| /* |
| * 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.wayang.core.util; |
| |
| import java.util.stream.Stream; |
| |
| /** |
| * Utility similar to {@link java.util.Optional}. However, it supports {@code null} as a valid value. |
| */ |
| public abstract class Optional<T> { |
| |
| /** |
| * @return whether this instance contains a value. |
| */ |
| public abstract boolean isAvailable(); |
| |
| /** |
| * @return the value contained by this instance |
| * @throws IllegalStateException if {@link #isAvailable()} is {@code false} |
| */ |
| public abstract T getValue(); |
| |
| /** |
| * @return a {@link Stream} containing the optional value |
| */ |
| public abstract Stream<T> stream(); |
| |
| private static final Optional NA = new Optional() { |
| @Override |
| public boolean isAvailable() { |
| return false; |
| } |
| |
| @Override |
| public Object getValue() { |
| throw new IllegalStateException("No value available."); |
| } |
| |
| @Override |
| public Stream stream() { |
| return Stream.empty(); |
| } |
| }; |
| |
| /** |
| * Create a n/a instance. |
| * |
| * @return an {@link Optional} without a value |
| */ |
| @SuppressWarnings("unchecked") |
| public static <T> Optional<T> na() { |
| return NA; |
| } |
| |
| /** |
| * Create a new instance with the given {@code value}. |
| * |
| * @param value the value of the new {@link Optional} |
| * @return the new instance |
| */ |
| public static <T> Optional<T> of(final T value) { |
| return new Optional<T>() { |
| @Override |
| public boolean isAvailable() { |
| return true; |
| } |
| |
| @Override |
| public T getValue() { |
| return value; |
| } |
| |
| @Override |
| public Stream<T> stream() { |
| return Stream.of(value); |
| } |
| }; |
| } |
| } |