String concatenation is not fully specified. Specifically, in this case

    String s1 = null;
    Object s2 = "foo";

    System.out.println(s1 + s2);

The current spec applies the string conversion to s2, but not to s1. This suggests that this should throw a NullPointerException, because

    s1.append(s2)

would throw such an error.

It has been suggested that javac should use .append instead of a StringBuffer when the static type of the two operands are both String, but the behavior is obviously not equivalent to the current behavior (using StringBuffer).

Can you please update JLS3 to more clearly specify the behavior.