Java Coding Problems

116. Copying HashMap

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") ...