src/main/java/org/apache/commons/configuration2/convert/ListDelimiterHandler.java

/*
 * 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.commons.configuration2.convert;

import java.util.Collection;
import java.util.Collections;
import java.util.IdentityHashMap;
import java.util.List;

/**
 * <p>
 * Definition of an interface that controls the handling of list delimiters in configuration properties.
 * </p>
 * <p>
 * {@link org.apache.commons.configuration2.AbstractConfiguration AbstractConfiguration} supports list delimiters in
 * property values. If such a delimiter is found, the value actually contains multiple values and has to be split. This
 * is useful for instance for {@link org.apache.commons.configuration2.PropertiesConfiguration PropertiesConfiguration}:
 * properties files that have to be compatible with the {@link java.util.Properties} class cannot have multiple
 * occurrences of a single property key, therefore a different storage scheme for multi-valued properties is needed. A
 * possible storage scheme could look as follows:
 * </p>
 *
 * <pre>
 * myProperty=value1,value2,value3
 * </pre>
 *
 * <p>
 * Here a comma is used as list delimiter. When parsing this property (and using a corresponding
 * {@code ListDelimiterHandler} implementation) the string value is split, and three values are added for the property
 * key.
 * </p>
 * <p>
 * A {@code ListDelimiterHandler} knows how to parse and to escape property values. It is called by concrete
 * {@code Configuration} implementations when they have to deal with properties with multiple values.
 * </p>
 *
 * @since 2.0
 */
public interface ListDelimiterHandler {
    /**
     * A specialized {@code ValueTransformer} implementation which does no transformation. The {@code transformValue()}
     * method just returns the passed in object without changes. This instance can be used by configurations which do not
     * require additional encoding.
     */
    ValueTransformer NOOP_TRANSFORMER = value -> value;

    /**
     * Escapes the specified single value object. This method is called for properties containing only a single value. So
     * this method can rely on the fact that the passed in object is not a list. An implementation has to check whether the
     * value contains list delimiter characters and - if so - escape them accordingly.
     *
     * @param value the value to be escaped
     * @param transformer a {@code ValueTransformer} for an additional encoding (must not be <b>null</b>)
     * @return the escaped value
     */
    Object escape(Object value, ValueTransformer transformer);

    /**
     * Escapes all values in the given list and concatenates them to a single string. This operation is required by
     * configurations that have to represent properties with multiple values in a single line in their external
     * configuration representation. This may require an advanced escaping in some cases.
     *
     * @param values the list with all the values to be converted to a single value
     * @param transformer a {@code ValueTransformer} for an additional encoding (must not be <b>null</b>)
     * @return the resulting escaped value
     */
    Object escapeList(List<?> values, ValueTransformer transformer);

    /**
     * Extracts all values contained in the specified object up to the given limit. The passed in object is evaluated (if
     * necessary in a recursive way). If it is a complex object (e.g. a collection or an array), all its elements are
     * processed recursively and added to a target collection. The process stops if the limit is reached, but depending on
     * the input object, it might be exceeded. (The limit is just an indicator to stop the process to avoid unnecessary work
     * if the caller is only interested in a few values.)
     *
     * @param value the value to be processed
     * @param limit the limit for aborting the processing
     * @return a &quot;flat&quot; collection containing all primitive values of the passed in object
     * @since 2.9.0
     */
    default Collection<?> flatten(final Object value, final int limit) {
        return AbstractListDelimiterHandler.flatten(this, value, limit, Collections.newSetFromMap(new IdentityHashMap<>()));
    }

    /**
     * Parses the specified value for list delimiters and splits it if necessary. The passed in object can be either a
     * single value or a complex one, e.g. a collection, an array, or an {@code Iterable}. It is the responsibility of this
     * method to return an {@code Iterable} which contains all extracted values.
     *
     * @param value the value to be parsed
     * @return an {@code Iterable} allowing access to all extracted values
     */
    Iterable<?> parse(Object value);

    /**
     * Splits the specified string at the list delimiter and returns a collection with all extracted components. A concrete
     * implementation also has to deal with escape characters which might mask a list delimiter character at certain
     * positions. The boolean {@code trim} flag determines whether each extracted component should be trimmed. This
     * typically makes sense as the list delimiter may be surrounded by whitespace. However, there may be specific use cases
     * in which automatic trimming is not desired.
     *
     * @param s the string to be split
     * @param trim a flag whether each component of the string is to be trimmed
     * @return a collection with all components extracted from the string
     */
    Collection<String> split(String s, boolean trim);

}