The Collections Framework - Beginning Java - page 638

To add a constant to an existing set, you use the add() method:
options.add(READ);
This adds the StandardOpenOption.READ constant to options .
The static copyOf() method returns a set that is the copy of its argument. The static allOf() method
returns a set that contains all of the constants of the enumeration type you specify as the argument. For ex-
ample:
EnumSet<StandardOpenOption> optionsCopy = EnumSet.copyOf(options);
EnumSet<StandardOpenOption> allOptions = EnumSet.allOf(StandardOpenOption.class);
The optionsCopy variable references a new set identical to options . The allOptions variable referen-
ces a set containing all the constants from the StandardOpenOption enumeration.
Next, I'll present a collection class, which you will note is close to the notion of an array, as you've now
become very familiar with arrays.
ARRAY COLLECTION CLASSES
There are two collection classes that emulate arrays: Vector<T> and ArrayList<T> . Both types define a
sequence collection of elements of any type T . They work rather like an array, but with the additional feature
that they can grow automatically when you need more capacity. They also implement the List<T> interface,
so you can also access the contents of containers of these types as a list.
These collection classes are similar in function and operation. They have the same base class and imple-
ment the same set of interfaces, so understanding the differences between them is useful.
• Vector<T> is synchronized so it is safe for concurrent use by more than one thread. Ar-
rayList<T> is not synchronized so you should not allow it to be accessed by more than one
thread. You can convert an ArrayList<T> into a synchronized list using the static synchron-
izedList() method in the Collections class.
• You can specify how a Vector<T> object should increase its capacity when it is necessary whereas
with an ArrayList<T> you cannot. You can have some control over how an ArrayList<T> in-
creases in capacity by using its ensureCapacity() method that sets a given capacity.
• You can discover the current capacity of a Vector<T> by calling its capacity() method. You
have no way to know the current capacity of an ArrayList<T> unless you have set its capacity
explicitly.
• ArrayList<T> is faster in general than a Vector<T> because it does not include the overhead
for synchronized operation. However, beware of default increments in capacity with an Ar-
rayList<T> .
I describe how you use array collection classes in the context of the Vector<T> class. However, except
where noted, the methods and usage are the same with the ArrayList<T> class.