org.apache.commons.configuration2

## Interface ImmutableConfiguration

- All Known Subinterfaces:
    Configuration, FileBasedConfiguration, HierarchicalConfiguration<T>, ImmutableHierarchicalConfiguration
  All Known Implementing Classes:
    AbstractConfiguration, AbstractHierarchicalConfiguration, AppletConfiguration, BaseConfiguration, BaseHierarchicalConfiguration, CombinedConfiguration, CompositeConfiguration, DatabaseConfiguration, DataConfiguration, DynamicCombinedConfiguration, EnvironmentConfiguration, INIConfiguration, JNDIConfiguration, MapConfiguration, PatternSubtreeConfigurationWrapper, PropertiesConfiguration, PropertyListConfiguration, ServletConfiguration, ServletContextConfiguration, ServletFilterConfiguration, ServletRequestConfiguration, SubnodeConfiguration, SubsetConfiguration, SystemConfiguration, XMLConfiguration, XMLPropertiesConfiguration, XMLPropertyListConfiguration

      public interface ImmutableConfiguration

  The main interface for accessing configuration data in a read-only fashion.

  The major part of the methods defined in this interface deals with accessing properties of various data types. There is a generic `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 `ConversionException` will be thrown.

  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 `AbstractConfiguration` class, which is the base class of most configuration implementations provided by this package, per default returns null if a property is not found, but provides the `setThrowExceptionOnMissing()` method, with which it can be configured to throw a `NoSuchElementException` exception in that case. (Note that getter methods for primitive types in `AbstractConfiguration` always throw an exception for missing properties because there is no way of overloading the return value.)

  Since:
    2.0
  Version:
    $Id: ImmutableConfiguration.java 1624601 2014-09-12 18:04:36Z oheger $

- - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `containsKey(String key)` Check if the configuration contains the specified key.
    `<T> T` | `get(Class<T> cls, String key)` Get an object of the specified type associated with the given configuration key.
    `<T> T` | `get(Class<T> cls, String key, T defaultValue)` Get an object of the specified type associated with the given configuration key using a default value.
    `Object` | `getArray(Class<?> cls, String key)` Get an array of typed objects associated with the given configuration key.
    `Object` | `getArray(Class<?> cls, String key, Object defaultValue)` Get an array of typed objects associated with the given configuration key.
    `BigDecimal` | `getBigDecimal(String key)` Get a `BigDecimal` associated with the given configuration key.
    `BigDecimal` | `getBigDecimal(String key, BigDecimal defaultValue)` Get a `BigDecimal` associated with the given configuration key.
    `BigInteger` | `getBigInteger(String key)` Get a `BigInteger` associated with the given configuration key.
    `BigInteger` | `getBigInteger(String key, BigInteger defaultValue)` Get a `BigInteger` associated with the given configuration key.
    `boolean` | `getBoolean(String key)` Get a boolean associated with the given configuration key.
    `boolean` | `getBoolean(String key, boolean defaultValue)` Get a boolean associated with the given configuration key.
    `Boolean` | `getBoolean(String key, Boolean defaultValue)` Get a `Boolean` associated with the given configuration key.
    `byte` | `getByte(String key)` Get a byte associated with the given configuration key.
    `byte` | `getByte(String key, byte defaultValue)` Get a byte associated with the given configuration key.
    `Byte` | `getByte(String key, Byte defaultValue)` Get a `Byte` associated with the given configuration key.
    `<T> Collection<T>` | `getCollection(Class<T> cls, String key, Collection<T> target)` Get a collection of typed objects associated with the given configuration key.
    `<T> Collection<T>` | `getCollection(Class<T> cls, String key, Collection<T> target, Collection<T> defaultValue)` Get 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.
    `double` | `getDouble(String key)` Get a double associated with the given configuration key.
    `double` | `getDouble(String key, double defaultValue)` Get a double associated with the given configuration key.
    `Double` | `getDouble(String key, Double defaultValue)` Get a `Double` associated with the given configuration key.
    `float` | `getFloat(String key)` Get a float associated with the given configuration key.
    `float` | `getFloat(String key, float defaultValue)` Get a float associated with the given configuration key.
    `Float` | `getFloat(String key, Float defaultValue)` Get a `Float` associated with the given configuration key.
    `int` | `getInt(String key)` Get a int associated with the given configuration key.
    `int` | `getInt(String key, int defaultValue)` Get a int associated with the given configuration key.
    `Integer` | `getInteger(String key, Integer defaultValue)` Get an `Integer` associated with the given configuration key.
    `Iterator<String>` | `getKeys()` Get the list of the keys contained in the configuration.
    `Iterator<String>` | `getKeys(String prefix)` Get the list of the keys contained in the configuration that match the specified prefix.
    `<T> List<T>` | `getList(Class<T> cls, String key)` Get a list of typed objects associated with the given configuration key returning an empty list if the key doesn't map to an existing object.
    `<T> List<T>` | `getList(Class<T> cls, String key, List<T> defaultValue)` Get 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.
    `List<Object>` | `getList(String key)` Get a List of the values associated with the given configuration key.
    `List<Object>` | `getList(String key, List<Object> defaultValue)` Get a List of strings associated with the given configuration key.
    `long` | `getLong(String key)` Get a long associated with the given configuration key.
    `long` | `getLong(String key, long defaultValue)` Get a long associated with the given configuration key.
    `Long` | `getLong(String key, Long defaultValue)` Get a `Long` associated with the given configuration key.
    `Properties` | `getProperties(String key)` Get a list of properties associated with the given configuration key.
    `Object` | `getProperty(String key)` Gets a property from the configuration.
    `short` | `getShort(String key)` Get a short associated with the given configuration key.
    `short` | `getShort(String key, short defaultValue)` Get a short associated with the given configuration key.
    `Short` | `getShort(String key, Short defaultValue)` Get a `Short` associated with the given configuration key.
    `String` | `getString(String key)` Get a string associated with the given configuration key.
    `String` | `getString(String key, String defaultValue)` Get a string associated with the given configuration key.
    `String[]` | `getStringArray(String key)` Get an array of strings associated with the given configuration key.
    `ImmutableConfiguration` | `immutableSubset(String prefix)` Return a decorator immutable Configuration containing every key from the current Configuration that starts with the specified prefix.
    `boolean` | `isEmpty()` Check if the configuration is empty.

