8.4.3

Method Modifiers

CLASSES

subclass

C

 of the

abstract

 class

A

, if

C

 is not

abstract

, then there must be some

class

B

 such that all of the following are true:

B

 is a superclass of

C

or is

C

 itself.

B

 is a subclass of

A

.

B

 provides a declaration of the method

m

 that is not

abstract

, and this decla 

ration is inherited by

C

, thereby providing an implementation of method

m

that is visible to

C

.

If there is no such class

B

, then a compile time error occurs.

It is a compile time error for a

private

 method to be declared

abstract

. It

would be impossible for a subclass to implement a

private abstract

 method,

because

private

 methods are not visible to subclasses; therefore such a method

could never be used.

It is a compile time error for a

static

 method to be declared

abstract

.

It is a compile time error for a

final

 method to be declared

abstract

.

An

abstract

 class can override an

abstract

 method by providing another

abstract

 method declaration. This can provide a place to put a documentation

comment ( 18), or to declare that the set of checked exceptions ( 11.2) that can

be thrown by that method, when it is implemented by its subclasses, is to be more

limited. For example, consider this code:

class BufferEmpty extends Exception {

BufferEmpty() { super(); }

BufferEmpty(String s) { super(s); }

}

class BufferError extends Exception {

BufferError() { super(); }

BufferError(String s) { super(s); }

}

public interface Buffer {

char get() throws BufferEmpty, BufferError;

}

public abstract class InfiniteBuffer implements Buffer {

abstract char get() throws BufferError;

}

The overriding declaration of method

get

 in class

InfiniteBuffer

 states that

method

get

 in any subclass of

InfiniteBuffer

 never throws a

BufferEmpty

exception, putatively because it generates the data in the buffer, and thus can never

run out of data.

158