The Java Language Specification, Third Edition

8.4.10 Examples of Method Declarations CLASSES
236
The subclass LineBufferOutput 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 BufferOutput.
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 subclass 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 implementation of putstr in a future implementation
of BufferOutput not to use the method putchar, because this would break
the preexisting contract with subclasses. See the further discussion of binary compatibility
in §13, especially §13.2.
8.4.10.7 Example: Incorrect Overriding because of Throws
This example uses the usual and conventional form for declaring a new exception
type, in its declaration of the class BadPointException:
class BadPointException extends Exception {
BadPointException() { super(); }
BadPointException(String s) { super(s); }
}
class Point {
int x, y;
void move(int dx, int dy) { x += dx; y += dy; }
}
class CheckedPoint extends Point {
void move(int dx, int dy) throws BadPointException {
if ((x + dx) < 0 || (y + dy) < 0)
throw new BadPointException();
x += dx; y += dy;
}
}
DRAFT