Java Reference
In-Depth Information
}
class Test {
public static void main(String[] args) throws IOException {
LineBufferOutput lbo = new LineBufferOutput(System.out);
lbo.putstr("lbo\nlbo");
System.out.print("print\n");
lbo.putstr("\n");
}
}
This program produces the output:
lbo
print
lbo
The class
BufferOutput
implements a very simple buffered version of an
OutputStream
,
flushing the output when the buffer is full or
flush
is invoked. The subclass
LineBuffer-
Output
declares only a constructor and a single method
putchar
, which overrides the
method
putchar
of
BufferOutput
. It inherits the methods
putstr
and
flush
from class
Buffer-
Output
.
In the
putchar
method of a
LineBufferOutput
object, if the character argument is a
newline, then it invokes the
flush
method. The critical point about overriding in this
example is that the method
putstr
, which is declared in class
BufferOutput
, invokes the
putchar
method defined by the current object
this
, which is not necessarily the
putchar
method declared in class
BufferOutput
.
Thus, when
putstr
is invoked in
main
using the
LineBufferOutput
object
lbo
, the invocation
of
putchar
in the body of the
putstr
method is an invocation of the
putchar
of the object
lbo
, the overriding declaration of
putchar
that checks for a newline. This allows a sub-
class of
BufferOutput
to change the behavior of the
putstr
method without redefining it.
Documentation for a class such as
BufferOutput
, which is designed to be extended,
should clearly indicate what is the contract between the class and its subclasses, and
should clearly indicate that subclasses may override the
putchar
method in this way.
The implementor of the
BufferOutput
class would not, therefore, want to change the im-
plementation of
putstr
in a future implementation of
BufferOutput
not to use the meth-
od
putchar
, because this would break the pre-existing contract with subclasses. See the
8.4.8.2. Hiding (by Class Methods)
If a class declares a
static
method
m
, then the declaration
m
is said to
hide
any method
m'
,
where the signature of
m
is a subsignature (§
8.4.2
) of the signature of
m'
, in the super-
