ImmutableConfiguration.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;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.time.Duration;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Properties;
import org.apache.commons.configuration2.convert.PropertyConverter;
import org.apache.commons.configuration2.ex.ConversionException;
/**
* <p>
* The main interface for accessing configuration data in a read-only fashion.
* </p>
* <p>
* The major part of the methods defined in this interface deals with accessing properties of various data types. There
* is a generic {@code getProperty()} method, which returns the value of the queried property in its raw data type.
* Other getter methods try to convert this raw data type into a specific data type. If this fails, a
* {@code ConversionException} will be thrown.
* </p>
* <p>
* For most of the property getter methods an overloaded version exists that allows to specify a default value, which
* will be returned if the queried property cannot be found in the configuration. The behavior of the methods that do
* not take a default value in case of a missing property is not defined by this interface and depends on a concrete
* implementation. E.g. the {@link AbstractConfiguration} class, which is the base class of most configuration
* implementations provided by this package, per default returns <b>null</b> if a property is not found, but provides
* the {@link AbstractConfiguration#setThrowExceptionOnMissing(boolean) setThrowExceptionOnMissing()} method, with which
* it can be configured to throw a {@code NoSuchElementException} exception in that case. (Note that getter methods for
* primitive types in {@code AbstractConfiguration} always throw an exception for missing properties because there is no
* way of overloading the return value.)
* </p>
*
* @since 2.0
*/
public interface ImmutableConfiguration {
/**
* Checks if the configuration contains the specified key.
*
* @param key the key whose presence in this configuration is to be tested
*
* @return {@code true} if the configuration contains a value for this key, {@code false} otherwise
*/
boolean containsKey(String key);
/**
* Tests whether this configuration contains one or more matches to this value. This operation stops at first
* match but may be more expensive than the {@link #containsKey containsKey} method.
*
* @param value value whose presence in this configuration is to be tested
* @return {@code true} if this configuration maps one or more keys to the specified value, false otherwise.
* @since 2.11.0
*/
default boolean containsValue(final Object value) {
final Iterator<String> keys = getKeys();
while (keys.hasNext()) {
if (Objects.equals(value, getProperty(keys.next()))) {
return true;
}
}
return false;
}
/**
* Gets an object of the specified type associated with the given configuration key. If the key doesn't map to an
* existing object, the method returns null unless {@link AbstractConfiguration#isThrowExceptionOnMissing()} is set to
* {@code true}.
*
* @param <T> the target type of the value
* @param cls the target class of the value
* @param key the key of the value
* @return the value of the requested type for the key
* @throws java.util.NoSuchElementException if the key doesn't map to an existing object and
* {@code throwExceptionOnMissing=true}
* @throws org.apache.commons.configuration2.ex.ConversionException if the value is not compatible with the requested
* type
* @since 2.0
*/
<T> T get(Class<T> cls, String key);
/**
* Gets an object of the specified type associated with the given configuration key using a default value. If the key
* doesn't map to an existing object, the default value is returned.
*
* @param <T> the target type of the value
* @param cls the target class of the value
* @param key the key of the value
* @param defaultValue the default value
*
* @return the value of the requested type for the key
*
* @throws org.apache.commons.configuration2.ex.ConversionException if the value is not compatible with the requested
* type
*
* @since 2.0
*/
<T> T get(Class<T> cls, String key, T defaultValue);
/**
* Gets an array of typed objects associated with the given configuration key. If the key doesn't map to an existing
* object, an empty list is returned.
*
* @param cls the type expected for the elements of the array
* @param key The configuration key.
* @return The associated array if the key is found, and the value compatible with the type specified.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not
* compatible with a list of the specified class.
*
* @since 2.0
*/
Object getArray(Class<?> cls, String key);
/**
* Gets an array of typed objects associated with the given configuration key. If the key doesn't map to an existing
* object, the default value is returned.
*
* @param cls the type expected for the elements of the array
* @param key the configuration key.
