Package org.apache.commons.configuration2

# Class BaseHierarchicalConfiguration

java.lang.Object
org.apache.commons.configuration2.event.BaseEventSource
org.apache.commons.configuration2.AbstractConfiguration
org.apache.commons.configuration2.AbstractHierarchicalConfiguration<ImmutableNode>
org.apache.commons.configuration2.BaseHierarchicalConfiguration

All Implemented Interfaces:
  `Cloneable`, `Configuration`, `EventSource`, `HierarchicalConfiguration<ImmutableNode>`, `ImmutableConfiguration`, `ImmutableHierarchicalConfiguration`, `SynchronizerSupport`, `InMemoryNodeModelSupport`, `NodeKeyResolver<ImmutableNode>`, `NodeModelSupport<ImmutableNode>`

Direct Known Subclasses:
  `AbstractYAMLBasedConfiguration`, `CombinedConfiguration`, `INIConfiguration`, `PatternSubtreeConfigurationWrapper`, `PropertyListConfiguration`, `SubnodeConfiguration`, `XMLConfiguration`, `XMLPropertyListConfiguration`

public class BaseHierarchicalConfiguration extends AbstractHierarchicalConfiguration<ImmutableNode> implements InMemoryNodeModelSupport

A specialized hierarchical configuration implementation that is based on a structure of `ImmutableNode` objects.

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `protected static class`
  `BaseHierarchicalConfiguration.BuilderVisitor`
  A specialized visitor base class that can be used for storing the tree of configuration nodes.

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  ``
  `BaseHierarchicalConfiguration()`
  Creates a new instance of `BaseHierarchicalConfiguration`.
  ``
  `BaseHierarchicalConfiguration(HierarchicalConfiguration<ImmutableNode> c)`
  Creates a new instance of `BaseHierarchicalConfiguration` and copies all data contained in the specified configuration into the new one.
  `protected`
  `BaseHierarchicalConfiguration(NodeModel<ImmutableNode> model)`
  Creates a new instance of `BaseHierarchicalConfiguration` and initializes it with the given `NodeModel`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `List<HierarchicalConfiguration<ImmutableNode>>`
  `childConfigurationsAt(String key)`
  Returns a list with sub configurations for all child nodes of the node selected by the given key.
  `List<HierarchicalConfiguration<ImmutableNode>>`
  `childConfigurationsAt(String key, boolean supportUpdates)`
  Returns a list with sub configurations for all child nodes of the node selected by the given key allowing the caller to specify the `supportUpdates` flag.
  `protected NodeModel<ImmutableNode>`
  `cloneNodeModel()`
  Creates a clone of the node model.
  `HierarchicalConfiguration<ImmutableNode>`
  `configurationAt(String key)`
  Returns a hierarchical subnode configuration for the node specified by the given key.
  `HierarchicalConfiguration<ImmutableNode>`
  `configurationAt(String key, boolean supportUpdates)`
  Returns a hierarchical sub configuration object that wraps the configuration node specified by the given key.
  `List<HierarchicalConfiguration<ImmutableNode>>`
  `configurationsAt(String key)`
  Returns a list of sub configurations for all configuration nodes selected by the given key.
  `List<HierarchicalConfiguration<ImmutableNode>>`
  `configurationsAt(String key, boolean supportUpdates)`
  Returns a list of sub configurations for all configuration nodes selected by the given key allowing the caller to specify the `supportUpdates` flag.
  `protected SubnodeConfiguration`
  `createSubConfigurationForTrackedNode(NodeSelector selector, InMemoryNodeModelSupport parentModelSupport)`
  Creates a connected sub configuration based on a selector for a tracked node.
  `InMemoryNodeModel`
  `getNodeModel()`
  Gets the `NodeModel` supported by this object.
  `protected NodeSelector`
  `getSubConfigurationNodeSelector(String key)`
  Gets the `NodeSelector` to be used for a sub configuration based on the passed in key.
  `protected InMemoryNodeModel`
  `getSubConfigurationParentModel()`
  Gets the `InMemoryNodeModel` to be used as parent model for a new sub configuration.
  `List<ImmutableHierarchicalConfiguration>`
  `immutableChildConfigurationsAt(String key)`
  Returns a list of immutable configurations for all direct child elements of the node selected by the given key.
  `ImmutableHierarchicalConfiguration`
  `immutableConfigurationAt(String key)`
  Returns an immutable hierarchical configuration for the node specified by the given key.
  `ImmutableHierarchicalConfiguration`
  `immutableConfigurationAt(String key, boolean supportUpdates)`
  Returns an immutable hierarchical configuration object that wraps the configuration node specified by the given key.
  `List<ImmutableHierarchicalConfiguration>`
  `immutableConfigurationsAt(String key)`
  Returns a list of immutable configurations for all configuration nodes selected by the given key.
  `protected void`
  `initSubConfigurationForThisParent(SubnodeConfiguration subConfig)`
  Initializes a `SubnodeConfiguration` object.
  `Configuration`
  `interpolatedConfiguration()`
  Returns a configuration with the same content as this configuration, but with all variables replaced by their actual values.
  `protected void`
  `subnodeConfigurationChanged(ConfigurationEvent event)`
  This method is always called when a subnode configuration created from this configuration has been modified.
  `Configuration`
  `subset(String prefix)`
  Creates a new `Configuration` object containing all keys that start with the specified prefix.

