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 java.util.concurrent.atomic.AtomicReference;
  import java.util.function.Supplier;
  
  /**
   * This class can is able to do a lazy initiaization
   * of an object based on a {@code Supplier}.
   *
   * <p>In contrast to {@code Suppliers.memoize}, the supplier is passed
   * at the later point in time, during the call to {@code getOrCompute}.
   * This allows the usage in abstract base classes with a supplier
   * method, implemented in a derived class.
   *
   * @param <T> Element Type
   */
  public class LazyReference<T> {
  
    private final AtomicReference<T> value = new AtomicReference<>();
  
    /**
     * Atomically sets the value to {@code supplier.get()}
     * if the current value was not set yet.
     *
     * <p>This method is reentrant. Different threads will
     * get the same result.
     *
     * @param supplier supplier for the new value
     * @return the current value.
     */
    public T getOrCompute(Supplier<T> supplier) {
      while (true) {
        T result = value.get();
        if (result != null) {
          return result;
        }
        T computed = supplier.get();
        if (value.compareAndSet((T) null, computed)) {
          return computed;
        }
      }
    }
  
    /**
     * Resets the current value.
     */
    public void reset() {
      value.set((T) null);
    }
  }
  