@Test
  public void testKeySetIterator() {
    mapCache.put("A", "A_value");
    mapCache.put("B", "B_value");
    mapCache.put("C", "C_value");

    assertThat(mapCache.unmodifiableKeySet()).hasSize(3);
    for (String key : mapCache.unmodifiableKeySet()) {
      assertThat(mapCache.get(key)).isEqualTo(key + "_value");
    }
  }

  @Test
  public void testPutNewValue() {
    assertThat(mapCache.put("key", "value")).isNull();

  @Test
  public void testKeySetIterator() {
    mapCache.put("A", "A_value");
    mapCache.put("B", "B_value");
    mapCache.put("C", "C_value");

    assertThat(mapCache.unmodifiableKeySet()).hasSize(3);
    for (String key : mapCache.unmodifiableKeySet()) {
      assertThat(mapCache.get(key)).isEqualTo(key + "_value");
    }
  }

  @Test
  public void testPutNewValue() {
    assertThat(mapCache.put("key", "value")).isNull();

import java.util.Map.Entry;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * A map-like data structure that wraps a backing map and caches values while iterating through
 * {@link #unmodifiableKeySet()}. By design, the cache is cleared when this structure is mutated. If
 * this structure is never mutated, it provides a thread-safe view of the backing map.
 *

    this.edgeToReferenceNode = new MapIteratorCache<>(edgeToReferenceNode);
  }

  @Override
  public Set<N> nodes() {
    return nodeConnections.unmodifiableKeySet();
  }

  @Override
  public Set<E> edges() {
    return edgeToReferenceNode.unmodifiableKeySet();
  }

  @Override
  public boolean isDirected() {
    return isDirected;
  }

  @Override
  public boolean allowsParallelEdges() {

    this.edgeToReferenceNode = new MapIteratorCache<>(edgeToReferenceNode);
  }

  @Override
  public Set<N> nodes() {
    return nodeConnections.unmodifiableKeySet();
  }

  @Override
  public Set<E> edges() {
    return edgeToReferenceNode.unmodifiableKeySet();
  }

  @Override
  public boolean isDirected() {
    return isDirected;
  }

  @Override
  public boolean allowsParallelEdges() {

            : new MapIteratorCache<N, GraphConnections<N, V>>(nodeConnections);
    this.edgeCount = checkNonNegative(edgeCount);
  }

  @Override
  public Set<N> nodes() {
    return nodeConnections.unmodifiableKeySet();
  }

  @Override
  public boolean isDirected() {
    return isDirected;
  }

  @Override
  public boolean allowsSelfLoops() {
    return allowsSelfLoops;
  }

  @Override