To be specific

310 * @throws IllegalArgumentException if a malformed Unicode escape
311 * appears in the input.
312 * @throws NullPointerException if {@code reader} is null.

and

336 * @throws IllegalArgumentException if the input stream contains a
337 * malformed Unicode escape sequence.
338 * @throws NullPointerException if {@code inStream} is null.

(It does not show up well above, but there are two and five spaces before the IAE clauses, respectively, and the same before the respective NPE clauses.)

While we're at it, the j.u.Properties "Basic.java" test and its @summary tag should be changed to something specific to what is being tested.