RFR: 8270490: Charset.forName() taking fallback default value [v2]

[Glavo]

On Wed, 20 Oct 2021 19:02:30 GMT, Naoto Sato <naoto at openjdk.org> wrote:

>> During the review of JEP 400, a proposal to provide an overloaded method to `Charset.forName()` was suggested [[1]](https://github.com/openjdk/jdk/pull/4733#discussion_r669693954). This PR is to implement the proposal. A CSR is also drafted as https://bugs.openjdk.java.net/browse/JDK-8275348
>
> Naoto Sato has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Moved the null sentence into @param tag.

I'm not reviewer.

I think maybe we should check all the usage of `Charset.isSupported`.

At present, `Charset.isSupported` and `Charset.forName` are often used continuously, such as in `StringCoding`:


class StringCoding {
    // ...
    private static Charset lookupCharset(String csn) {
        if (Charset.isSupported(csn)) {
            try {
                return Charset.forName(csn);
            } catch (UnsupportedCharsetException x) {
                throw new Error(x);
            }
        }
        return null;
    }
    //...
}


This calls `Charset.lookup` twice. Replacing it with such code should eliminate unnecessary lookup:


private static Charset lookupCharset(String csn) {
    return Charset.forName(csn, null);
}

-------------

PR: https://git.openjdk.java.net/jdk/pull/6045