private static Pair<String, String> getKeyValueForClass(TableDescriptorBuilder builder,
    Class<? extends Constraint> clazz) {
    // get the serialized version of the constraint
    String key = serializeConstraintClass(clazz);
    String value = builder.getValue(key);

    return value == null ? null : new Pair<>(key, value);
  }
Get the kv {@link Entry} in the descriptor builder for the specified class
@param builder {@link TableDescriptorBuilder} to read
@param clazz   To search for
@return The {@link Pair} of {@literal <key, value>} in the table, if that class is present.
        {@code null} otherwise.
hbase-server/src/main/java/org/apache/hadoop/hbase/constraint/Constraints.java