Package org.apache.commons.configuration2

# Class DynamicCombinedConfiguration

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
org.apache.commons.configuration2.CombinedConfiguration
org.apache.commons.configuration2.DynamicCombinedConfiguration

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

public class DynamicCombinedConfiguration extends CombinedConfiguration

DynamicCombinedConfiguration allows a set of CombinedConfigurations to be used.

Each CombinedConfiguration is referenced by a key that is dynamically constructed from a key pattern on each call. The key pattern will be resolved using the configured ConfigurationInterpolator.

This Configuration implementation uses the configured `Synchronizer` to guard itself against concurrent access. If there are multiple threads accessing an instance concurrently, a fully functional `Synchronizer` implementation (for example `ReadWriteSynchronizer`) has to be used to ensure consistency and to avoid exceptions. The `Synchronizer` assigned to an instance is also passed to child configuration objects when they are created.

Since:
  1.6

- ## Nested Class Summary

  ## Nested classes/interfaces inherited from class org.apache.commons.configuration2.BaseHierarchicalConfiguration

  `BaseHierarchicalConfiguration.BuilderVisitor`

- ## Field Summary

  ### Fields inherited from class org.apache.commons.configuration2.CombinedConfiguration

  `COMBINED_INVALIDATE`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `DynamicCombinedConfiguration()`
  Creates a new instance of `DynamicCombinedConfiguration` that uses a union combiner.
  `DynamicCombinedConfiguration(NodeCombiner comb)`
  Creates a new instance of `DynamicCombinedConfiguration` and initializes the combiner to be used.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addConfiguration(Configuration config, String name, String at)`
  Adds a new configuration to this combined configuration.
  `<T extends Event>
  void`
  `addEventListener(EventType<T> eventType, EventListener<? super T> listener)`
  Adds an event listener for the specified event type.
  `protected void`
  `addNodesInternal(String key, Collection<? extends ImmutableNode> nodes)`
  Actually adds a collection of new nodes to this configuration.
  `protected void`
  `addPropertyInternal(String key, Object value)`
  Adds the property with the specified key.
  `protected void`
  `beginRead(boolean optimize)`
  Notifies this configuration's `Synchronizer` that a read operation is about to start.
  `protected void`
  `beginWrite(boolean optimize)`
  Notifies this configuration's `Synchronizer` that an update operation is about to start.
  `void`
  `clearErrorListeners()`
  Removes all registered error listeners.
  `void`
  `clearEventListeners()`
  Removes all registered event listeners.
  `protected void`
  `clearInternal()`
  Clears this configuration.
  `protected void`
  `clearPropertyDirect(String key)`
  Removes the property with the given key.
  `protected Object`
  `clearTreeInternal(String key)`
  Actually clears the tree of elements referenced by the given key.
  `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.
  `protected boolean`
  `containsKeyInternal(String key)`
  Checks if the specified key is contained in this configuration.
  `protected boolean`
  `containsValueInternal(Object value)`
  Tests whether this configuration contains one or more matches to this value.
  `protected void`
  `endRead()`
  Notifies this configuration's `Synchronizer` that a read operation has finished.
  `protected void`
  `endWrite()`
  Notifies this configuration's `Synchronizer` that an update operation has finished.
  `BigDecimal`
  `getBigDecimal(String key)`
  Gets a `BigDecimal` associated with the given configuration key.
  `BigDecimal`
  `getBigDecimal(String key, BigDecimal defaultValue)`
  Gets a `BigDecimal` associated with the given configuration key.
  `BigInteger`
  `getBigInteger(String key)`
  Gets a `BigInteger` associated with the given configuration key.
  `BigInteger`
  `getBigInteger(String key, BigInteger defaultValue)`
  Gets a `BigInteger` associated with the given configuration key.
  `boolean`
  `getBoolean(String key)`
  Gets a boolean associated with the given configuration key.
  `boolean`
  `getBoolean(String key, boolean defaultValue)`
  Gets a boolean associated with the given configuration key.
  `Boolean`
  `getBoolean(String key, Boolean defaultValue)`
  Obtains the value of the specified key and tries to convert it into a `Boolean` object.
  `byte`
  `getByte(String key)`
  Gets a byte associated with the given configuration key.
  `byte`
  `getByte(String key, byte defaultValue)`
  Gets a byte associated with the given configuration key.
  `Byte`
  `getByte(String key, Byte defaultValue)`
  Gets a `Byte` associated with the given configuration key.
  `Configuration`
  `getConfiguration(int index)`
  Gets the configuration at the specified index.
  `Configuration`
  `getConfiguration(String name)`
