Avoid creation of duplicate vertices in digraph (using jgrapht)

Question

I am looking for a way to avoid creating duplicates in my digraph (I use the jgrapht library).

I read some topics which said to use: directedGraph.setCloneable(false); But it doesn't seem to be right, can't find it in the library's documentation and I get an error on this line saying it doesn't exists.

I created my graph using:

public static DirectedGraph<Point, DefaultEdge> directedGraph = new DefaultDirectedGraph<Point, DefaultEdge>(DefaultEdge.class);

And then it adds vertices to it based on the flood fill algorithm (adds vertices and edges as the algorithm go through every point, below is a part of it):

// Up
    xToFillNext = x-1;
    yToFillNext = y;
    if (xToFillNext==targetX && yToFillNext==targetY && !forbiddenDirection.equals(Direction.UP)) {
      Point myPoint = new Point(x, y);
      Point myNextPoint = new Point(xToFillNext, yToFillNext);

      directedGraph.addVertex(myPoint);
      directedGraph.addVertex(myNextPoint);
      directedGraph.addEdge(myPoint, myNextPoint);
      return true;
    } else if (xToFillNext>=0 && originValue==matrix[xToFillNext][yToFillNext] && !forbiddenDirection.equals(Direction.UP)) {  
      Point myPoint = new Point(x, y);
      Point myNextPoint = new Point(xToFillNext, yToFillNext);

      directedGraph.addVertex(myPoint);
      directedGraph.addVertex(myNextPoint);
      directedGraph.addEdge(myPoint, myNextPoint);   
      fillingReachedTargetPosition = 
        fillReachesTargetPosition(matrix, xToFillNext, yToFillNext, targetX, targetY, fillValue, Direction.DOWN );
      if (fillingReachedTargetPosition) {
        return true;
      }
    }

But when I print the list of vertices, there are duplicates which I either need to get rid of, or avoid them to be created. Is there a way to do it ?

EDIT: I created a Point class:

public static class Point {

  public int x;
  public int y;

  public  Point(int x, int y) 
  {

    this.x = x;
    this.y = y;
  }
  @Override
    public String toString() {
    return ("[x="+x+" y="+y+"]");
  }
}

score 2 · Accepted Answer · edited Jul 27 '15 at 02:38

2

In order to detect duplicate vertices you need to supply also method for equals and hashCode. Ohterwise JVM does not know, how to compare the objects, and uses object id (==) which would not identify two different object of class Point as duplicate or equal.

e.g. ( The hashCode has countless number of possible implementations)

@Override
public int hashCode() {
    int hash = 7;
    hash = 71 * hash + this.x;
    hash = 71 * hash + this.y;
    return hash;
}



@Override
public boolean equals(Object other) 
{
    if (this == other)
       return true;

    if (!(other instanceof Point))
       return false;

    Point otherPoint = (Point) other;
    return otherPoint.x == x && otherPoint.y == y;
}

edited Jul 27 '15 at 02:38

v.k.

2,826
2
20
29

answered Jul 26 '15 at 22:15

adamliesko

1,887
1
14
21

Should I add this inside my Point class ? – Graham Slick Jul 26 '15 at 22:18
1

yes, just add those methods into your existing Point class – adamliesko Jul 26 '15 at 22:19
Worked perfectly with the creation of vertices, but now if I create an ArrayList of the same Point objects, it is possible to have duplicates, even when I added the piece of code you provided. Isn't it supposed to apply to every kind of list ? – Graham Slick Jul 29 '15 at 17:22
Nope, ArrayList is not specific in this. You can use some kind of java Set. – adamliesko Jul 29 '15 at 17:34

score 1 · Answer 2 · answered Jul 26 '15 at 22:00

I tried creating some duplicate vertices and edges with the following program:

import org.jgrapht.DirectedGraph;
import org.jgrapht.graph.DefaultDirectedGraph;
import org.jgrapht.graph.DefaultEdge;

import java.awt.Point;

public class JgraphtTest {
    public static void main(String[] args) {
        DirectedGraph<Point, DefaultEdge> directedGraph = new DefaultDirectedGraph<Point, DefaultEdge>(DefaultEdge.class);
        Point zero = new Point(0, 0), zero2 = new Point(0, 0), one = new Point(1, 1), two = new Point(2, 2);
        directedGraph.addVertex(zero);
        directedGraph.addVertex(one);
        directedGraph.addVertex(two);
        directedGraph.addVertex(zero2);   // should be a dup
        directedGraph.addEdge(zero, one);
        directedGraph.addEdge(one, two);
        directedGraph.addEdge(zero2, one); // should be a dup

        for (Point vertex : directedGraph.vertexSet()) {
            System.out.format("vertex: %s\n", vertex);
        }
        for (DefaultEdge edge : directedGraph.edgeSet()) {
            System.out.format("edge: %s\n", edge);
        }
    }
}

But it showed no duplicates in the output:

vertex: java.awt.Point[x=0,y=0]
vertex: java.awt.Point[x=1,y=1]
vertex: java.awt.Point[x=2,y=2]
edge: (java.awt.Point[x=0,y=0] : java.awt.Point[x=1,y=1])
edge: (java.awt.Point[x=1,y=1] : java.awt.Point[x=2,y=2])

So I'm guessing that you are using your own Point class, but you're not properly overriding equals and hashCode. Without those methods being overridden, DirectedGraph won't be able to tell if a vertex exists already or not.

thanks for your help ! What do you mean by overriding equals and hashCode ... ? I don't know how to do this — Graham Slick, Jul 26 '15 at 22:02
Are you using your own `Point` class? If so, can you post its source code? — heenenee, Jul 26 '15 at 22:03

Avoid creation of duplicate vertices in digraph (using jgrapht)

2 Answers2