content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to you under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.calcite.rel.metadata;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /** Placeholder for null values. */
  public enum NullSentinel {
    /** Placeholder for a null value. */
    INSTANCE {
      @Override public String toString() {
        return "NULL";
      }
    },
  
    /** Placeholder that means that a request for metadata is already active,
     * therefore this request forms a cycle. */
    ACTIVE;
  
    public static Comparable mask(@Nullable Comparable value) {
      if (value == null) {
        return INSTANCE;
      }
      return value;
    }
  
    public static Object mask(@Nullable Object value) {
      if (value == null) {
        return INSTANCE;
      }
      return value;
    }
  }
  