Summary

Provide mapping between SourceVersion values and Runtime.Version values.

Problem

Helpful to be able to map between SourceVersion values and Runtime.Version in various contexts. Indirectly, this provides a mapping between version strings and SourceVersion values via the Runtime.Version.parse() method.

Solution

Add two methods to SourceVersion, one mapping SourceVersion to Runtime.Version, the other mapping Runtime.Version to SourceVersion.

Specification

+
+    /**
+     * {@return the latest source version that is usable under the
+     * runtime version argument} If the runtime version's {@linkplain
+     * Runtime.Version#feature() feature} is greater than the feature
+     * of the {@linkplain #runtimeVersion() runtime version} of the
+     * {@linkplain #latest() latest source version}, an {@code
+     * IllegalArgumentException} is thrown.
+     *
+     * <p>Because the source versions of the Java programming language
+     * have so far followed a linear progression, only the feature
+     * component of a runtime version is queried to determine the
+     * mapping to a source version. If that linearity changes in the
+     * future, other components of the runtime version may influence
+     * the result.
+     *
+     * @apiNote
+     * An expression to convert from a string value, for example
+     * {@code "17"}, to the corresponding source version, {@code
+     * RELEASE_17}, is:
+     *
+     * <pre>{@code SourceVersion.valueOf(Runtime.Version.parse("17"))}</pre>
+     *
+     * @param rv runtime version to map to a source version
+     * @throws IllegalArgumentException if the feature of version
+     * argument is greater than the feature of the platform version.
+     */
+    public static SourceVersion valueOf(Runtime.Version rv) {
+        // Could also implement this as a switch where a case was
+        // added with each new release.
+        return valueOf("RELEASE_" + rv.feature());
+    }
+
+    /**
+     * {@return the least runtime version that supports this source
+     * version; otherwise {@code null}} The returned runtime version
+     * has a {@linkplain Runtime.Version#feature() feature} large
+     * enough to support this source version and has no other elements
+     * set.
+     *
+     * Source versions greater than or equal to {@link RELEASE_6}
+     * have non-{@code null} results.
+     */
+    public Runtime.Version runtimeVersion() {
+        // The javax.lang.model API was added in JDK 6; for now,
+        // limiting supported range to 6 and up.
+        if (this.compareTo(RELEASE_6) >= 0) {
+            return Runtime.Version.parse(Integer.toString(ordinal()));
+        } else {
+            return null;
+        }
+    }