  ### Methods inherited from class org.apache.commons.configuration2.AbstractHierarchicalConfiguration

  `addNodes, addNodesInternal, addPropertyDirect, addPropertyInternal, clearInternal, clearPropertyDirect, clearTree, clearTreeInternal, clone, containsKeyInternal, containsValueInternal, fetchNodeList, getExpressionEngine, getKeysInternal, getKeysInternal, getKeysInternal, getMaxIndex, getMaxIndexInternal, getModel, getPropertyInternal, getRootElementName, getRootElementNameInternal, isEmptyInternal, nodeDefined, nodeKey, resolveAddKey, resolveKey, resolveNodeKey, resolveUpdateKey, setExpressionEngine, setPropertyInternal, sizeInternal, toString`

  ### Methods inherited from class org.apache.commons.configuration2.AbstractConfiguration

  `addErrorLogListener, addProperty, append, beginRead, beginWrite, clear, clearProperty, cloneInterpolator, contains, containsKey, containsValue, copy, endRead, endWrite, get, get, getArray, getArray, getBigDecimal, getBigDecimal, getBigInteger, getBigInteger, getBoolean, getBoolean, getBoolean, getByte, getByte, getByte, getCollection, getCollection, getConfigurationDecoder, getConversionHandler, getDouble, getDouble, getDouble, getDuration, getDuration, getEncodedString, getEncodedString, getFloat, getFloat, getFloat, getInt, getInt, getInteger, getInterpolator, getKeys, getKeys, getKeys, getList, getList, getList, getList, getListDelimiterHandler, getLogger, getLong, getLong, getLong, getProperties, getProperties, getProperty, getShort, getShort, getShort, getString, getString, getStringArray, getSynchronizer, immutableSubset, initLogger, installInterpolator, interpolate, interpolate, isEmpty, isScalarValue, isThrowExceptionOnMissing, lock, setConfigurationDecoder, setConversionHandler, setDefaultLookups, setInterpolator, setListDelimiterHandler, setLogger, setParentInterpolator, setPrefixLookups, setProperty, setSynchronizer, setThrowExceptionOnMissing, size, unlock`

  ### Methods inherited from class org.apache.commons.configuration2.event.BaseEventSource

  `addEventListener, clearErrorListeners, clearEventListeners, copyEventListeners, createErrorEvent, createEvent, fireError, fireEvent, getEventListenerRegistrations, getEventListeners, isDetailEvents, removeEventListener, setDetailEvents`

  ### Methods inherited from class java.lang.Object

  `equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods inherited from interface org.apache.commons.configuration2.Configuration

  `addProperty, clear, clearProperty, getInterpolator, installInterpolator, setInterpolator, setProperty`