  Gets the configuration with the given name.
  `Set<String>`
  `getConfigurationNames()`
  Gets a set with the names of all configurations contained in this combined configuration.
  `double`
  `getDouble(String key)`
  Gets a double associated with the given configuration key.
  `double`
  `getDouble(String key, double defaultValue)`
  Gets a double associated with the given configuration key.
  `Double`
  `getDouble(String key, Double defaultValue)`
  Gets a `Double` associated with the given configuration key.
  `float`
  `getFloat(String key)`
  Gets a float associated with the given configuration key.
  `float`
  `getFloat(String key, float defaultValue)`
  Gets a float associated with the given configuration key.
  `Float`
  `getFloat(String key, Float defaultValue)`
  Gets a `Float` associated with the given configuration key.
  `int`
  `getInt(String key)`
  Gets a int associated with the given configuration key.
  `int`
  `getInt(String key, int defaultValue)`
  Gets a int associated with the given configuration key.
  `Integer`
  `getInteger(String key, Integer defaultValue)`
  Gets an `Integer` associated with the given configuration key.
  `String`
  `getKeyPattern()`
  Gets the key pattern for the CombinedConfiguration map.
  `protected Iterator<String>`
  `getKeysInternal()`
  Gets an iterator with all keys defined in this configuration.
  `protected Iterator<String>`
  `getKeysInternal(String prefix)`
  Gets an iterator with all keys defined in this configuration that start with the given prefix.
  `List<Object>`
  `getList(String key)`
  Gets a List of the values associated with the given configuration key.
  `List<Object>`
  `getList(String key, List<?> defaultValue)`
  Gets a List of strings associated with the given configuration key.
  `long`
  `getLong(String key)`
  Gets a long associated with the given configuration key.
  `long`
  `getLong(String key, long defaultValue)`
  Gets a long associated with the given configuration key.
  `Long`
  `getLong(String key, Long defaultValue)`
  Gets a `Long` associated with the given configuration key.
  `protected int`
  `getMaxIndexInternal(String key)`
  Actually retrieves the maximum defined index for the given key.
  `NodeCombiner`
  `getNodeCombiner()`
  Gets the node combiner that is used for creating the combined node structure.
  `int`
  `getNumberOfConfigurations()`
  Gets the number of configurations that are contained in this combined configuration.
  `Properties`
  `getProperties(String key)`
  Gets a list of properties associated with the given configuration key.
  `protected Object`
  `getPropertyInternal(String key)`
  Fetches the specified property.
  `short`
  `getShort(String key)`
  Gets a short associated with the given configuration key.
  `short`
  `getShort(String key, short defaultValue)`
  Gets a short associated with the given configuration key.
  `Short`
  `getShort(String key, Short defaultValue)`
  Gets a `Short` associated with the given configuration key.
  `Configuration`
  `getSource(String key)`
  Gets the configuration source, in which the specified key is defined.
  `String`
  `getString(String key)`
  Gets a string associated with the given configuration key.
  `String`
  `getString(String key, String defaultValue)`
  Gets a string associated with the given configuration key.
  `String[]`
  `getStringArray(String key)`
  Gets an array of strings associated with the given configuration key.
  `Configuration`
  `interpolatedConfiguration()`
  Returns a configuration with the same content as this configuration, but with all variables replaced by their actual values.
  `void`
  `invalidate()`
  Invalidates the current combined configuration.
  `void`
  `invalidateAll()`
  Invalidates all CombinedConfigurations.
  `protected boolean`
  `isEmptyInternal()`
  Checks if this configuration is empty.
  `Configuration`
  `removeConfiguration(String name)`
  Removes the configuration with the specified name.
  `boolean`
  `removeConfiguration(Configuration config)`
  Removes the specified configuration from this combined configuration.
  `Configuration`
  `removeConfigurationAt(int index)`
  Removes the configuration at the specified index.
  `<T extends Event>
  boolean`
  `removeEventListener(EventType<T> eventType, EventListener<? super T> listener)`
  Removes the event listener registration for the given event type and listener.
  `void`
  `setKeyPattern(String pattern)`
  Sets the key pattern for the CombinedConfiguration map.
  `void`
  `setLoggerName(String name)`
  Sets the name of the Logger to use on each CombinedConfiguration.
  `void`
  `setNodeCombiner(NodeCombiner nodeCombiner)`
  Sets the node combiner.
  `protected void`
  `setPropertyInternal(String key, Object value)`
  Sets the value of the specified property.
  `protected int`
  `sizeInternal()`
  Actually calculates the size of this configuration.
  `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.CombinedConfiguration

