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.Consumer;
  import java.util.function.Supplier;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Supplier that awaits a value and allows the value to be set, once,
   * to a not-null value. The value supplied by {@link #get} is never null.
   *
   * <p>Not thread-safe.
   *
   * @param <E> Element type
   */
  public class MonotonicSupplier<E> implements Consumer<E>, Supplier<E> {
    private @Nullable E e = null;
  
    /** Creates a MonotonicSupplier. */
    public static <E> MonotonicSupplier<E> empty() {
      return new MonotonicSupplier<>();
    }
  
    /**
     * {@inheritDoc}
     *
     * <p>Sets the value once and for all.
     */
    @Override public void accept(E e) {
      if (this.e != null) {
        throw new IllegalArgumentException("accept has been called already");
      }
      this.e = requireNonNull(e, "element must not be null");
    }
  
    @Override public E get() {
      if (e == null) {
        throw new IllegalArgumentException("accept has not been called");
      }
      return e;
    }
  }
  