* @param defaultValue the default value
* @return The associated array if the key is found, and the value compatible with the type specified.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not
* compatible with an array of the specified class.
* @throws IllegalArgumentException if the default value is not an array of the specified type
*
* @since 2.0
* @deprecated This method should not be used any more because its signature does not allow type-safe invocations; use
* {@link #get(Class, String, Object)} instead which offers the same functionality; for instance, to query
* for an array of ints use {@code int[] result = config.get(int[].class, "myArrayKey", someDefault);}.
*/
@Deprecated
Object getArray(Class<?> cls, String key, Object defaultValue);
/**
* Gets a {@link BigDecimal} associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated BigDecimal if key is found and has valid format
*/
BigDecimal getBigDecimal(String key);
/**
* Gets a {@link BigDecimal} associated with the given configuration key. If the key doesn't map to an existing object,
* the default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
*
* @return The associated BigDecimal if key is found and has valid format, default value otherwise.
*/
BigDecimal getBigDecimal(String key, BigDecimal defaultValue);
/**
* Gets a {@link BigInteger} associated with the given configuration key.
*
* @param key The configuration key.
*
* @return The associated BigInteger if key is found and has valid format
*/
BigInteger getBigInteger(String key);
/**
* Gets a {@link BigInteger} associated with the given configuration key. If the key doesn't map to an existing object,
* the default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
*
* @return The associated BigInteger if key is found and has valid format, default value otherwise.
*/
BigInteger getBigInteger(String key, BigInteger defaultValue);
/**
* Gets a boolean associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated boolean.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Boolean.
*/
boolean getBoolean(String key);
/**
* Gets a boolean associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated boolean.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Boolean.
*/
boolean getBoolean(String key, boolean defaultValue);
/**
* Gets a {@link Boolean} associated with the given configuration key.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated boolean if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Boolean.
*/
Boolean getBoolean(String key, Boolean defaultValue);
/**
* Gets a byte associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated byte.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Byte.
*/
byte getByte(String key);
/**
* Gets a byte associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated byte.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Byte.
*/
byte getByte(String key, byte defaultValue);
/**
* Gets a {@link Byte} associated with the given configuration key.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated byte if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Byte.
*/
Byte getByte(String key, Byte defaultValue);
/**
* Gets a collection of typed objects associated with the given configuration key. This method works like
* {@link #getCollection(Class, String, Collection, Collection)} passing in <b>null</b> as default value.
*
* @param <T> the element type of the result list
* @param cls the element class of the result list
* @param key the configuration key
* @param target the target collection (may be <b>null</b>)
* @return the collection to which data was added
* @throws org.apache.commons.configuration2.ex.ConversionException if the conversion is not possible
* @since 2.0
*/
<T> Collection<T> getCollection(Class<T> cls, String key, Collection<T> target);
/**
* Gets a collection of typed objects associated with the given configuration key using the values in the specified
* default collection if the key does not map to an existing object. This method is similar to {@code getList()},
* however, it allows specifying a target collection. Results are added to this collection. This is useful if the data
* retrieved should be added to a specific kind of collection, e.g. a set to remove duplicates. The return value is as
* follows:
* <ul>
* <li>If the key does not map to an existing object and the default value is <b>null</b>, the method returns
* <b>null</b>.</li>
* <li>If the target collection is not <b>null</b> and data has been added (either from the resolved property value or
* from the default collection), the target collection is returned.</li>
* <li>If the target collection is <b>null</b> and data has been added (either from the resolved property value or from
* the default collection), return value is the target collection created by this method.</li>
* </ul>
*
* @param <T> the element type of the result list
* @param cls the element class of the result list
* @param key the configuration key
* @param target the target collection (may be <b>null</b>)
* @param defaultValue the default value (may be <b>null</b>)
* @return the collection to which data was added
* @throws org.apache.commons.configuration2.ex.ConversionException if the conversion is not possible
* @since 2.0
*/
<T> Collection<T> getCollection(Class<T> cls, String key, Collection<T> target, Collection<T> defaultValue);
/**
* Gets a double associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated double.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Double.
