Collections, Maps and Iterators - Java - page 845

classes should implement. A summary of these operations (method headings) for the

interface are given in Display 16.2. A more complete description can
be found in Appendix 5. Because an interface is a type, you can define methods with
a parameter of type

Collection<T>

. That parameter can be filled in with an argu-
ment that is an object of any class in the collection framework (that is, any class that
implements the

Collection<T>

interface). This turns out to be a very powerful tool.
Let's explore the possibilities. So far, we have seen one class that implements the

Collection<T>

interface, namely the class . In addition to the meth-

Collection<T> ArrayList<T>

ods given in Chapter 14 for the class also

implements all the methods given in Display 16.2. There are a number of different
predefined classes that implement the

class, the ArrayList<T> ArrayList<T>

interface, and you can define Collection<T>

your own classes that implement the interface. If you write a method

Collection<T>

to manipulate a parameter of type , it will work for all of these Collection<T>

classes. Also, the methods in the interface ensure that you can inter-

mix the use of different collection classes. For example, consider the method

Collection<T>

public boolean containsAll(Collection<?> collectionOfTargets)

You can use this with two objects (one the calling object and one the argu-

ment) 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.

ArrayList<T>

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.

(continued)