  `addConfiguration, addConfiguration, clone, getConfigurationNameList, getConfigurations, getConversionExpressionEngine, getSources, onEvent, setConversionExpressionEngine`

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

  `childConfigurationsAt, childConfigurationsAt, cloneNodeModel, configurationsAt, createSubConfigurationForTrackedNode, getNodeModel, getSubConfigurationNodeSelector, getSubConfigurationParentModel, immutableChildConfigurationsAt, immutableConfigurationAt, immutableConfigurationAt, immutableConfigurationsAt, initSubConfigurationForThisParent, subnodeConfigurationChanged`

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

  `addNodes, addPropertyDirect, clearTree, fetchNodeList, getExpressionEngine, getKeysInternal, getMaxIndex, getModel, getRootElementName, getRootElementNameInternal, nodeDefined, nodeKey, resolveAddKey, resolveKey, resolveNodeKey, resolveUpdateKey, setExpressionEngine, toString`

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

  `addErrorLogListener, addProperty, append, clear, clearProperty, cloneInterpolator, contains, containsKey, containsValue, copy, get, get, getArray, getArray, getCollection, getCollection, getConfigurationDecoder, getConversionHandler, getDuration, getDuration, getEncodedString, getEncodedString, getInterpolator, getKeys, getKeys, getKeys, getList, getList, getListDelimiterHandler, getLogger, getProperties, getProperty, 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

  `copyEventListeners, createErrorEvent, createEvent, fireError, fireEvent, getEventListenerRegistrations, getEventListeners, isDetailEvents, 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, getCollection, getCollection, getDuration, getDuration, getEncodedString, getEncodedString, getEnum, getEnum, getKeys, getKeys, getKeys, getList, getList, getProperty, immutableSubset, isEmpty, size`

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

  `getSynchronizer, lock, setSynchronizer, unlock`

- ## Constructor Details

  - ### DynamicCombinedConfiguration

    public DynamicCombinedConfiguration()
    Creates a new instance of `DynamicCombinedConfiguration` that uses a union combiner.
    See Also:
      - `UnionCombiner`

  - ### DynamicCombinedConfiguration

    public DynamicCombinedConfiguration(NodeCombiner comb)
    Creates a new instance of `DynamicCombinedConfiguration` and initializes the combiner to be used.
    Parameters:
      `comb` - the node combiner (can be null, then a union combiner is used as default)

- ## Method Details

  - ### addConfiguration

    public void addConfiguration(Configuration config, String name, String at)
    Adds a new configuration to this combined configuration. It is possible (but not mandatory) to give the new configuration a name. This name must be unique, otherwise a `ConfigurationRuntimeException` will be thrown. With the optional `at` argument you can specify where in the resulting node structure the content of the added configuration should appear. This is a string that uses dots as property delimiters (independent on the current expression engine). For instance if you pass in the string `"database.tables"`, all properties of the added configuration will occur in this branch.
    Overrides:
      `addConfiguration` in class `CombinedConfiguration`
    Parameters:
      `config` - the configuration to add (must not be null)
      `name` - the name of this configuration (can be null)
      `at` - the position of this configuration in the combined tree (can be null)

  - ### addEventListener

    public <T extends Event> void addEventListener(EventType<T> eventType, EventListener<? super T> listener)
    Description copied from interface: `EventSource`
    Adds an event listener for the specified event type. This listener is notified about events of this type and all its sub types.
    Specified by:
      `addEventListener` in interface `EventSource`
    Overrides:
      `addEventListener` in class `BaseEventSource`
    Type Parameters:
      `T` - the type of events processed by this listener
    Parameters:
      `eventType` - the event type (must not be null)
      `listener` - the listener to be registered (must not be null)

  - ### addNodesInternal

    protected void addNodesInternal(String key, Collection<? extends ImmutableNode> nodes)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Actually adds a collection of new nodes to this configuration. This method is called by `addNodes()`. It can be overridden by subclasses that need to adapt this operation.
    Overrides:
      `addNodesInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key where the nodes are to be added; can be null, then they are added to the root node
      `nodes` - a collection with the `Node` objects to be added

  - ### addPropertyInternal

    protected void addPropertyInternal(String key, Object value)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Adds the property with the specified key. This task will be delegated to the associated `ExpressionEngine`, so the passed in key must match the requirements of this implementation.
    Overrides:
      `addPropertyInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key of the new property
      `value` - the value of the new property

