Lazy.java

/**
 * Licensed to The Apereo Foundation under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.
 *
 *
 * The Apereo Foundation licenses this file to you under the Educational
 * Community 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://opensource.org/licenses/ecl2.txt
 *
 * 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.opencastproject.util.data;

/**
 * The value of a lazy is calculated on first access.
 * <h3>Implementation note</h3>
 * The calculation is not synchronized for performance reasons. As a trade off the value may be calculated
 * multiple times if many threads concurrently access the lazy in uninitialized state.
 * Since <code>f</code> is expected to be a <em>pure</em> function this is not a serious problem despite the
 * fact that calculation <em>may</em> happens multiple times.
 */
public final class Lazy<A> {
  private volatile A a;
  private final Function0<A> f;

  /**
   * Create a new lazy value.
   *
   * @param f
   *        a <em>pure</em>, referentially transparent function returning the lazy's value
   */

  public Lazy(Function0<A> f) {
    this.f = f;
  }

  public static <A> Lazy<A> lazy(Function0<A> f) {
    return new Lazy<A>(f);
  }

  public A value() {
    if (a == null) {
      a = f.apply();
    }
    return a;
  }
}