How safe is it to access an element in HashMap by a key that does not exist?

Accessing an element in a HashMap with a non-existent key using the get() method returns null. This is safe in the sense that it will not throw an IndexOutOfBoundsException, as might happen with an array or list when accessing an invalid index.

However, if subsequent code assumes that the result will not be null (for example, when calling methods on this object), it can lead to a NullPointerException.

For safe operation, it is recommended to:

• Check the result for null before using it:

  // Null check example
  Object value = hashMap.get(nonExistentKey);
  if (value != null) {
      // Use value
  } else {
      // Handle the case where the key is missing
  }
  

• Use methods that provide a default value or safer behavior, such as getOrDefault() (available from Java 8):

  // Using getOrDefault
  Object value = hashMap.getOrDefault(nonExistentKey, defaultValue);
  

• Use the Optional construct (from Java 8), if appropriate, to explicitly indicate the possible absence of a value:

  // Using Optional
  Optional<Object> optionalValue = Optional.ofNullable(hashMap.get(nonExistentKey));
  if (optionalValue.isPresent()) {
      // Use the value: optionalValue.get()
  }
  

Thus, directly calling get() is safe, but working with the resulting null value without proper checks can cause a NullPointerException.