The field sun.security.rsa.RSAPSSSignature#DIGEST_LENGTHS is modified only in <clinit>. It means we can use Immutable Map instead of Hashtable.

    private static final Hashtable<KnownOIDs, Integer> DIGEST_LENGTHS =
            new Hashtable<>();
    static {
        DIGEST_LENGTHS.put(KnownOIDs.SHA_1, 20);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_224, 28);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_256, 32);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_384, 48);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_512, 64);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_512$224, 28);
        DIGEST_LENGTHS.put(KnownOIDs.SHA_512$256, 32);
        DIGEST_LENGTHS.put(KnownOIDs.SHA3_224, 28);
        DIGEST_LENGTHS.put(KnownOIDs.SHA3_256, 32);
        DIGEST_LENGTHS.put(KnownOIDs.SHA3_384, 48);
        DIGEST_LENGTHS.put(KnownOIDs.SHA3_512, 64);
    }


Hashtable is legacy synchronized class, which have synchronized on its get method. It's not needed in our case.