// The purpose of V and W are explained below
public interface Graph<V, W extends Comparable<W>> {
    // Some methods
 
    // This represents an edge (note that it is an inner interface)
    public interface Edge<V, W extends Comparable<W>> {
        //  Some methods...
    }
 
    // This represents a node in the graph (note that it is an inner interface)
    public interface Node<V> {
 
        // Returns the edges going out or into this node
        public <X extends Comparable<X>> Collection<? extends Edge<V, X>> edges();
    }
}

public class GraphImpl<V, W extends Comparable<W>> implements Graph<V, W>  {
    // Graph methods implemented here
 
    public class EdgeImpl implements Graph.Edge<V, W> {
        // Some methods...
    }
 
    public class NodeImpl implements Graph.Node<V> {
        // This return type results in a warning
        public Collection<? extends Edge<V, W>> edges() {
             // ... implementation
        }
    }
}