001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.juneau.commons.settings;
018
019import static org.apache.juneau.commons.utils.Utils.*;
020
021import java.util.*;
022import java.util.function.*;
023
024/**
025 * A functional interface for creating read-only {@link SettingSource} instances from a function.
026 *
027 * <p>
028 * This functional interface allows you to create setting sources directly from lambda expressions or method references,
029 * making it easy to wrap existing property sources (e.g., {@link System#getProperty(String)},
030 * {@link System#getenv(String)}) as {@link SettingSource} instances.
031 *
032 * <p>
033 * Functional sources are read-only and do not implement {@link SettingStore}. If you need a writable source,
034 * use {@link MapStore} or {@link FunctionalStore} instead.
035 *
036 * <h5 class='section'>Return Value Semantics:</h5>
037 * <ul class='spaced-list'>
038 *    <li>If the function returns <c>null</c>, this source returns <c>null</c> (key doesn't exist).
039 *    <li>If the function returns a non-null value, this source returns <c>Optional.of(value)</c>.
040 * </ul>
041 *
042 * <p>
043 * Note: This source cannot distinguish between a key that doesn't exist and a key that exists with a null value,
044 * since the function only returns a <c>String</c>. If you need to distinguish these cases, use {@link MapStore} instead.
045 *
046 * <h5 class='section'>Example:</h5>
047 * <p class='bjava'>
048 *    <jc>// Create a read-only source directly from a lambda (returns Optional)</jc>
049 *    Settings.<jsf>get</jsf>().addSource(name -&gt; opt(System.getProperty(name)));
050 *
051 *    <jc>// Using the static factory method (takes Function&lt;String, String&gt;)</jc>
052 *    Settings.<jsf>get</jsf>().addSource(FunctionalSource.<jsf>of</jsf>(System::getProperty));
053 *
054 *    <jc>// Create a read-only source from System.getenv</jc>
055 *    Settings.<jsf>get</jsf>().addSource(FunctionalSource.<jsf>of</jsf>(System::getenv));
056 *
057 *    <jc>// Explicit creation for reuse</jc>
058 *    FunctionalSource <jv>sysProps</jv> = FunctionalSource.<jsf>of</jsf>(System::getProperty);
059 *    Settings.<jsf>get</jsf>().addSource(<jv>sysProps</jv>);
060 * </p>
061 */
062@FunctionalInterface
063public interface FunctionalSource extends SettingSource {
064
065   /**
066    * Returns a setting by applying the function.
067    *
068    * <p>
069    * If the function returns <c>null</c>, this method returns <c>null</c> (indicating the key doesn't exist).
070    * If the function returns a non-null value, this method returns <c>Optional.of(value)</c>.
071    *
072    * @param name The property name.
073    * @return The property value, or <c>null</c> if the function returns <c>null</c>.
074    */
075   @Override
076   Optional<String> get(String name);
077
078   /**
079    * Creates a functional source from a function that returns a string.
080    *
081    * <p>
082    * This is a convenience factory method for creating functional sources from functions that return
083    * <c>String</c> values. The function's return value is converted to an <c>Optional</c> as follows:
084    * <ul>
085    *    <li>If the function returns <c>null</c>, the source returns <c>null</c> (key doesn't exist).
086    *    <li>If the function returns a non-null value, the source returns <c>Optional.of(value)</c>.
087    * </ul>
088    *
089    * <h5 class='section'>Example:</h5>
090    * <p class='bjava'>
091    *    <jc>// Create from a lambda</jc>
092    *    FunctionalSource <jv>source1</jv> = FunctionalSource.<jsf>of</jsf>(name -&gt; System.getProperty(name));
093    *
094    *    <jc>// Create from a method reference</jc>
095    *    FunctionalSource <jv>source2</jv> = FunctionalSource.<jsf>of</jsf>(System::getProperty);
096    * </p>
097    *
098    * @param function The function to delegate property lookups to. Must not be <c>null</c>.
099    * @return A new functional source instance.
100    */
101   static FunctionalSource of(Function<String, String> function) {
102      return name -> {
103         var v = function.apply(name);
104         return v == null ? null : opt(v);
105      };
106   }
107}