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.util;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.function.UnaryOperator;
  
  /**
   * A mutable slot that can contain one object.
   *
   * <p>A holder is useful for implementing OUT or IN-OUT parameters.
   *
   * <p>It is possible to sub-class to receive events on get or set.
   *
   * @param <E> Element type
   */
  public class Holder<E> {
    private E e;
  
    /** Creates a Holder containing a given value.
     *
     * <p>Call this method from a derived constructor or via the {@link #of}
     * method. */
    protected Holder(E e) {
      this.e = e;
    }
  
    /** Sets the value. */
    public void set(E e) {
      this.e = e;
    }
  
    /** Gets the value. */
    public E get() {
      return e;
    }
  
    /** Applies a transform to the value. */
    public Holder<E> accept(UnaryOperator<E> transform) {
      e = transform.apply(e);
      return this;
    }
  
    /** Creates a holder containing a given value. */
    public static <E> Holder<E> of(E e) {
      return new Holder<>(e);
    }
  
    /** Creates a holder containing null. */
    @SuppressWarnings("ConstantConditions")
    public static <E> Holder<@Nullable E> empty() {
      return new Holder<@Nullable E>(null);
    }
  }
  