Builtin charsets can be trusted more than user-provided charsets,
and this provides optimization opportunities.


/**
 * A {@link Charset} that is provided as part of the JDK.
 * Such a charset guarantees that the following pathological
 * conditions never occur:
 *
 * <ul>
 * <li>no collisions among builtin charset names or aliases
 * <li>the charset encoders and decoders never retain references to
 * the source or destination arrays after a coding operation
 * <li>the {@code Charset} constructors are always called with valid
 * arguments
 * </ul>
 *
 * <p>The default security manager enforces that user-provided
 * charsets do not subclass this class.
 */
public abstract class BuiltinCharset extends Charset {
    protected BuiltinCharset(String canonicalName, String[] aliases) {
        super(canonicalName, aliases);
    }
}

In particular, do the CharsetEncoder and CharsetDecoder constructors really need to
validate their input every single time, given that their parameters are build-time
constants in practice?

    {#if[encoder]?protected:private}
    Charset$Coder$(Charset cs,
float average$ItypesPerOtype$,
float max$ItypesPerOtype$,
$replType$ replacement)
    {
this.charset = cs;
this.average$ItypesPerOtype$ = average$ItypesPerOtype$;
this.max$ItypesPerOtype$ = max$ItypesPerOtype$;
this.replacement = replacement;

        if (cs instanceof sun.nio.cs.BuiltinCharset)
            assertConstructorPreconditions(cs,
                                           average$ItypesPerOtype$,
                                           max$ItypesPerOtype$,
                                           replacement);
        else
            checkConstructorPreconditions(cs,
                                          average$ItypesPerOtype$,
                                          max$ItypesPerOtype$,
                                          replacement);
    }

Also, the StringCoding class could do this to check for whether defensive copies
are necessary:

    private static boolean charsetIsTrusted(Charset cs) {
        return System.getSecurityManager() == null ||
            cs instanceof sun.nio.cs.BuiltinCharset ||
            cs.getClass().getClassLoader0() == null;
    }