| /* |
| * 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.reef.util; |
| |
| /** |
| * A reference to a value that can be set exactly once. |
| */ |
| public final class SetOnce<T> { |
| |
| private Optional<T> value; |
| |
| public SetOnce(final T value) { |
| this.set(value); |
| } |
| |
| public SetOnce() { |
| this.value = Optional.empty(); |
| } |
| |
| public synchronized T get() { |
| return value.get(); |
| } |
| |
| @SuppressWarnings("checkstyle:hiddenfield") |
| public synchronized void set(final T value) { |
| if (this.value.isPresent()) { |
| throw new IllegalStateException("Trying to set new value " + value + |
| " while an old value was already present: " + this.value); |
| } |
| this.value = Optional.of(value); |
| } |
| |
| public synchronized boolean isSet() { |
| return this.value.isPresent(); |
| } |
| } |