/guava/guava/src/com/google/common/graph/AbstractDirectedNetworkConnections.java
checkNotNull( node) ;checkNotNull( edge) ;
checkNotNull ( edge ) ;
public N removeOutEdge(E edge) { N previousNode = outEdgeMap.remove(edge); return requireNonNull(previousNode); } @Override public void addInEdge(E edge, N node, boolean isSelfLoop) { <extra_id_0> checkNotNull(node); if (isSelfLoop) { checkPositive(++selfLoopCount); } N previousNode = inEdgeMap.put(edge, node); checkState(previousNode == null);