  - ### beginRead

    protected void beginRead(boolean optimize)
    Notifies this configuration's `Synchronizer` that a read operation is about to start. This method is called by all methods which access this configuration in a read-only mode. Subclasses may override it to perform additional actions before this read operation. The boolean optimize argument can be evaluated by overridden methods in derived classes. Some operations which require a lock do not need a fully initialized configuration object. By setting this flag to true, such operations can give a corresponding hint. An overridden implementation of `beginRead()` can then decide to skip some initialization steps. All basic operations in this class (and most of the basic `Configuration` implementations) call this method with a parameter value of false. In any case the inherited method must be called! Otherwise, proper synchronization is not guaranteed. This implementation checks whether a combined root node is available. If not, it is constructed by requesting a write lock. This implementation ensures that the current configuration is initialized. The lock counter is increased.
    Overrides:
      `beginRead` in class `CombinedConfiguration`
    Parameters:
      `optimize` - a flag whether optimization can be performed

  - ### beginWrite

    protected void beginWrite(boolean optimize)
    Notifies this configuration's `Synchronizer` that an update operation is about to start. This method is called by all methods which modify this configuration. Subclasses may override it to perform additional operations before an update. For a description of the boolean optimize argument refer to the documentation of `beginRead()`. In any case the inherited method must be called! Otherwise, proper synchronization is not guaranteed. This implementation checks whether a combined root node is available. If not, it is constructed now. This implementation ensures that the current configuration is initialized. If necessary, a new child configuration instance is created.
    Overrides:
      `beginWrite` in class `CombinedConfiguration`
    Parameters:
      `optimize` - a flag whether optimization can be performed
    See Also:
      - `AbstractConfiguration.beginRead(boolean)`

  - ### clearErrorListeners

    public void clearErrorListeners()
    Description copied from class: `BaseEventSource`
    Removes all registered error listeners.
    Overrides:
      `clearErrorListeners` in class `BaseEventSource`

  - ### clearEventListeners

    public void clearEventListeners()
    Description copied from class: `BaseEventSource`
    Removes all registered event listeners.
    Overrides:
      `clearEventListeners` in class `BaseEventSource`

  - ### clearInternal

    protected void clearInternal()
    Description copied from class: `CombinedConfiguration`
    Clears this configuration. All contained configurations will be removed.
    Overrides:
      `clearInternal` in class `CombinedConfiguration`

  - ### clearPropertyDirect

    protected void clearPropertyDirect(String key)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Removes the property with the given key. Properties with names that start with the given key (i.e. properties below the specified key in the hierarchy) won't be affected. This implementation delegates to the node+ model.
    Overrides:
      `clearPropertyDirect` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key of the property to be removed

  - ### clearTreeInternal

    protected Object clearTreeInternal(String key)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Actually clears the tree of elements referenced by the given key. This method is called by `clearTree()`. Subclasses that need to adapt this operation can override this method. This base implementation delegates to the node model.
    Overrides:
      `clearTreeInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key of the property to be removed
    Returns:
      an object with information about the nodes that have been removed (this is needed for firing a meaningful event of type CLEAR_TREE)

  - ### configurationAt

    public HierarchicalConfiguration<ImmutableNode> configurationAt(String key)
    Description copied from class: `BaseHierarchicalConfiguration`
    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>`
    Overrides:
      `configurationAt` in class `BaseHierarchicalConfiguration`
    Parameters:
      `key` - the key that selects the sub tree
    Returns:
      a hierarchical configuration that contains this sub tree
    See Also:
      - `SubnodeConfiguration`

  - ### configurationAt

    public HierarchicalConfiguration<ImmutableNode> configurationAt(String key, boolean supportUpdates)
    Description copied from class: `BaseHierarchicalConfiguration`

    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>`
    Overrides:
      `configurationAt` in class `BaseHierarchicalConfiguration`
    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
    See Also:
      - `SubnodeConfiguration`

  - ### configurationsAt

    public List<HierarchicalConfiguration<ImmutableNode>> configurationsAt(String key)
    Description copied from class: `BaseHierarchicalConfiguration`
    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 `BaseHierarchicalConfiguration.configurationAt(String)`.
    Specified by:
      `configurationsAt` in interface `HierarchicalConfiguration<ImmutableNode>`
    Overrides:
      `configurationsAt` in class `BaseHierarchicalConfiguration`
    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

  - ### containsKeyInternal

    protected boolean containsKeyInternal(String key)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Checks if the specified key is contained in this configuration. Note that for this configuration the term "contained" means that the key has an associated value. If there is a node for this key that has no value but children (either defined or undefined), this method will still return false .
    Overrides:
      `containsKeyInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key to be checked
    Returns:
      a flag if this key is contained in this configuration