- - ### Method Detail

    - #### isEmpty

          boolean isEmpty()

      Check if the configuration is empty.
      Returns:
        `true` if the configuration contains no property, `false` otherwise.

    - #### containsKey

          boolean containsKey(String key)

      Check if the configuration contains the specified key.
      Parameters:
        `key` - the key whose presence in this configuration is to be tested
      Returns:
        `true` if the configuration contains a value for this key, `false` otherwise

    - #### getProperty

          Object getProperty(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 `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 `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).
      Parameters:
        `key` - property to retrieve
      Returns:
        the value to which this configuration maps the specified key, or null if the configuration contains no mapping for this key.

    - #### getKeys

          Iterator<String> getKeys(String prefix)

      Get the list of the keys contained in the configuration that match the specified prefix. For instance, if the configuration contains the following keys:
      `db.user, db.pwd, db.url, window.xpos, window.ypos`,
      an invocation of `getKeys("db");`
      will return the keys below:
      `db.user, db.pwd, db.url`.
      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.
      Parameters:
        `prefix` - The prefix to test against.
      Returns:
        An Iterator of keys that match the prefix.
      See Also:
        `getKeys()`

    - #### getKeys

          Iterator<String> getKeys()

      Get the list of the keys contained in the configuration. The returned iterator can be used to obtain all defined keys. Note that the exact behavior of the iterator's `remove()` method is specific to a concrete implementation. It may remove the corresponding property from the configuration, but this is not guaranteed. In any case it is no replacement for calling `#clearProperty(String)` for this property. So it is highly recommended to avoid using the iterator's `remove()` method.
      Returns:
        An Iterator.

    - #### getProperties

          Properties getProperties(String key)

      Get 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 `key=value`. These strings are split at the equals sign, and the key parts will become keys of the returned `Properties` object, the value parts become values.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated properties if key is found.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a String/List.
        `IllegalArgumentException` - if one of the tokens is malformed (does not contain an equals sign).

    - #### getBoolean

          boolean getBoolean(String key)

      Get a boolean associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated boolean.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Boolean.