  ### Methods inherited from interface org.apache.commons.configuration2.ImmutableConfiguration

  `containsKey, containsValue, get, get, getArray, getArray, getBigDecimal, getBigDecimal, getBigInteger, getBigInteger, getBoolean, getBoolean, getBoolean, getByte, getByte, getByte, getCollection, getCollection, getDouble, getDouble, getDouble, getDuration, getDuration, getEncodedString, getEncodedString, getEnum, getEnum, getFloat, getFloat, getFloat, getInt, getInt, getInteger, getKeys, getKeys, getKeys, getList, getList, getList, getList, getLong, getLong, getLong, getProperties, getProperty, getShort, getShort, getShort, getString, getString, getStringArray, immutableSubset, isEmpty, size`

  ### Methods inherited from interface org.apache.commons.configuration2.sync.SynchronizerSupport

  `getSynchronizer, lock, setSynchronizer, unlock`

- ## Constructor Details

  - ### BaseHierarchicalConfiguration

    public BaseHierarchicalConfiguration()
    Creates a new instance of `BaseHierarchicalConfiguration`.

  - ### BaseHierarchicalConfiguration

    public BaseHierarchicalConfiguration(HierarchicalConfiguration<ImmutableNode> c)
    Creates a new instance of `BaseHierarchicalConfiguration` and copies all data contained in the specified configuration into the new one.
    Parameters:
      `c` - the configuration that is to be copied (if null, this constructor will behave like the standard constructor)
    Since:
      1.4

  - ### BaseHierarchicalConfiguration

    protected BaseHierarchicalConfiguration(NodeModel<ImmutableNode> model)
    Creates a new instance of `BaseHierarchicalConfiguration` and initializes it with the given `NodeModel`.
    Parameters:
      `model` - the `NodeModel`

- ## Method Details

  - ### childConfigurationsAt

    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(String key)
    Returns a list with sub configurations for all child nodes of the node selected by the given key. This method works like `ImmutableHierarchicalConfiguration.immutableChildConfigurationsAt(String)`, but returns a list with mutable configuration objects. The configuration objects returned are not connected to the parent configuration. This implementation resolves the node(s) selected by the given key. If not a single node is selected, an empty list is returned. Otherwise, sub configurations for each child of the node are created.
    Specified by:
      `childConfigurationsAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key for selecting the desired parent node
    Returns:
      a collection with `HierarchicalConfiguration` objects for all child nodes of the selected parent node

  - ### childConfigurationsAt

    public List<HierarchicalConfiguration<ImmutableNode>> childConfigurationsAt(String key, boolean supportUpdates)
    Returns a list with sub configurations for all child nodes of the node selected by the given key allowing the caller to specify the `supportUpdates` flag. This method works like `childConfigurationsAt(String)`; however, depending on the value of the `supportUpdates` flag, connected sub configurations may be created.
    Specified by:
      `childConfigurationsAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key for selecting the desired parent node
      `supportUpdates` - a flag whether the returned sub configuration should be directly connected to its parent
    Returns:
      a collection with `HierarchicalConfiguration` objects for all child nodes of the selected parent node

  - ### cloneNodeModel

    protected NodeModel<ImmutableNode> cloneNodeModel()
    Creates a clone of the node model. This method is called by `clone()`. This implementation creates a new instance of `InMemoryNodeModel`, initialized with this configuration's root node. This has the effect that although the same nodes are used, the original and copied configurations are independent on each other.
    Specified by:
      `cloneNodeModel` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Returns:
      the clone of the `NodeModel`

  - ### configurationAt

    public HierarchicalConfiguration<ImmutableNode> configurationAt(String key)
    Returns a hierarchical subnode configuration for the node specified by the given key. This is a short form for `configurationAt(key, <strong>false</strong>)`. This is a short form for `configurationAt(key, <strong>false</strong>)`.
    Specified by:
      `configurationAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key that selects the sub tree
    Returns:
      a hierarchical configuration that contains this sub tree
    Throws:
      `ConfigurationRuntimeException` - if the key does not select a single node
    See Also:
      - `SubnodeConfiguration`

  - ### configurationAt

    public HierarchicalConfiguration<ImmutableNode> configurationAt(String key, boolean supportUpdates)

    Returns a hierarchical sub configuration object that wraps the configuration node specified by the given key. This method provides an easy means of accessing sub trees of a hierarchical configuration. In the returned configuration the sub tree can directly be accessed, it becomes the root node of this configuration. Because of this the passed in key must select exactly one configuration node; otherwise an `IllegalArgumentException` will be thrown.