  - ### containsValueInternal

    protected boolean containsValueInternal(Object value)
    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 containsKey method.
    Overrides:
      `containsValueInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `value` - the value in question
    Returns:
      `true` if and only if some key maps to the `value` argument in this configuration as determined by the `equals` method; `false` otherwise.
    Since:
      2.11.0

  - ### endRead

    protected void endRead()
    Notifies this configuration's `Synchronizer` that a read operation has finished. This method is called by all methods which access this configuration in a read-only manner at the end of their execution. Subclasses may override it to perform additional actions after this read operation. In any case the inherited method must be called! Otherwise, the read lock will not be released. This implementation clears the current configuration if necessary.
    Overrides:
      `endRead` in class `AbstractConfiguration`

  - ### endWrite

    protected void endWrite()
    Notifies this configuration's `Synchronizer` that an update operation has finished. This method is called by all methods which modify this configuration at the end of their execution. Subclasses may override it to perform additional operations after an update. In any case the inherited method must be called! Otherwise, the write lock will not be released. This implementation clears the current configuration if necessary.
    Overrides:
      `endWrite` in class `AbstractConfiguration`

  - ### getBigDecimal

    public BigDecimal getBigDecimal(String key)
    Description copied from class: `AbstractConfiguration`
    Gets a `BigDecimal` associated with the given configuration key.
    Specified by:
      `getBigDecimal` in interface `ImmutableConfiguration`
    Overrides:
      `getBigDecimal` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated BigDecimal if key is found and has valid format
    See Also:
      - `AbstractConfiguration.setThrowExceptionOnMissing(boolean)`

  - ### getBigDecimal

    public BigDecimal getBigDecimal(String key, BigDecimal defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `BigDecimal` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getBigDecimal` in interface `ImmutableConfiguration`
    Overrides:
      `getBigDecimal` in class `AbstractConfiguration`
    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

    public BigInteger getBigInteger(String key)
    Description copied from class: `AbstractConfiguration`
    Gets a `BigInteger` associated with the given configuration key.
    Specified by:
      `getBigInteger` in interface `ImmutableConfiguration`
    Overrides:
      `getBigInteger` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated BigInteger if key is found and has valid format
    See Also:
      - `AbstractConfiguration.setThrowExceptionOnMissing(boolean)`

  - ### getBigInteger

    public BigInteger getBigInteger(String key, BigInteger defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `BigInteger` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getBigInteger` in interface `ImmutableConfiguration`
    Overrides:
      `getBigInteger` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated BigInteger if key is found and has valid format, default value otherwise.

  - ### getBoolean

    public boolean getBoolean(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a boolean associated with the given configuration key.
    Specified by:
      `getBoolean` in interface `ImmutableConfiguration`
    Overrides:
      `getBoolean` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated boolean.

  - ### getBoolean

    public boolean getBoolean(String key, boolean defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a boolean associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getBoolean` in interface `ImmutableConfiguration`
    Overrides:
      `getBoolean` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated boolean.

  - ### getBoolean

    public Boolean getBoolean(String key, Boolean defaultValue)
    Description copied from class: `AbstractConfiguration`
    Obtains the value of the specified key and tries to convert it into a `Boolean` object. If the property has no value, the passed in default value will be used.
    Specified by:
      `getBoolean` in interface `ImmutableConfiguration`
    Overrides:
      `getBoolean` in class `AbstractConfiguration`
    Parameters:
      `key` - the key of the property
      `defaultValue` - the default value
    Returns:
      the value of this key converted to a `Boolean`

  - ### getByte

    public byte getByte(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a byte associated with the given configuration key.
    Specified by:
      `getByte` in interface `ImmutableConfiguration`
    Overrides:
      `getByte` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated byte.

  - ### getByte

    public byte getByte(String key, byte defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a byte associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getByte` in interface `ImmutableConfiguration`
    Overrides:
      `getByte` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated byte.

  - ### getByte

    public Byte getByte(String key, Byte defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `Byte` associated with the given configuration key.
    Specified by:
      `getByte` in interface `ImmutableConfiguration`
    Overrides:
      `getByte` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated byte if key is found and has valid format, default value otherwise.

