Here are a number of API specification problems/requests. Filling in some missing information will enable the development of better JCK tests (and better Java portability)

1) Please document IllegalArgumentExceptions wherever these are thrown
2) Please document when NullPointerException may be expected
3) IndexOutOfBounds should be documented as thrown when read or write requests are out of bounds
4) If I open a Writer or Reader on a closed stream, the behavior is at the discretion of the implementor. The spec should document this.
5) StringReader.ready() is always true. This should be specified.
6) StringWriter(int) - please document what happens if int = 0 or int is negative.
7) CharArrayReader's constructor takes integers. Please document the legal values. Similarly document CharArrayWriter(int) legal values...
8) The behavior of PipedReader/PipedWriter.connect when already connected should be documented.
9) Reader does not support mark. It should be documented.
10) please document what happens if you call an i/o method on an unconnected PipedReader/PipedWriter...
11) Here is an attempt at an exhaustive list of the parameter values that need to be specified, along with the exceptions you see if they are invalid: (apologies if there is some duplication with the above, here)
BufferedReader: read(char[], int, int)
  skip(long)
BufferedWriter: write(char[], int,int), write(String, int,int)
CharArrayReader: CharArrayReader(char[],int,int) read(char[],int,int) skip(long) mark(int)
CharArrayWriter: CharArrayWriter(int) write(char[],int,int) write(String,int,int)
FilterReader: skip(long)
FilterWriter: write(char[],int,int) write(String,int,int)
InputStreamReader: read(char[],int,int)
LineNumberReader: LineNumberReader(Reader, size) setLineNumber(int) read(char[],int,int) skip(long) mark(int)
OutputStreamWriter: write(char[], int,int) write(String,int,int)
PipedReader: read(char[], int,int)
PipedWriter: write(char[],int,int)
PrintWriter: write(char[],int,int) write(String,int,int)
PushbackReader: PushbackReader(Reader,int) read(char[] int int) unread(int) unread(char[], int,int)
Reader: read(char[], int,int) skip(long) mark(int)
StringReader: read(char[] int int) skip(long)
StringWriter: StringWriter(int) write(char[] int int) write(String, int,int)
Writer: write(char[],int,int) write(String,int,int)