*/
double getDouble(String key);
/**
* Gets a double associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated double.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Double.
*/
double getDouble(String key, double defaultValue);
/**
* Gets a {@link Double} associated with the given configuration key.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated double if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Double.
*/
Double getDouble(String key, Double defaultValue);
/**
* Gets a {@link Duration} associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated Duration if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Duration.
* @since 2.8.0
*/
default Duration getDuration(final String key) {
final String string = getString(key);
if (string == null) {
throw new NoSuchElementException(key);
}
return PropertyConverter.toDuration(string);
}
/**
* Gets a {@link Duration} associated with the given configuration key.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated Duration if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Duration.
* @since 2.8.0
*/
default Duration getDuration(final String key, final Duration defaultValue) {
final Object value = getProperty(key);
return value == null ? defaultValue : PropertyConverter.toDuration(value);
}
/**
* Gets the value of a string property that is stored in encoded form in this configuration using a default
* {@code ConfigurationDecoder}. This method works like the method with the same name, but it uses a default
* {@code ConfigurationDecoder} associated with this configuration. It depends on a specific implementation how this
* default decoder is obtained.
*
* @param key the configuration key
* @return the plain string value of the specified encoded property
*/
String getEncodedString(String key);
/**
* Gets the value of a string property that is stored in encoded form in this configuration. This method obtains the
* value of the string property identified by the given key. This value is then passed to the provided
* {@code ConfigurationDecoder}. The value returned by the {@code ConfigurationDecoder} is passed to the caller. If the
* key is not associated with a value, the decoder is not invoked; depending on this configuration's settings either
* <b>null</b> is returned or an exception is thrown.
*
* @param key the configuration key
* @param decoder the {@code ConfigurationDecoder} (must not be <b>null</b>)
* @return the plain string value of the specified encoded property
* @throws IllegalArgumentException if a <b>null</b> decoder is passed
*/
String getEncodedString(String key, ConfigurationDecoder decoder);
/**
* Gets an enum associated with the given configuration key.
*
* @param <T> The enum type whose constant is to be returned.
* @param enumType the {@code Class} object of the enum type from which to return a constant
* @param key The configuration key.
* @return The associated enum.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* String.
* @since 2.8.0
*/
default <T extends Enum<T>> T getEnum(final String key, final Class<T> enumType) {
try {
return Enum.valueOf(enumType, getString(key));
} catch (final IllegalArgumentException e) {
throw new ConversionException(e);
}
}
/**
* Gets the enum associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param <T> The enum type whose constant is to be returned.
* @param key The configuration key.
* @param enumType the {@code Class} object of the enum type from which to return a constant
* @param defaultValue The default value.
* @return The associated enum if key is found and has valid format, default value otherwise.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Enum.
* @since 2.8.0
*/
default <T extends Enum<T>> T getEnum(final String key, final Class<T> enumType, final T defaultValue) {
final String strValue = getString(key, null);
if (strValue == null) {
return defaultValue;
}
try {
return Enum.valueOf(enumType, strValue);
} catch (final IllegalArgumentException e) {
throw new ConversionException(e);
}
}
/**
* Gets a float associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated float.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Float.
*/
float getFloat(String key);
/**
* Gets a float associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated float.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Float.
*/
float getFloat(String key, float defaultValue);
/**
* Gets a {@link Float} associated with the given configuration key. If the key doesn't map to an existing object, the
* default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated float if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Float.
*/
Float getFloat(String key, Float defaultValue);
/**
* Gets a int associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated int.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Integer.
*/
int getInt(String key);
/**
* Gets a int associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated int.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Integer.
