Section 12.3. Collection Constructors

12.3. Collection Constructors

We will go on to look at these three main kinds of collection in the next three chapters, but we should first explain two common forms of constructor which are shared by most collection implementations. Taking HashSet as an example, these are:

 public HashSet() public HashSet(Collection<? extends E> c) 

The first of these creates an empty set, and the second a set that will contain the elements of any collection of the parametric typeor one of its subtypes, of course. Using this constructor has the same effect as creating an empty set with the default constructor, and then adding the contents of a collection using addAll. This is sometimes called a "copy constructor", but that term should really be reserved for constructors which make a copy of an object of the same class, whereas constructors of the second form can take any object which implements the interface Collection<? extends E>. Joshua Bloch has suggested the term "conversion constructor".

Not all collection classes have constructors of both formsArrayBlockingQueue, for example, cannot be created without fixing its capacity, and SynchronousQueue cannot hold any elements at all, so no constructor of the second form is appropriate. In addition, many collection classes have other constructors besides these two, but which ones they have depends not on the interface they implement but on the underlying implementation; these additional constructors are used to configure the implementation.