    The difference between this method and the `Configuration.subset(String)` method is that `subset()` supports arbitrary subsets of configuration nodes while `configurationAt()` only returns a single sub tree. Please refer to the documentation of the `SubnodeConfiguration` class to obtain further information about sub configurations and when they should be used.

    With the `supportUpdate` flag the behavior of the returned sub configuration regarding updates of its parent configuration can be determined. If set to false, the configurations return on independent nodes structures. So changes made on one configuration cannot be seen by the other one. A value of true in contrast creates a direct connection between both configurations - they are then using the same underlying data structures as much as possible. There are however changes which break this connection; for instance, if the sub tree the sub configuration belongs to is completely removed from the parent configuration. If such a change happens, the sub configuration becomes detached from its parent. It can still be used in a normal way, but changes on it are not reflected by the parent and vice verse. Also, it is not possible to reattach a once detached sub configuration.

    The result of this implementation depends on the `supportUpdates` flag: If it is false, a plain `BaseHierarchicalConfiguration` is returned using the selected node as root node. This is suitable for read-only access to properties. Because the configuration returned in this case is not connected to the parent configuration, updates on properties made by one configuration are not reflected by the other one. A value of true for this parameter causes a tracked node to be created, and result is a `SubnodeConfiguration` based on this tracked node. This configuration is really connected to its parent, so that updated properties are visible on both.
    Specified by:
      `configurationAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key that selects the sub tree
      `supportUpdates` - a flag whether the returned sub configuration should be directly connected to its parent
    Returns:
      a hierarchical configuration that contains this sub tree
    Throws:
      `ConfigurationRuntimeException` - if the key does not select a single node
    See Also:
      - `SubnodeConfiguration`

  - ### configurationsAt

    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(String key)
    Returns a list of sub configurations for all configuration nodes selected by the given key. This method will evaluate the passed in key (using the current `ExpressionEngine`) and then create a sub configuration for each returned node (like `HierarchicalConfiguration.configurationAt(String)` ). This is especially useful when dealing with list-like structures. As an example consider the configuration that contains data about database tables and their fields. If you need access to all fields of a certain table, you can simply do

         List fields = config.configurationsAt("tables.table(0).fields.field");
         for(Iterator it = fields.iterator(); it.hasNext();)
         {
             BaseHierarchicalConfiguration sub = (BaseHierarchicalConfiguration) it.next();
             // now the children and attributes of the field node can be
             // directly accessed
             String fieldName = sub.getString("name");
             String fieldType = sub.getString("type");
             ...

    The configuration objects returned are not connected to the parent configuration. This implementation creates sub configurations in the same way as described for `configurationAt(String)`.
    Specified by:
      `configurationsAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key for selecting the desired nodes
    Returns:
      a list with hierarchical configuration objects; each configuration represents one of the nodes selected by the passed in key

  - ### configurationsAt

    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(String key, boolean supportUpdates)
    Returns a list of sub configurations for all configuration nodes selected by the given key allowing the caller to specify the `supportUpdates` flag. This method works like `HierarchicalConfiguration.configurationsAt(String)`, but with the additional boolean parameter it can be specified whether the returned configurations react on updates of the parent configuration. This implementation creates tracked nodes for the specified key. Then sub configurations for these nodes are created and returned.
    Specified by:
      `configurationsAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key for selecting the desired nodes
      `supportUpdates` - a flag whether the returned sub configuration should be directly connected to its parent
    Returns:
      a list with hierarchical configuration objects; each configuration represents one of the nodes selected by the passed in key
    See Also:
      - `HierarchicalConfiguration.configurationsAt(String, boolean)`

