3

Each node is named uniquely and knows who it is connected to. The List of connections allows for a Node to be connected to an arbitrary number of other nodes.

public class Node {
    public String name;
    public List<Edge> connections;
}

Each connection is directed, has a start and an end, and is weighted.

public class Edge {
    public Node start;
    public Node end;
    public double weight;
}

A graph is just your collection of nodes. Instead of List<Node> consider Map<String, Node> for fast lookup by name.

public class Graph {
    List<Node> nodes;
}

Answer 4

When learning algorithms, the programming language (Java) should not be considered in deciding the representation. Each problem could benefit from a unique representation, and moreover designing it can add a bit of learning. Solve the problem first without relying on a particular language, then the representation for any particular language will flow naturally.

Of course, general representations and libraries are useful in real-world applications. But some of them could benefit from some customization as well. Use the other answers to know the different techniques available, but consider customization when appropriate.

Answer 5

class Graph<E> {
    private List<Vertex<E>> vertices;

    private static class Vertex<E> {
        E elem;
        List<Vertex<E>> neighbors;
    }
}

Answer 6

A simple representation written by 'Robert Sedgwick' and 'Kevin Wayne' is available at http://algs4.cs.princeton.edu/41graph/Graph.java.html

Explanation copied from the above page.

The Graph class represents an undirected graph of vertices named 0 through V - 1.

It supports the following two primary operations: add an edge to the graph, iterate over all of the vertices adjacent to a vertex. It also provides methods for returning the number of vertices V and the number of edges E. Parallel edges and self-loops are permitted. By convention, a self-loop v-v appears in the adjacency list of v twice and contributes two to the degree of v.

This implementation uses an adjacency-lists representation, which is a vertex-indexed array of Bag objects. All operations take constant time (in the worst case) except iterating over the vertices adjacent to a given vertex, which takes time proportional to the number of such vertices.