RFR: 8250802: Refactor StringConverter and its subclasses

[Nir Lisker]

On Sun, 24 Aug 2025 20:14:47 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> Refactoring of all `StringConverter`s and their tests. General notes:
>> * The documentation language has been unified and `null` parameter rules have been documented.
>> *  Tests have been cleaned up in the vein of https://github.com/openjdk/jfx/pull/1759 and unneeded `@BeforeAll`s were removed.
>> * Internal fields were made `private final` to guarantee immutability.
>> 
>>  Incremental commits are provided for easier reviewing:
>> 
>> ### Parent classes
>> * `StringConverter`: updated documentation
>> * `BaseStringConverter`: a new internal class that implements repeated code from converter implementations and serves as an intermediate superclass. It does empty and `null` string checks that are handled uniformly, except for `DefaultStringConverter`, which has a different formatting mechanism.
>> 
>> ### Primitive-related converters
>> * All primitive (wrapper) converters also document their formatting and parsing mechanism since these are "well-established".
>> 
>> ### Format converter
>> * Checked for `null` during constriction time to avoid runtime NPEs.
>> * There is no test class for this converter. A followup might be desirable.
>> * A followup should deprecate for removal `protected Format getFormat()` (as in [JDK-8314597](https://bugs.openjdk.org/browse/JDK-8314597) and [JDK-8260475](https://bugs.openjdk.org/browse/JDK-8260475).
>> 
>> ### Number and subclasses converters
>> * The intermediate `locale` and `pattern` fields were removed (along with their tests). The class generated a new formatter from these on each call. This only makes sense for mutable fields where the resulting formatter can change, but here the formatter can be computed once on construction and stored.
>> * The only difference between these classes is a single method for creating a format from a `null` pattern, which was encapsulated in the `getSpecializedNumberFormat` method.
>> * The terminally deprecated `protected NumberFormat getNumberFormat()` was removed. Can be split to its own issue if preferred. In my opinion, it shouldn't exist even internally since testing the internal formatter doesn't help. The only tests here should be for to/from strings, and these are lacking. A followup can be filed for adding more conversion tests.
>> 
>> ### Date/Time converters
>> * Added a documentation note advising users to use the `java.time` classes instead of the old `Date` class.
>> * As with Number converters, only the `dateFormat` field was kept, which is created once on construction instead of on each ...
>
> modules/javafx.base/src/main/java/javafx/util/converter/DateTimeStringConverter.java line 99:
> 
>> 97:     ///        both date and time styles.
>> 98:     public DateTimeStringConverter(Locale locale, String pattern) {
>> 99:         dateFormat = create(locale, DateFormat.DEFAULT, DateFormat.DEFAULT, pattern);
> 
> Just noting inconsistent style here (see below `this.dateFormat = ` or `dateFormat = `) -- I personally favor prefixing with `this.` when it is a write action.

I only use `this` to avoid shadowing, or when in the same constructor for uniformity with other field assignment. I don't really mind it either way. I try to avoid "this" noise.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1880#discussion_r2297721228