In which situations is it recommended to create a custom implementation of the hashCode method?

It is recommended to create a custom implementation of the hashCode method when you override the equals method in a class. This is necessary to maintain the contract between equals and hashCode: if two objects are equal according to equals, then their hashCode must be the same.

It is also advisable to implement hashCode if objects of your class will be used in hash-based collections, such as HashMap, HashSet, or Hashtable. A proper implementation of hashCode will improve the performance of these collections and ensure correct behavior.

Here is a simple example of implementing hashCode for a class with two fields:

@Override
public int hashCode() {
    int result = 17;
    result = 31 * result + (field1 != null ? field1.hashCode() : 0);
    result = 31 * result + field2;
    return result;
}