8.4.8

Examples of Method Declarations

CLASSES

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 implementa 

tion of

BufferOutput

 not to use the method

putchar

, because this would break

the preexisting contract with subclasses. See the further discussion of binary com 

patibility in  13, especially  13.2.

8.4.8.7

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;

}

}

This example results in a compile time error, because the override of method

move

 in class

CheckedPoint

 declares that it will throw a checked exception that

the

move

 in class

Point

 has not declared. If this were not considered an error, an

invoker of the method

move

 on a reference of type

Point

 could find the contract

between it and

Point

 broken if this exception were thrown.

Removing the

throws

 clause does not help:

class CheckedPoint extends Point {

void move(int dx, int dy) {

if ((x + dx) < 0 || (y + dy) < 0)

throw new BadPointException();

x += dx; y += dy;

}

}

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