A handy solution for performing a shallow copy of HashMap relies on the HashMap constructor, HashMap​(Map<? extends K,​? extends V> m). The following code is self-explanatory:

Map<K, V> mapToCopy = new HashMap<>();
Map<K, V> shallowCopy = new HashMap<>(mapToCopy);

Another solution may rely on the putAll​(Map<? extends K,​? extends V> m) method. This method copies all of the mappings from the specified map to this map, as shown in the following helper method:

@SuppressWarnings("unchecked")
public static <K, V> HashMap<K, V> shallowCopy(Map<K, V> map) {

  HashMap<K, V> copy = new HashMap<>();
  copy.putAll(map);

  return copy;
}

We can also write a helper method in Java 8 functional style as follows:

@SuppressWarnings("unchecked")
public static <K, V> HashMap<K, V> shallowCopy(Map<K, V> map) {

  Set<Entry<K, V>> entries = map.entrySet();
  HashMap<K, V> copy = (HashMap<K, V>) entries.stream()
    .collect(Collectors.toMap(
       Map.Entry::getKey, Map.Entry::getValue));

  return copy;
}

However, these three solutions only provide a shallow copy of the map. A solution for obtaining a deep copy can rely on the Cloning library (https://github.com/kostaskougios/cloning) introduced in Chapter 2, Objects, Immutability, and Switch Expressions. A helper method that will use Cloning can be written as follows:

@SuppressWarnings("unchecked") 
public static <K, V> HashMap<K, V> deepCopy(Map<K, V> map) {
  Cloner cloner = new Cloner();
  HashMap<K, V> copy = (HashMap<K, V>) cloner.deepClone(map);

  return copy;
}