Source code

001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *   http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing,
013 * software distributed under the License is distributed on an
014 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015 * KIND, either express or implied.  See the License for the
016 * specific language governing permissions and limitations
017 * under the License.
018 */
019package org.apache.reef.util;
020
021/**
022 * A reference to a value that can be set exactly once.
023 */
024public final class SetOnce<T> {
025
026  private Optional<T> value;
027
028  public SetOnce(final T value) {
029    this.set(value);
030  }
031
032  public SetOnce() {
033    this.value = Optional.empty();
034  }
035
036  public synchronized T get() {
037    return value.get();
038  }
039
040  @SuppressWarnings("checkstyle:hiddenfield")
041  public synchronized void set(final T value) {
042    if (this.value.isPresent()) {
043      throw new IllegalStateException("Trying to set new value " + value +
044          " while an old value was already present: " + this.value);
045    }
046    this.value = Optional.of(value);
047  }
048
049  public synchronized boolean isSet() {
050    return this.value.isPresent();
051  }
052}