    - #### getBoolean

          boolean getBoolean(String key,
                           boolean defaultValue)

      Get a boolean associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated boolean.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Boolean.

    - #### getBoolean

          Boolean getBoolean(String key,
                           Boolean defaultValue)

      Get a `Boolean` associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated boolean if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Boolean.

    - #### getByte

          byte getByte(String key)

      Get a byte associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated byte.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Byte.

    - #### getByte

          byte getByte(String key,
                     byte defaultValue)

      Get a byte associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated byte.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Byte.

    - #### getByte

          Byte getByte(String key,
                     Byte defaultValue)

      Get a `Byte` associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated byte if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Byte.

    - #### getDouble

          double getDouble(String key)

      Get a double associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated double.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Double.

    - #### getDouble

          double getDouble(String key,
                         double defaultValue)

      Get a double associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated double.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Double.

    - #### getDouble

          Double getDouble(String key,
                         Double defaultValue)

      Get a `Double` associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated double if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Double.

    - #### getFloat

          float getFloat(String key)

      Get a float associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated float.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Float.

    - #### getFloat

          float getFloat(String key,
                       float defaultValue)

      Get a float associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated float.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Float.

    - #### getFloat

          Float getFloat(String key,
                       Float defaultValue)

      Get a `Float` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated float if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Float.

    - #### getInt

          int getInt(String key)

      Get a int associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated int.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Integer.

    - #### getInt

          int getInt(String key,
                   int defaultValue)

      Get a int associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated int.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Integer.

    - #### getInteger

          Integer getInteger(String key,
                           Integer defaultValue)

      Get an `Integer` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated int if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Integer.

    - #### getLong

          long getLong(String key)

      Get a long associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated long.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Long.

    - #### getLong

          long getLong(String key,
                     long defaultValue)

      Get a long associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated long.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Long.

    - #### getLong

          Long getLong(String key,
                     Long defaultValue)

      Get a `Long` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated long if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Long.

    - #### getShort

          short getShort(String key)

      Get a short associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated short.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Short.

    - #### getShort

          short getShort(String key,
                       short defaultValue)

      Get a short associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated short.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Short.

    - #### getShort

          Short getShort(String key,
                       Short defaultValue)

      Get a `Short` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated short if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a Short.

    - #### getBigDecimal

          BigDecimal getBigDecimal(String key)

      Get a `BigDecimal` associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated BigDecimal if key is found and has valid format

    - #### getBigDecimal

          BigDecimal getBigDecimal(String key,
                                 BigDecimal defaultValue)

      Get a `BigDecimal` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated BigDecimal if key is found and has valid format, default value otherwise.

    - #### getBigInteger

          BigInteger getBigInteger(String key)

      Get a `BigInteger` associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated BigInteger if key is found and has valid format

    - #### getBigInteger

          BigInteger getBigInteger(String key,
                                 BigInteger defaultValue)

      Get a `BigInteger` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated BigInteger if key is found and has valid format, default value otherwise.

    - #### getString

          String getString(String key)

      Get a string associated with the given configuration key.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated string.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a String.

    - #### getString

          String getString(String key,
                         String defaultValue)

      Get a string associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated string if key is found and has valid format, default value otherwise.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a String.

    - #### getStringArray

          String[] getStringArray(String key)

      Get 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
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated string array if key is found.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a String/List of Strings.

    - #### getList

          List<Object> getList(String key)

      Get a List of the values associated with the given configuration key. This method is different from the generic `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.
      Parameters:
        `key` - The configuration key.
      Returns:
        The associated List.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a List.