  - ### getConfiguration

    public Configuration getConfiguration(int index)
    Gets the configuration at the specified index. The contained configurations are numbered in the order they were added to this combined configuration. The index of the first configuration is 0.
    Overrides:
      `getConfiguration` in class `CombinedConfiguration`
    Parameters:
      `index` - the index
    Returns:
      the configuration at this index

  - ### getConfiguration

    public Configuration getConfiguration(String name)
    Gets the configuration with the given name. This can be null if no such configuration exists.
    Overrides:
      `getConfiguration` in class `CombinedConfiguration`
    Parameters:
      `name` - the name of the configuration
    Returns:
      the configuration with this name

  - ### getConfigurationNames

    public Set<String> getConfigurationNames()
    Gets a set with the names of all configurations contained in this combined configuration. Of course here are only these configurations listed, for which a name was specified when they were added.
    Overrides:
      `getConfigurationNames` in class `CombinedConfiguration`
    Returns:
      a set with the names of the contained configurations (never null)

  - ### getDouble

    public double getDouble(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a double associated with the given configuration key.
    Specified by:
      `getDouble` in interface `ImmutableConfiguration`
    Overrides:
      `getDouble` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated double.

  - ### getDouble

    public double getDouble(String key, double defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a double associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getDouble` in interface `ImmutableConfiguration`
    Overrides:
      `getDouble` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated double.

  - ### getDouble

    public Double getDouble(String key, Double defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `Double` associated with the given configuration key.
    Specified by:
      `getDouble` in interface `ImmutableConfiguration`
    Overrides:
      `getDouble` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated double if key is found and has valid format, default value otherwise.

  - ### getFloat

    public float getFloat(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a float associated with the given configuration key.
    Specified by:
      `getFloat` in interface `ImmutableConfiguration`
    Overrides:
      `getFloat` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated float.

  - ### getFloat

    public float getFloat(String key, float defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a float associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getFloat` in interface `ImmutableConfiguration`
    Overrides:
      `getFloat` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated float.

  - ### getFloat

    public Float getFloat(String key, Float defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `Float` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getFloat` in interface `ImmutableConfiguration`
    Overrides:
      `getFloat` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated float if key is found and has valid format, default value otherwise.

  - ### getInt

    public int getInt(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a int associated with the given configuration key.
    Specified by:
      `getInt` in interface `ImmutableConfiguration`
    Overrides:
      `getInt` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated int.

  - ### getInt

    public int getInt(String key, int defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a int associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getInt` in interface `ImmutableConfiguration`
    Overrides:
      `getInt` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated int.

  - ### getInteger

    public Integer getInteger(String key, Integer defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets an `Integer` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getInteger` in interface `ImmutableConfiguration`
    Overrides:
      `getInteger` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated int if key is found and has valid format, default value otherwise.

  - ### getKeyPattern

    public String getKeyPattern()
    Gets the key pattern for the CombinedConfiguration map.
    Returns:
      the key pattern for the CombinedConfiguration map.

  - ### getKeysInternal

    protected Iterator<String> getKeysInternal()
    Description copied from class: `AbstractHierarchicalConfiguration`
    Gets an iterator with all keys defined in this configuration. Note that the keys returned by this method will not contain any indices. This means that some structure will be lost.
    Overrides:
      `getKeysInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Returns:
      an iterator with the defined keys in this configuration

  - ### getKeysInternal

    protected Iterator<String> getKeysInternal(String prefix)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Gets an iterator with all keys defined in this configuration that start with the given prefix. The returned keys will not contain any indices. This implementation tries to locate a node whose key is the same as the passed in prefix. Then the subtree of this node is traversed, and the keys of all nodes encountered (including attributes) are added to the result set.
    Overrides:
      `getKeysInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `prefix` - the prefix of the keys to start with
    Returns:
      an iterator with the found keys

  - ### getList

    public List<Object> getList(String key)
    Description copied from class: `AbstractConfiguration`
    Gets 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.
    Specified by:
      `getList` in interface `ImmutableConfiguration`
    Overrides:
      `getList` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated List.
    See Also:
      - `AbstractConfiguration.getStringArray(String)`

  - ### getList

    public List<Object> getList(String key, List<?> defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    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.
    Specified by:
      `getList` in interface `ImmutableConfiguration`
    Overrides:
      `getList` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated List of strings.
    See Also:
      - `ImmutableConfiguration.getList(Class, String, List)`

  - ### getLong

    public long getLong(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a long associated with the given configuration key.
    Specified by:
      `getLong` in interface `ImmutableConfiguration`
    Overrides:
      `getLong` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated long.

  - ### getLong

    public long getLong(String key, long defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a long associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getLong` in interface `ImmutableConfiguration`
    Overrides:
      `getLong` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated long.

  - ### getLong

    public Long getLong(String key, Long defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `Long` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getLong` in interface `ImmutableConfiguration`
    Overrides:
      `getLong` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated long if key is found and has valid format, default value otherwise.

