Java Reference
In-Depth Information
Collection<T>
interface). This turns out to be a very powerful tool. Let us explore the
possibilities. So far, we have seen one class that implements the
Collection<T>
interface,
namely the class
ArrayList<T>
. In addition to the methods given in Chapter 14 for the
ArrayList<T>
class, the
ArrayList<T>
class also implements all the methods given
in Display 16.2. There are a number of different predefined classes that implement the
Collection<T>
interface, and you can define your own classes to do this. If you write a
method to manipulate a parameter of type
Collection<T>
, it will work for all of these
classes. Also, the methods in the
Collection<T>
interface ensure that you can intermix
the use of different collection classes. For example, consider the method
public boolean
containsAll(Collection<?> collectionOfTargets)
You can use this with two
ArrayList<T>
objects (the calling object and the argument)
to see if one contains all the elements of the other. The two
ArrayList<T>
objects
do not even have to have the same base type. Moreover, you can also use it with
an
ArrayList<T>
object and an object of any other class that implements the
Collection<T>
interface to compare the elements in these two different kinds of
Collection<T>
objects.
Display 16.2
Method Headings in the
Collection<T>
Interface
(part 1 of 3)
The
Collection<T>
interface is in the
java.util
package.
CONSTRUCTORS
Although not officially required by the interface, any class that implements the
Collection<T>
interface should have at least two constructors: a no-argument constructor that creates an empty
Collection<T>
object, and a constructor with one parameter of type
Collection<? extends
T>
that creates a
Collection<T>
object with the same elements as the constructor argument.
The interface does not specify whether the copy produced by the one-argument constructor is a
shallow copy or a deep copy of its argument.
METHODS
boolean
isEmpty()
Returns
true
if the calling object is empty; otherwise returns
false
.
public boolean
contains(Object target)
Returns
true
if the calling object contains at least one instance of
target
. Uses
target.
equals
to determine if
target
is in the calling object.
public boolean
containsAll(Collection<?> collectionOfTargets)
Returns
true
if the calling object contains all of the elements in
collectionOfTargets
. For
an element in
collectionOfTargets
, this method uses
element.equals
to determine if
element
is in the calling object.
(continued)