*/
int getInt(String key, int defaultValue);
/**
* Gets an {@link Integer} associated with the given configuration key. If the key doesn't map to an existing object,
* the default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated int if key is found and has valid format, default value otherwise.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Integer.
*/
Integer getInteger(String key, Integer defaultValue);
/**
* Gets the list of the keys contained in the configuration. The returned iterator can be used to obtain all defined
* keys. It does not allow removing elements from this configuration via its {@code remove()} method. Note that the keys
* of this configuration are returned in a form, so that they can be directly evaluated; escaping of special characters
* (if necessary) has already been performed.
*
* @return An Iterator.
*/
Iterator<String> getKeys();
/**
* Gets the list of the keys contained in the configuration that match the specified prefix. For instance, if the
* configuration contains the following keys:<br>
* {@code db.user, db.pwd, db.url, window.xpos, window.ypos},<br>
* an invocation of {@code getKeys("db");}<br>
* will return the keys below:<br>
* {@code db.user, db.pwd, db.url}.<br>
* Note that the prefix itself is included in the result set if there is a matching key. The exact behavior - how the
* prefix is actually interpreted - depends on a concrete implementation.
*
* @param prefix The prefix to test against.
* @return An Iterator of keys that match the prefix.
* @see #getKeys()
*/
Iterator<String> getKeys(String prefix);
/**
* Gets the list of the keys contained in the configuration that match the specified prefix. For instance, if the
* configuration contains the following keys:<br>
* {@code db@user, db@pwd, db@url, window.xpos, window.ypos},<br>
* an invocation of {@code getKeys("db","@");}<br>
* will return the keys below:<br>
* {@code db@user, db@pwd, db@url}.<br>
* Note that the prefix itself is included in the result set if there is a matching key. The exact behavior - how the
* prefix is actually interpreted - depends on a concrete implementation.
*
* @param prefix The prefix to test against.
* @param delimiter The prefix delimiter.
* @return An Iterator of keys that match the prefix.
* @see #getKeys()
* @since 2.10.0
*/
default Iterator<String> getKeys(final String prefix, final String delimiter) {
return null;
}
/**
* Gets a list of typed objects associated with the given configuration key returning a null if the key doesn't map to
* an existing object.
*
* @param <T> the type expected for the elements of the list
* @param cls the class expected for the elements of the list
* @param key The configuration key.
* @return The associated list if the key is found.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not
* compatible with a list of the specified class.
*
* @since 2.0
*/
<T> List<T> getList(Class<T> cls, String key);
/**
* Gets a list of typed objects associated with the given configuration key returning the specified default value if the
* key doesn't map to an existing object. This method recursively retrieves all values stored for the passed in key,
* i.e. if one of these values is again a complex object like an array or a collection (which may be the case for some
* concrete subclasses), all values are extracted and added to the resulting list - performing a type conversion if
* necessary.
*
* @param <T> the type expected for the elements of the list
* @param cls the class expected for the elements of the list
* @param key the configuration key.
* @param defaultValue the default value.
* @return The associated List.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not
* compatible with a list of the specified class.
*
* @since 2.0
*/
<T> List<T> getList(Class<T> cls, String key, List<T> defaultValue);
/**
* Gets a List of the values associated with the given configuration key. This method is different from the generic
* {@code getList()} method in that it does not recursively obtain all values stored for the specified property key.
* Rather, only the first level of the hierarchy is processed. So the resulting list may contain complex objects like
* arrays or collections - depending on the storage structure used by a concrete subclass. If the key doesn't map to an
* existing object, an empty List is returned.
*
* @param key The configuration key.
* @return The associated List.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* List.
*/
List<Object> getList(String key);
/**
* Gets a List of strings associated with the given configuration key. If the key doesn't map to an existing object, the
* default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated List of strings.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* List.
* @see #getList(Class, String, List)
*/
List<Object> getList(String key, List<?> defaultValue);
/**
* Gets a long associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated long.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Long.