  - ### getMaxIndexInternal

    protected int getMaxIndexInternal(String key)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Actually retrieves the maximum defined index for the given key. This method is called by `getMaxIndex()`. Subclasses that need to adapt this operation have to override this method.
    Overrides:
      `getMaxIndexInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key to be checked
    Returns:
      the maximum defined index for this key

  - ### getNodeCombiner

    public NodeCombiner getNodeCombiner()
    Gets the node combiner that is used for creating the combined node structure.
    Overrides:
      `getNodeCombiner` in class `CombinedConfiguration`
    Returns:
      the node combiner

  - ### getNumberOfConfigurations

    public int getNumberOfConfigurations()
    Gets the number of configurations that are contained in this combined configuration.
    Overrides:
      `getNumberOfConfigurations` in class `CombinedConfiguration`
    Returns:
      the number of contained configurations

  - ### getProperties

    public Properties getProperties(String key)
    Description copied from interface: `ImmutableConfiguration`
    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 `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.
    Specified by:
      `getProperties` in interface `ImmutableConfiguration`
    Overrides:
      `getProperties` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated properties if key is found.

  - ### getPropertyInternal

    protected Object getPropertyInternal(String key)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Fetches the specified property. This task is delegated to the associated expression engine.
    Overrides:
      `getPropertyInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key to be looked up
    Returns:
      the found value

  - ### getShort

    public short getShort(String key)
    Description copied from interface: `ImmutableConfiguration`
    Gets a short associated with the given configuration key.
    Specified by:
      `getShort` in interface `ImmutableConfiguration`
    Overrides:
      `getShort` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated short.

  - ### getShort

    public short getShort(String key, short defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a short associated with the given configuration key.
    Specified by:
      `getShort` in interface `ImmutableConfiguration`
    Overrides:
      `getShort` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated short.

  - ### getShort

    public Short getShort(String key, Short defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a `Short` associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getShort` in interface `ImmutableConfiguration`
    Overrides:
      `getShort` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated short if key is found and has valid format, default value otherwise.

  - ### getSource

    public Configuration getSource(String key)
    Gets the configuration source, in which the specified key is defined. This method will determine the configuration node that is identified by the given key. The following constellations are possible:
    - If no node object is found for this key, null is returned.
    - If the key maps to multiple nodes belonging to different configuration sources, a `IllegalArgumentException` is thrown (in this case no unique source can be determined).
    - If exactly one node is found for the key, the (child) configuration object, to which the node belongs is determined and returned.
    - For keys that have been added directly to this combined configuration and that do not belong to the namespaces defined by existing child configurations this configuration will be returned.
    Overrides:
      `getSource` in class `CombinedConfiguration`
    Parameters:
      `key` - the key of a configuration property
    Returns:
      the configuration, to which this property belongs or null if the key cannot be resolved
    Throws:
      `IllegalArgumentException` - if the key maps to multiple properties and the source cannot be determined, or if the key is null

  - ### getString

    public String getString(String key)
    Description copied from class: `AbstractConfiguration`
    Gets a string associated with the given configuration key.
    Specified by:
      `getString` in interface `ImmutableConfiguration`
    Overrides:
      `getString` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated string.
    See Also:
      - `AbstractConfiguration.setThrowExceptionOnMissing(boolean)`

  - ### getString

    public String getString(String key, String defaultValue)
    Description copied from interface: `ImmutableConfiguration`
    Gets a string associated with the given configuration key. If the key doesn't map to an existing object, the default value is returned.
    Specified by:
      `getString` in interface `ImmutableConfiguration`
    Overrides:
      `getString` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
      `defaultValue` - The default value.
    Returns:
      The associated string if key is found and has valid format, default value otherwise.

  - ### getStringArray

    public String[] getStringArray(String key)
    Description copied from class: `AbstractConfiguration`
    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. When a property is added to a configuration, it is checked whether it contains multiple values. This is obvious if the added object is a list or an array. For strings the association `ListDelimiterHandler` is consulted to find out whether the string can be split into multiple values.
    Specified by:
      `getStringArray` in interface `ImmutableConfiguration`
    Overrides:
      `getStringArray` in class `AbstractConfiguration`
    Parameters:
      `key` - The configuration key.
    Returns:
      The associated string array if key is found.
    See Also:
      - `AbstractConfiguration.setListDelimiterHandler(ListDelimiterHandler)`

  - ### interpolatedConfiguration

    public Configuration interpolatedConfiguration()
    Description copied from class: `BaseHierarchicalConfiguration`
    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 `BaseHierarchicalConfiguration`
    Returns:
      a configuration with all variables interpolated

  - ### invalidate

    public void invalidate()
    Invalidates the current combined configuration. This means that the next time a property is accessed the combined node structure must be re-constructed. Invalidation of a combined configuration also means that an event of type `EVENT_COMBINED_INVALIDATE` is fired. Note that while other events most times appear twice (once before and once after an update), this event is only fired once (after update).
    Overrides:
      `invalidate` in class `CombinedConfiguration`

  - ### invalidateAll

    public void invalidateAll()
    Invalidates all CombinedConfigurations.

