Java Reference
In-Depth Information
Let's examine
arraysEqual( )
closely. First, notice how it is declared by this line:
The type parameters are declared
before
the return type of the method. Also note that
T
extends
Comparable<T>
.
Comparable
is an interface declared in
java.lang
. A class that
implements
Comparable
defines objects that can be ordered. Thus, requiring an upper
bound of
Comparable
ensures that
arraysEqual( )
can be used only with objects that are
capable of being compared.
Comparable
is generic, and its type parameter specifies the
type of objects that it compares. (Shortly, you will see how to create a generic interface.)
Next, notice that the type
V
is upper-bounded by
T
. Thus,
V
must be either the same as
type
T
or a subclass of
T
. This relationship enforces that
arraysEqual( )
can be called only
with arguments that are comparable with each other. Also notice that
arraysEqual( )
is
static, enabling it to be called independently of any object. Understand, though, that gener-
ic methods can be either static or nonstatic. There is no restriction in this regard.
Now, notice how
arraysEqual( )
is called within
main( )
by use of the normal call syn-
tax, without the need to specify type arguments. This is because the types of the arguments
are automatically discerned, and the types of
T
and
V
are adjusted accordingly. For ex-
ample, in the first call:
the element type of the first argument is
Integer
, which causes
Integer
to be substituted
for
T
. The element type of the second argument is also
Integer
, which makes
Integer
a
substitute for
V
, too. Thus, the call to
arraysEqual( )
is legal, and the two arrays can be
compared.
Now, notice the commented-out code, shown here:
If you remove the comments and then try to compile the program, you will receive an error.
The reason is that the type parameter
V
is bounded by
T
in the
extends
clause in
V
's de-
claration. This means that
V
must be either type
T
or a subclass of
T
. In this case, the first
argument is of type
Integer
, making
T
into
Integer
, but the second argument is of type
