How to properly define hash function for a list of objects?

I have a data structure containing a list of objects, like this:

class A {
  private List<Object> list;
}

How to properly define a hash function for the list, assuming each element of the list has 开发者_运维问答a correct hashCode()?

If the actual List implementation is fully conformant to the interface, the provided hashCode implementation should be sufficient:

Returns the hash code value for this list. The hash code of a list is defined to be the result of the following calculation:

hashCode = 1;
  Iterator i = list.iterator();
  while (i.hasNext()) {
      Object obj = i.next();
      hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
  }

(List documentation)

The List interface requires conforming implementations to provide equals based on the elements of the list. Thus, they had to specify the hashCode algorithm explicitely

Why do you want to define hashCode for your list, when it already has it implemented (along with equals)?

(Provided it is java.util.List of course - however if not, the link above shows you the exact implementation you can use for your own list type.)

The hash code of a list is defined by the List interface of List. This can be used as part of your object's hash code, though there are a couple of cases where you might not want to use it - if the elements of your list have an expensive hash code function, or if the list can hold a reference to the object, and you would then get a stack overflow if the list's algorithm was used. In that case, just use the length of the list or another hash value.

In the Java library, List implementations (LinkedList, ArrayList) use the default hashCode implementation provided by AbstractList. which is defined as:

int hashCode = 1;
Iterator<E> i = iterator();
while (i.hasNext()) {
    E obj = i.next();
    hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
}
return hashCode;

Any specific reason why you just wouldn't do:

Arrays.hashCode(<cast list to array>);

Something like:

Arrays.hashCode((String []) myList.toArray());

Maybe the question should have been "how to compute a hashcode of an object containing a list".

class A {
  private List<Object> list;
  
  @Override
  public int hashCode() {
    return list.hashCode();
  }

  // ... don't forget to implement custom equals ...
}