Update Network.asGraph()'s edge value methods to be consistent with h…

…ow they're defined in the interfaces. Update tests to fail with the old implementation.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130683572

guava-tests/test/com/google/common/graph/AbstractGraphTest.java

@@ -165,6 +165,14 @@ static <N> void validateGraph(Graph<N, ?> graph) {
         assertThat(graph.outDegree(node)).isEqualTo(graph.degree(node));
       }
 
+      for (N otherNode : graph.nodes()) {
+        if (graph.successors(node).contains(otherNode)) {
+          assertThat(graph.edgeValue(node, otherNode)).isNotNull();
+        } else {
+          assertThat(graph.edgeValueOrDefault(node, otherNode, null)).isNull();
+        }
+      }
+
       for (N adjacentNode : graph.adjacentNodes(node)) {
         if (!graph.allowsSelfLoops()) {
           assertThat(node).isNotEqualTo(adjacentNode);

guava-tests/test/com/google/common/graph/AbstractNetworkTest.java

@@ -231,7 +231,6 @@ static <N, E> void validateNetwork(Network<N, E> network) {
         Set<E> edgesConnecting = network.edgesConnecting(node, otherNode);
         boolean isSelfLoop = node.equals(otherNode);
         if (network.isDirected() || !isSelfLoop) {
-          assertThat(edgesConnecting).isEqualTo(asGraph.edgeValue(node, otherNode));
           assertThat(edgesConnecting).isEqualTo(
               Sets.intersection(network.outEdges(node), network.inEdges(otherNode)));
         }
@@ -279,8 +278,10 @@ static <N, E> void validateNetwork(Network<N, E> network) {
       }
 
       for (N successor : network.successors(node)) {
+        Set<E> edgesConnecting = network.edgesConnecting(node, successor);
         assertThat(network.predecessors(successor)).contains(node);
-        assertThat(network.edgesConnecting(node, successor)).isNotEmpty();
+        assertThat(edgesConnecting).isNotEmpty();
+        assertThat(edgesConnecting).isEqualTo(asGraph.edgeValue(node, successor));
       }
     }
   }

guava/src/com/google/common/graph/AbstractNetwork.java

@@ -16,7 +16,8 @@
 
 package com.google.common.graph;
 
-import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.graph.GraphConstants.EDGE_CONNECTING_NOT_IN_GRAPH;
 import static com.google.common.graph.GraphConstants.GRAPH_STRING_FORMAT;
 
 import com.google.common.annotations.Beta;
@@ -120,12 +121,15 @@ public Set<N> successors(Object node) {
 
       @Override
       public Set<E> edgeValue(Object nodeA, Object nodeB) {
-        return checkNotNull(edgesConnecting(nodeA, nodeB));
+        Set<E> edges = edgesConnecting(nodeA, nodeB);
+        checkArgument(!edges.isEmpty(), EDGE_CONNECTING_NOT_IN_GRAPH, nodeA, nodeB);
+        return edges;
       }
 
       @Override
       public Set<E> edgeValueOrDefault(Object nodeA, Object nodeB, Set<E> defaultValue) {
-        return checkNotNull(edgesConnecting(nodeA, nodeB));
+        Set<E> edges = edgesConnecting(nodeA, nodeB);
+        return edges.isEmpty() ? defaultValue : edges;
       }
     };
   }

guava/src/com/google/common/graph/Network.java

@@ -111,7 +111,7 @@ public interface Network<N, E> {
    * an edge connecting node A to node B iff this {@link Network} has an edge connecting A to B.
    *
    * <p>{@link Graph#edgeValue(Object, Object)} will return the set of edges connecting node A to
-   * node B. It will return the empty set if there are no edges connecting A to B.
+   * node B if the set is non-empty, otherwise, it will throw {@link IllegalArgumentException}.
    *
    * <p>If this network {@link #allowsParallelEdges()}, parallel edges will treated as if collapsed
    * into a single edge. For example, the {@link #degree(Object)} of a node in the {@link Graph}