Implementation of undirected graph in java

A minimal implementation of undirected graph in Java :

import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * @author Narendra Yadala
 * Implementation of undirected graph represented using adjacency list.
 */
public final class UndirectedGraph<T> implements Iterable<T>{
 private final Map<T, Set<T>> graph = new HashMap<T, Set<T>>();

 public boolean addNode(T node) {
  if (graph.containsKey(node))
   return false;

  graph.put(node, new HashSet<T>());
  return true;
 }

 public void addEdge(T start, T dest) {
  if (!graph.containsKey(start) || !graph.containsKey(dest))
   throw new NoSuchElementException("Nodes must be in the graph.");
  
  graph.get(start).add(dest);
  graph.get(dest).add(start);
 }

 public void removeEdge(T start, T dest) {
  if (!graph.containsKey(start) || !graph.containsKey(dest))
   throw new NoSuchElementException("Nodes must be in the graph.");

  graph.get(start).remove(dest);
  graph.get(dest).remove(start);
 }

 public boolean isEdgeExists(T start, T end) {
  if (!graph.containsKey(start) || !graph.containsKey(end))
   throw new NoSuchElementException("Nodes must be in the graph.");

  return graph.get(start).contains(end);
 }

 public Set<T> getNeighbors(T node) {
  Set<T> neighbors = graph.get(node);
  if (neighbors == null)
   throw new NoSuchElementException("Node does not exist.");

  return Collections.unmodifiableSet(neighbors);
 }

 public Iterator<T> iterator() {
  return graph.keySet().iterator();
 }
 public Iterable<T> getNodes() {
  return graph.keySet();
 }
 public int size() {
  return graph.size();
 }
 public boolean isEmpty() {
  return graph.isEmpty();
 }
}