  - ### createSubConfigurationForTrackedNode

    protected SubnodeConfiguration createSubConfigurationForTrackedNode(NodeSelector selector, InMemoryNodeModelSupport parentModelSupport)
    Creates a connected sub configuration based on a selector for a tracked node.
    Parameters:
      `selector` - the `NodeSelector`
      `parentModelSupport` - the `InMemoryNodeModelSupport` object for the parent node model
    Returns:
      the newly created sub configuration
    Since:
      2.0

  - ### getNodeModel

    public InMemoryNodeModel getNodeModel()
    Gets the `NodeModel` supported by this object. This implementation returns the configuration's `NodeModel`. It is guarded by the current `Synchronizer`. This implementation returns the `InMemoryNodeModel` used by this configuration.
    Specified by:
      `getNodeModel` in interface `InMemoryNodeModelSupport`
    Specified by:
      `getNodeModel` in interface `NodeModelSupport<ImmutableNode>`
    Overrides:
      `getNodeModel` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Returns:
      the `NodeModel`

  - ### getSubConfigurationNodeSelector

    protected NodeSelector getSubConfigurationNodeSelector(String key)
    Gets the `NodeSelector` to be used for a sub configuration based on the passed in key. This method is called whenever a sub configuration is to be created. This base implementation returns a new `NodeSelector` initialized with the passed in key. Sub classes may override this method if they have a different strategy for creating a selector.
    Parameters:
      `key` - the key of the sub configuration
    Returns:
      a `NodeSelector` for initializing a sub configuration
    Since:
      2.0

  - ### getSubConfigurationParentModel

    protected InMemoryNodeModel getSubConfigurationParentModel()
    Gets the `InMemoryNodeModel` to be used as parent model for a new sub configuration. This method is called whenever a sub configuration is to be created. This base implementation returns the model of this configuration. Sub classes with different requirements for the parent models of sub configurations have to override it.
    Returns:
      the parent model for a new sub configuration

  - ### immutableChildConfigurationsAt

    public List<ImmutableHierarchicalConfiguration> immutableChildConfigurationsAt(String key)
    Returns a list of immutable configurations for all direct child elements of the node selected by the given key. With this method it is possible to inspect the content of a hierarchical structure; all children of a given node can be queried without having to know their exact names. If the passed in key does not point to a single node, an empty list is returned. This is also the result if the node referred to by the key does not have child elements. This implementation first delegates to `childConfigurationsAt()` to create a list of mutable child configurations. Then a list with immutable wrapper configurations is created.
    Specified by:
      `immutableChildConfigurationsAt` in interface `ImmutableHierarchicalConfiguration`
    Parameters:
      `key` - the key for selecting the desired parent node
    Returns:
      a collection with immutable configurations for all child nodes of the selected parent node

  - ### immutableConfigurationAt

    public ImmutableHierarchicalConfiguration immutableConfigurationAt(String key)
    Returns an immutable hierarchical configuration for the node specified by the given key. This is a short form for `immutableConfigurationAt(key, <strong>false</strong>)`. This implementation creates a `SubnodeConfiguration` by delegating to `configurationAt()`. Then an immutable wrapper is created and returned.
    Specified by:
      `immutableConfigurationAt` in interface `ImmutableHierarchicalConfiguration`
    Parameters:
      `key` - the key that selects the sub tree
    Returns:
      a hierarchical configuration that contains this sub tree
    Throws:
      `ConfigurationRuntimeException` - if the key does not select a single node

  - ### immutableConfigurationAt

    public ImmutableHierarchicalConfiguration immutableConfigurationAt(String key, boolean supportUpdates)

    Returns an immutable hierarchical configuration object that wraps the configuration node specified by the given key. This method provides an easy means of accessing sub trees of a hierarchical configuration. In the returned configuration the sub tree can directly be accessed, it becomes the root node of this configuration. Because of this the passed in key must select exactly one configuration node; otherwise an `IllegalArgumentException` will be thrown.

    The difference between this method and the `ImmutableConfiguration.immutableSubset(String)` method is that `immutableSubset()` supports arbitrary subsets of configuration nodes while `immutableConfigurationAt()` only returns a single sub tree. Please refer to the documentation of the `SubnodeConfiguration` class to obtain further information about subnode configurations and when they should be used.

