7.1. Implementation

public class Edge implements Comparable<Edge> {
    private int source;
    private int target;
    private double weight;

    public Edge(int source, int target, double weight) {
        init(source, target, weight);
    }

    public Edge(int source, int target) {
        this(source, target, 0);
    }

    public Edge(Edge edge) {
        this(edge.getSource(), edge.getTarget(), edge.getWeight());
    }
}

private void init(int source, int target, double weight) {
    setSource(source);
    setTarget(target);
    setWeight(weight);
}

public int getSource() { return source; }
public int getTarget() { return target; }
public double getWeight() { return weight; }

public void setSource(int vertex) { source = vertex; }
public void setTarget(int vertex) { target = vertex; }
public void setWeight(double weight) { this.weight = weight; }
public void addWeight(double weight) { this.weight += weight; }

@Override
public int compareTo(Edge edge) {
    double diff = weight - edge.weight;
    if (diff > 0) return 1;
    else if (diff < 0) return -1;
    else return 0;
}

public class Graph {
    private final List<List<Edge>> incoming_edges;

    public Graph(int size) {
        incoming_edges = Stream.generate(ArrayList<Edge>::new).limit(size).collect(Collectors.toList());
    }
    
    public Graph(Graph g) {
        incoming_edges = g.incoming_edges.stream().map(ArrayList::new).collect(Collectors.toList());
    }
    
    public boolean hasNoEdge() {
        return IntStream.range(0, size()).allMatch(i -> getIncomingEdges(i).isEmpty());
    }
    
    public int size() {
        return incoming_edges.size();
    }
}

incoming_edges.set(0, new ArrayList());
incoming_edges.set(1, new ArrayList(new Edge(0, 1), new Edge(2, 1));
incoming_edges.set(2, new ArrayList(new Edge(0, 2)));

public Edge setDirectedEdge(int source, int target, double weight) {
    List<Edge> edges = getIncomingEdges(target);
    Edge edge = new Edge(source, target, weight);
    edges.add(edge);
    return edge;
}

public void setUndirectedEdge(int source, int target, double weight) {
    setDirectedEdge(source, target, weight);
    setDirectedEdge(target, source, weight);
}

public List<Edge> getIncomingEdges(int target) {
    return incoming_edges.get(target);
}

public List<Edge> getAllEdges() {
    return incoming_edges.stream().flatMap(List::stream).collect(Collectors.toList());
}

public Deque<Integer> getVerticesWithNoIncomingEdges() {
    return IntStream.range(0, size()).filter(i -> getIncomingEdges(i).isEmpty()).boxed().collect(Collectors.toCollection(ArrayDeque::new));
}

public List<Deque<Edge>> getOutgoingEdges() {
    List<Deque<Edge>> outgoing_edges = Stream.generate(ArrayDeque<Edge>::new).limit(size()).collect(Collectors.toList());

    for (int target = 0; target < size(); target++) {
        for (Edge incoming_edge : getIncomingEdges(target))
            outgoing_edges.get(incoming_edge.getSource()).add(incoming_edge);
    }

    return outgoing_edges;
}