The Java Language Specification, Third Edition

CLASSES Method Modifiers 8.4.3
an implementation. The declaration of an abstract method m must appear
directly within an abstract class (call it A) unless it occurs within an enum
(§8.9); otherwise a compile-time error results. Every subclass of A that is not
abstract must provide an implementation for m, or a compile-time error occurs
as specified in §8.1.1.1.
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 inherited by 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, to refine the return
type, 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 {
public abstract char get() throws BufferError;
}
DRAFT
The overriding declaration of method get in class InfiniteBuffer states
that method get in any subclass of InfiniteBuffer never throws a Buffer-
Empty exception, putatively because it generates the data in the buffer, and thus
can never run out of data.
An instance method that is not abstract can be overridden by an abstract
method.
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