RFR: 8246797: A convenient method to read OPTIONAL element

[Jamil Nimeh]

On Mon, 23 Aug 2021 20:23:44 GMT, Weijun Wang <weijun at openjdk.org> wrote:

>> src/java.base/share/classes/sun/security/util/DerInputStream.java line 353:
>> 
>>> 351:      */
>>> 352:     public boolean seeOptionalContextSpecific(int n) throws IOException {
>>> 353:         return checkNextTag(t -> (t & 0x0c0) == 0x080 && (t & 0x01f) == n);
>> 
>> Minor nit for clarity, the 0x080 could use the static DerValue.TAG_CONTEXT.
>> 
>> On an unrelated note, it's a bummer that we don't have names for the typical masking constants 0x0c0 and 0x01f for getting at the tag class and number values (or static methods maybe).  But those numeric literals are only in wide use in DerValue.java and maybe not worth changing since you don't touch that file.  It might make that code a bit clearer, but it is just a nit.
>
> In fact, I tried using DerValue.TAG_CONTEXT at the beginning but it's defined as a byte so it's actually -128, and will never equal to any `t & 0x0c0`. Even the `DerValue::isContextSpecific` uses 0x80 directly. (Unfortunately this is not a static method otherwise I'll use it).

Yep, you're right.  Sign extension strikes again!  Nevermind then, it's fine as-is.

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PR: https://git.openjdk.java.net/jdk/pull/5221