*/
long getLong(String key);
/**
* Gets a long associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated long.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Long.
*/
long getLong(String key, long defaultValue);
/**
* Gets a {@link Long} associated with the given configuration key. If the key doesn't map to an existing object, the
* default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated long if key is found and has valid format, default value otherwise.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Long.
*/
Long getLong(String key, Long defaultValue);
/**
* Gets a list of properties associated with the given configuration key. This method expects the given key to have an
* arbitrary number of String values, each of which is of the form {@code key=value}. These strings are split at the
* equals sign, and the key parts will become keys of the returned {@code Properties} object, the value parts become
* values.
*
* @param key The configuration key.
* @return The associated properties if key is found.
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* String/List.
* @throws IllegalArgumentException if one of the tokens is malformed (does not contain an equals sign).
*/
Properties getProperties(String key);
/**
* Gets a property from the configuration. This is the most basic get method for retrieving values of properties. In a
* typical implementation of the {@code Configuration} interface the other get methods (that return specific data types)
* will internally make use of this method. On this level variable substitution is not yet performed. The returned
* object is an internal representation of the property value for the passed in key. It is owned by the
* {@code Configuration} object. So a caller should not modify this object. It cannot be guaranteed that this object
* will stay constant over time (i.e. further update operations on the configuration may change its internal state).
*
* @param key property to retrieve
* @return the value to which this configuration maps the specified key, or null if the configuration contains no
* mapping for this key.
*/
Object getProperty(String key);
/**
* Gets a short associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated short.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Short.
*/
short getShort(String key);
/**
* Gets a short associated with the given configuration key.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated short.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Short.
*/
short getShort(String key, short defaultValue);
/**
* Gets a {@link Short} associated with the given configuration key. If the key doesn't map to an existing object, the
* default value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated short if key is found and has valid format, default value otherwise.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* Short.
*/
Short getShort(String key, Short defaultValue);
/**
* Gets a string associated with the given configuration key.
*
* @param key The configuration key.
* @return The associated string.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* String.
*/
String getString(String key);
/**
* Gets a string associated with the given configuration key. If the key doesn't map to an existing object, the default
* value is returned.
*
* @param key The configuration key.
* @param defaultValue The default value.
* @return The associated string if key is found and has valid format, default value otherwise.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* String.
*/
String getString(String key, String defaultValue);
/**
* Gets an array of strings associated with the given configuration key. If the key doesn't map to an existing object an
* empty array is returned
*
* @param key The configuration key.
* @return The associated string array if key is found.
*
* @throws org.apache.commons.configuration2.ex.ConversionException is thrown if the key maps to an object that is not a
* String/List of Strings.
*/
String[] getStringArray(String key);
/**
* Return a decorator immutable Configuration containing every key from the current Configuration that starts with the
* specified prefix. The prefix is removed from the keys in the subset. For example, if the configuration contains the
* following properties:
*
* <pre>
* prefix.number = 1
* prefix.string = Apache
* prefixed.foo = bar
* prefix = Jakarta
* </pre>
*
* the immutable Configuration returned by {@code subset("prefix")} will contain the properties:
*
* <pre>
* number = 1
* string = Apache
* = Jakarta
* </pre>
*
* (The key for the value "Jakarta" is an empty string)
*
* @param prefix The prefix used to select the properties.
* @return a subset immutable configuration
*/
ImmutableConfiguration immutableSubset(String prefix);
/**
* Checks if the configuration is empty.
*
* @return {@code true} if the configuration contains no property, {@code false} otherwise.
*/
boolean isEmpty();
/**
* Returns the number of keys stored in this configuration. Note that a concrete implementation is not guaranteed to be
* efficient; for some implementations it may be expensive to determine the size. Especially, if you just want to check
* whether a configuration is empty, it is preferable to use the {@link #isEmpty()} method.
*
* @return the number of keys stored in this configuration
*/
int size();
}