  - ### isEmptyInternal

    protected boolean isEmptyInternal()
    Description copied from class: `AbstractHierarchicalConfiguration`
    Checks if this configuration is empty. Empty means that there are no keys with any values, though there can be some (empty) nodes.
    Overrides:
      `isEmptyInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Returns:
      a flag if this configuration is empty

  - ### removeConfiguration

    public boolean removeConfiguration(Configuration config)
    Removes the specified configuration from this combined configuration.
    Overrides:
      `removeConfiguration` in class `CombinedConfiguration`
    Parameters:
      `config` - the configuration to be removed
    Returns:
      a flag whether this configuration was found and could be removed

  - ### removeConfiguration

    public Configuration removeConfiguration(String name)
    Removes the configuration with the specified name.
    Overrides:
      `removeConfiguration` in class `CombinedConfiguration`
    Parameters:
      `name` - the name of the configuration to be removed
    Returns:
      the removed configuration (null if this configuration was not found)

  - ### removeConfigurationAt

    public Configuration removeConfigurationAt(int index)
    Removes the configuration at the specified index.
    Overrides:
      `removeConfigurationAt` in class `CombinedConfiguration`
    Parameters:
      `index` - the index
    Returns:
      the removed configuration

  - ### removeEventListener

    public <T extends Event> boolean removeEventListener(EventType<T> eventType, EventListener<? super T> listener)
    Description copied from interface: `EventSource`
    Removes the event listener registration for the given event type and listener. An event listener instance may be registered multiple times for different event types. Therefore, when removing a listener the event type of the registration in question has to be specified. The return value indicates whether a registration was removed. A value of false means that no such combination of event type and listener was found.
    Specified by:
      `removeEventListener` in interface `EventSource`
    Overrides:
      `removeEventListener` in class `BaseEventSource`
    Type Parameters:
      `T` - the type of events processed by this listener
    Parameters:
      `eventType` - the event type
      `listener` - the event listener to be removed
    Returns:
      a flag whether a listener registration was removed

  - ### setKeyPattern

    public void setKeyPattern(String pattern)
    Sets the key pattern for the CombinedConfiguration map.
    Parameters:
      `pattern` - the key pattern for the CombinedConfiguration map.

  - ### setLoggerName

    public void setLoggerName(String name)
    Sets the name of the Logger to use on each CombinedConfiguration.
    Parameters:
      `name` - The Logger name.

  - ### setNodeCombiner

    public void setNodeCombiner(NodeCombiner nodeCombiner)
    Sets the node combiner. This object will be used when the combined node structure is to be constructed. It must not be null, otherwise an `IllegalArgumentException` exception is thrown. Changing the node combiner causes an invalidation of this combined configuration, so that the new combiner immediately takes effect.
    Overrides:
      `setNodeCombiner` in class `CombinedConfiguration`
    Parameters:
      `nodeCombiner` - the node combiner

  - ### setPropertyInternal

    protected void setPropertyInternal(String key, Object value)
    Description copied from class: `AbstractHierarchicalConfiguration`
    Sets the value of the specified property.
    Overrides:
      `setPropertyInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Parameters:
      `key` - the key of the property to set
      `value` - the new value of this property

  - ### sizeInternal

    protected int sizeInternal()
    Description copied from class: `AbstractHierarchicalConfiguration`
    Actually calculates the size of this configuration. This method is called by `size()` with a read lock held. The base implementation provided here calculates the size based on the iterator returned by `getKeys()`. Sub classes which can determine the size in a more efficient way should override this method. This implementation is slightly more efficient than the default implementation. It does not iterate over the key set, but directly queries its size after it has been constructed. Note that constructing the key set is still an O(n) operation.
    Overrides:
      `sizeInternal` in class `AbstractHierarchicalConfiguration<ImmutableNode>`
    Returns:
      the size of this configuration (i.e. the number of keys)

  - ### subset

    public Configuration subset(String prefix)
    Description copied from class: `BaseHierarchicalConfiguration`
    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 `BaseHierarchicalConfiguration`
    Parameters:
      `prefix` - the prefix of the keys for the subset
    Returns:
      a new configuration object representing the selected subset
    See Also:
      - `SubsetConfiguration`