    - #### getList

          List<Object> getList(String key,
                             List<Object> defaultValue)

      Get 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.
      Parameters:
        `key` - The configuration key.
        `defaultValue` - The default value.
      Returns:
        The associated List of strings.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not a List.
      See Also:
        `getList(Class, String, List)`

    - #### get

          <T> T get(Class<T> cls,
                  String key)

      Get 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 `AbstractConfiguration.isThrowExceptionOnMissing()` is set to `true`.
      Type Parameters:
        `T` - the target type of the value
      Parameters:
        `cls` - the target class of the value
        `key` - the key of the value
      Returns:
        the value of the requested type for the key
      Throws:
        `NoSuchElementException` - if the key doesn't map to an existing object and `throwExceptionOnMissing=true`
        `ConversionException` - if the value is not compatible with the requested type
      Since:
        2.0

    - #### get

          <T> T get(Class<T> cls,
                  String key,
                  T defaultValue)

      Get 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.
      Type Parameters:
        `T` - the target type of the value
      Parameters:
        `cls` - the target class of the value
        `key` - the key of the value
        `defaultValue` - the default value
      Returns:
        the value of the requested type for the key
      Throws:
        `ConversionException` - if the value is not compatible with the requested type
      Since:
        2.0

    - #### getArray

          Object getArray(Class<?> cls,
                        String key)

      Get 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.
      Parameters:
        `cls` - the type expected for the elements of the array
        `key` - The configuration key.
      Returns:
        The associated array if the key is found, and the value compatible with the type specified.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not compatible with a list of the specified class.
      Since:
        2.0

    - #### getArray

          Object getArray(Class<?> cls,
                        String key,
                        Object defaultValue)

      Get 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.
      Parameters:
        `cls` - the type expected for the elements of the array
        `key` - the configuration key.
        `defaultValue` - the default value
      Returns:
        The associated array if the key is found, and the value compatible with the type specified.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not compatible with an array of the specified class.
        `IllegalArgumentException` - if the default value is not an array of the specified type
      Since:
        2.0

    - #### getList

          <T> List<T> getList(Class<T> cls,
                            String key)

      Get a list of typed objects associated with the given configuration key returning an empty list if the key doesn't map to an existing object.
      Type Parameters:
        `T` - the type expected for the elements of the list
      Parameters:
        `cls` - the class expected for the elements of the list
        `key` - The configuration key.
      Returns:
        The associated list if the key is found.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not compatible with a list of the specified class.
      Since:
        2.0

    - #### getList

          <T> List<T> getList(Class<T> cls,
                            String key,
                            List<T> defaultValue)

      Get 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.
      Type Parameters:
        `T` - the type expected for the elements of the list
      Parameters:
        `cls` - the class expected for the elements of the list
        `key` - the configuration key.
        `defaultValue` - the default value.
      Returns:
        The associated List.
      Throws:
        `ConversionException` - is thrown if the key maps to an object that is not compatible with a list of the specified class.
      Since:
        2.0

    - #### getCollection

          <T> Collection<T> getCollection(Class<T> cls,
                                        String key,
                                        Collection<T> target)

      Get a collection of typed objects associated with the given configuration key. This method works like `getCollection(Class, String, Collection, Collection)` passing in null as default value.
      Type Parameters:
        `T` - the element type of the result list
      Parameters:
        `cls` - the the element class of the result list
        `key` - the configuration key
        `target` - the target collection (may be null)
      Returns:
        the collection to which data was added
      Throws:
        `ConversionException` - if the conversion is not possible
      Since:
        2.0

    - #### getCollection

          <T> Collection<T> getCollection(Class<T> cls,
                                        String key,
                                        Collection<T> target,
                                        Collection<T> defaultValue)

      Get 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 `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:
      - If the key does not map to an existing object and the default value is null, the method returns null.
      - If the target collection is not null and data has been added (either from the resolved property value or from the default collection), the target collection is returned.
      - If the target collection is null 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.
      Type Parameters:
        `T` - the element type of the result list
      Parameters:
        `cls` - the the element class of the result list
        `key` - the configuration key
        `target` - the target collection (may be null)
        `defaultValue` - the default value (may be null)
      Returns:
        the collection to which data was added
      Throws:
        `ConversionException` - if the conversion is not possible
      Since:
        2.0

    - #### immutableSubset

          ImmutableConfiguration immutableSubset(String prefix)

      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:

              prefix.number = 1
              prefix.string = Apache
              prefixed.foo = bar
              prefix = Jakarta

      the immutable Configuration returned by `subset("prefix")` will contain the properties:

              number = 1
              string = Apache
              = Jakarta

      (The key for the value "Jakarta" is an empty string)
      Parameters:
        `prefix` - The prefix used to select the properties.
      Returns:
        a subset immutable configuration

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