public void throwIOException(int code) throws IOException {
     throw new IOException(IO.getMessage(code));
   }
  

   public int read() throws IOException {
     int value=readAByte()
     if (value!=-1)
       return value;
     throwIOException(IO.errorCode);
   }
  

Because there is no way to say to the compiler something like "hey, the method throwIOEXception doesn't return", the compiler emit an error because the method read() could return without a value.

In the now famous closure proposal, there is a small section titled "The type of null" that i understand like this: if you use null as return type, you can indicate that the method never return.

   public null throwIOException(int code) throws IOException {
     throw new IOException(getMessage(code));
   }
  

Pretty cool !

update: see Function that does not return normally II (the return) for the part2 of this entry.