    This implementation creates a `SubnodeConfiguration` by delegating to `configurationAt()`. Then an immutable wrapper is created and returned.
    Specified by:
      `immutableConfigurationAt` in interface `ImmutableHierarchicalConfiguration`
    Parameters:
      `key` - the key that selects the sub tree
      `supportUpdates` - a flag whether the returned subnode configuration should be able to handle updates of its parent
    Returns:
      a hierarchical configuration that contains this sub tree

  - ### immutableConfigurationsAt

    public List<ImmutableHierarchicalConfiguration> immutableConfigurationsAt(String key)
    Returns a list of immutable configurations for all configuration nodes selected by the given key. This method will evaluate the passed in key (using the current `ExpressionEngine`) and then create an immutable subnode configuration for each returned node (like `ImmutableHierarchicalConfiguration.immutableConfigurationAt(String)`}). This is especially useful when dealing with list-like structures. As an example consider the configuration that contains data about database tables and their fields. If you need access to all fields of a certain table, you can simply do

         List<ImmutableHierarchicalConfiguration> fields =
           config.immutableConfigurationsAt("tables.table(0).fields.field");
         for(Iterator<ImmutableHierarchicalConfiguration> it = fields.iterator();
           it.hasNext();)
         {
             ImmutableHierarchicalConfiguration sub = it.next();
             // now the children and attributes of the field node can be
             // directly accessed
             String fieldName = sub.getString("name");
             String fieldType = sub.getString("type");
             ...

    This implementation first delegates to `configurationsAt()` to create a list of `SubnodeConfiguration` objects. Then for each element of this list an unmodifiable wrapper is created.
    Specified by:
      `immutableConfigurationsAt` in interface `ImmutableHierarchicalConfiguration`
    Parameters:
      `key` - the key for selecting the desired nodes
    Returns:
      a list with immutable hierarchical configuration objects; each configuration represents one of the nodes selected by the passed in key

  - ### initSubConfigurationForThisParent

    protected void initSubConfigurationForThisParent(SubnodeConfiguration subConfig)
    Initializes a `SubnodeConfiguration` object. This method should be called for each sub configuration created for this configuration. It ensures that the sub configuration is correctly connected to its parent instance and that update events are correctly propagated.
    Parameters:
      `subConfig` - the sub configuration to be initialized
    Since:
      2.0

  - ### interpolatedConfiguration

    public Configuration interpolatedConfiguration()
    Returns a configuration with the same content as this configuration, but with all variables replaced by their actual values. This implementation is specific for hierarchical configurations. It clones the current configuration and runs a specialized visitor on the clone, which performs interpolation on the single configuration nodes.
    Overrides:
      `interpolatedConfiguration` in class `AbstractConfiguration`
    Returns:
      a configuration with all variables interpolated
    Since:
      1.5

  - ### subnodeConfigurationChanged

    protected void subnodeConfigurationChanged(ConfigurationEvent event)
    This method is always called when a subnode configuration created from this configuration has been modified. This implementation transforms the received event into an event of type `SUBNODE_CHANGED` and notifies the registered listeners.
    Parameters:
      `event` - the event describing the change
    Since:
      1.5

  - ### subset

    public Configuration subset(String prefix)
    Creates a new `Configuration` object containing all keys that start with the specified prefix. This implementation will return a `BaseHierarchicalConfiguration` object so that the structure of the keys will be saved. The nodes selected by the prefix (it is possible that multiple nodes are selected) are mapped to the root node of the returned configuration, i.e. their children and attributes will become children and attributes of the new root node. However, a value of the root node is only set if exactly one of the selected nodes contain a value (if multiple nodes have a value, there is simply no way to decide how these values are merged together). Note that the returned `Configuration` object is not connected to its source configuration: updates on the source configuration are not reflected in the subset and vice versa. The returned configuration uses the same `Synchronizer` as this configuration.
    Specified by:
      `subset` in interface `Configuration`
    Overrides:
      `subset` in class `AbstractConfiguration`
    Parameters:
      `prefix` - the prefix of the keys for the subset
    Returns:
      a new configuration object representing the selected subset
    See Also:
      - `SubsetConfiguration`