Collections, Maps and Iterators - Java - page 852

TIP: (continued)

will be inherited, so you need not worry too much about it. If you are defining a col-
lection class from scratch and want your class to implement one of the collection inter-
faces, then you do need to throw suitable exceptions as specified for the interface.
With one exception (no pun intended), all the exception classes mentioned in this
chapter are in the package java.lang and so do not require any import statement. The
one exception is the NoSuchElementException , which is used with iterators in Section
16.3. The NoSuchElementException is in the java.util package, which requires an
import statement if your code mentions the NoSuchElementException class.

■

Self-Test Exercises
1. Give the definition of a boolean valued static generic method named inSome . The
method inSome has two parameters of type Collection<T> and one parameter of
type T . The method returns true if the parameter of type T is in either (or both)
collections; it returns false otherwise.
2. Give the definition of a static generic method named getFirst that has one
parameter of type List<T> and a return type of T . The method returns the first
element in the list or null if the list is empty.
3. Give the definition of a static boolean valued method named noNull . The
method noNull has one parameter of type Set<?> and removes null from the set
if null is in the set; otherwise it leaves the set unchanged. The method returns
true if the set is changed and false if it is not changed.

Concrete Collection Classes
The abstract classes AbstractSet<T> and AbstractList<T> are there for convenience
when implementing the Set<T> and List<T> interfaces, respectively. They have almost
no methods beyond those in the interfaces they implement. Although these two
abstract classes have only a few abstract methods, the other (nonabstract) methods
have fairly useless implementations that must be overridden. When defining a derived
class of either AbstractSet<T> or AbstractList<T> , you need to define not just the
abstract methods but also all the methods you intend to use. It usually makes more
sense to simply use (or define derived classes of ) the HashSet<T> , ArrayList<T> , or
Vector<T> classes, which are derived classes of AbstractSet<T> and AbstractList<T>
and are full implementations of the Set<T> and List<T> interfaces.
The abstract class AbstractCollection<T> is a skeleton class for the Collection<T>
interface. Although it is perfectly legal, you seldom, if ever, need to define a derived
class of the AbstractCollection<T> class. Instead, you normally define a derived class
of one of the descendent classes of the AbstractCollection<T> class.

Abstract-
Set<T>
Abstract-
List<T>

Abstract-
Collection<T>