Java tutorial
/* * 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.thrift; /** * Implementation of the Option type pattern */ public abstract class Option<T> { @SuppressWarnings("rawtypes") private static final Option NONE = new None(); /** * Whether the Option is defined or not * @return * true if the Option is defined (of type Some) * false if the Option is not defined (of type None) */ public abstract boolean isDefined(); /** * Get the value of the Option (if it is defined) * @return the value * @throws IllegalStateException if called on a None */ public abstract T get(); /** * Get the contained value (if defined) or else return a default value * @param other what to return if the value is not defined (a None) * @return either the value, or other if the value is not defined */ public T or(T other) { if (isDefined()) { return get(); } else { return other; } } /** * The None type, representing an absent value (instead of "null") */ public static class None<T> extends Option<T> { public boolean isDefined() { return false; } public T get() { throw new IllegalStateException("Cannot call get() on None"); } public String toString() { return "None"; } } /** * The Some type, representing an existence of some value * @param <T> The type of value */ public static class Some<T> extends Option<T> { private final T value; public Some(T value) { this.value = value; } public boolean isDefined() { return true; } public T get() { return value; } public String toString() { return "Some(" + value + ")"; } } /** * Wraps value in an Option type, depending on whether or not value is null * @param value * @param <T> type of value * @return Some(value) if value is not null, None if value is null */ public static <T> Option<T> fromNullable(T value) { if (value != null) { return some(value); } else { return none(); } } /** * Wrap value in a Some type (NB! value must not be null!) * @param value * @param <T> type of value * @return a new Some(value) */ public static <T> Some<T> some(T value) { return new Some<T>(value); } @SuppressWarnings("unchecked") public static <T> None<T> none() { return (None<T>) NONE; } }