Initialize a collection within an object?

Question

If an object has a property that is a collection, should the object create the collection object or make a consumer check for null? I know the consumer should not assume, just

Answer 1

As a general rule, I think the object should create the collection and provide methods for manipulating it; add/get/remove. When following this rule, you should not provide methods for setting the collection directly. If you want a method like setSomeCollection(Collection someCollection) for convenience, the implementation of this method should not set the reference within the object, but copy the values from the method parameter collection to the collection within the object;

public void setSomeCollection(Collection someCollection) {
   this.someCollection.clear();
   this.someCollection.addAll(someCollection);
}

You should do this, so clients of your API cannot get access to the collection that your class maintains. If they do, they can manipulate it without using objects of your class and violate constraints that your class guarantees.

The same is true if providing a getter; you should either return a copy of your internal colleciton, or a read-only version of it. Apache commons-collections and google-collections provide methods for creating immutable wrappers around a collection, aswell as the ones that come with Java in the java.util package; java.util.Collections.unmodifiable*.