Foo.java

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.regex.Pattern;

public final class Foo
{
    private static final Pattern PATTERN = Pattern.compile("\\s*:\\s*");

    private static final class AdjMatrix
        implements Iterable<String>
    {
        private final Set<String> edges = new HashSet<>();
        private final Map<Vertex, Integer> vertices = new HashMap<>();
        
        // PRIVATE!!
        private void addEdges(final String edge1, final String edge2)
        {
            edges.add(edge1);
            edges.add(edge2);
        }

        private void addVertex(final Vertex vertex, final int weight)
        {
            addEdges(vertex.from, vertex.to);
            if (vertices.put(vertex, weight) != null)
                throw new IllegalStateException();
        }
        
        @Override
        public Iterator<String> iterator()
        {
            return edges.iterator();
        }
    }

    public static void main(final String... args)
        throws IOException
    {
        final AdjMatrix matrix = new AdjMatrix();

        final Path path = Paths.get(System.getProperty("user.home"), "FOO.txt");

        try (
            final BufferedReader reader = Files.newBufferedReader(path);
        ) {
            String line;
            String[] parts;

            String edge1;
            String edge2;
            int weight;

            Vertex v;

            while ((line = reader.readLine()) != null) {
                parts = PATTERN.split(line);
                edge1 = parts[0];
                edge2 = parts[1];
                weight = Integer.parseInt(parts[2]);

                v = new Vertex(edge1, edge2);
                matrix.addVertex(v, weight);

                v = new Vertex(edge2, edge1);
                matrix.addVertex(v, weight);
            }
        }
    }
    
    private static final class Vertex
    {
        private final String from;
        private final String to;

        Vertex(final String from, final String to)
        {
            this.from = from;
            this.to = to;
        }

        @Override
        public int hashCode()
        {
            return Objects.hash(from, to);
        }

        @Override
        public boolean equals(final Object obj)
        {
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;
            final Vertex other = (Vertex) obj;
            return from.equals(other.from)
                && to.equals(other.to);
        }
    }
}