Package com.tangosol.net.cache

Class OverflowMap

  • All Implemented Interfaces:
    CacheMap, ObservableMap, Map
    public class OverflowMap
extends AbstractKeySetBasedMap
implements CacheMap
    An Observable Map implementation that wraps two maps - a front map (assumed to be fast but limited in its maximum size) and a back map (assumed to be slower but much less limited in its maximum size).

    The OverflowMap is observable, and as such it is allowed to be an active data structure. In other words, it supports item expiration, eviction, and other types of "self-generating" events. (The SimpleOverflowMap) is an alternative that may be more efficient if the overflow map is passive.) As a consequence of the requirements for managing the overflow map as an active data structure, the OverflowMap maintains information about each entry in the map; in practice, this will limit the size of the OverflowMap since all keys and some additional information for each key will be maintained in memory. On the other hand, by maintaining this information in memory, certain operations will be more efficient, such as the methods containsKey(), size(), keySet().iterator(), etc. (Again, for an alternative, see SimpleOverflowMap, which does not maintain the entire set of keys in memory.)

    The primary reason why OverflowMap is based on the AbstractKeySetBasedMap is that the set of keys within the OverflowMap is considered to be "known", although not altogether stable due to concurrency issues (the OverflowMap is designed to support a high level of concurrency). AbstractKeySetBasedMap conceptually delegates many operations down to its key Set, which the OverflowMap implementation reverses (by having the implementations on the OverflowMap itself). However, the key and entry Sets and the values Collection benefit significantly in terms of their ability to optimize the Collection.iterator() and Collection.toArray() methods.

    Further, as a result (and sometimes as a consequence) of its observability, the OverflowMap supports (and has to support) a number of capabilities that the SimpleOverflowMap does not:

    • Expiry - the entries in the OverflowMap can each have their own expiry, which is respected regardless of whether the entry is being managed in the front and/or back map.
    • Reentrancy - since an event listener on the OverflowMap can respond to an event in an unpredictable manner, including by attempting to undo or compensate for or react to the event, the OverflowMap is forced to support reentrancy.

    Since:
    Coherence 2.2
    Author:
    cp 2003.05.24, cp 2005.08.11 re-architected and split out SimpleOverflowMap

    • Field Detail

      • ENTRY_INSERTED

        public static final int ENTRY_INSERTED
        This event indicates that an entry has been added to the map.
        See Also:
        Constant Field Values

      • ENTRY_UPDATED

        public static final int ENTRY_UPDATED
        This event indicates that an entry has been updated in the map.
        See Also:
        Constant Field Values

      • ENTRY_DELETED

        public static final int ENTRY_DELETED
        This event indicates that an entry has been removed from the map.
        See Also:
        Constant Field Values

    • Constructor Detail

      • OverflowMap

        public OverflowMap​(ObservableMap mapFront,
                   Map mapBack)
        Construct a OverflowMap using two specified maps:
        • FrontMap (aka "cache" or "shallow") and
        • BackMap (aka "file" or "deep")
        If the BackMap implements the ObservableMap interface a listener will be added to the BackMap to invalidate items updated [externally] in the back map.
        Parameters:
        mapBack - back map
        mapFront - front map

    • Method Detail

      • containsKey

        public boolean containsKey​(Object oKey)
        Returns true if this map contains a mapping for the specified key.
        Specified by:
        containsKey in interface Map
        Overrides:
        containsKey in class AbstractKeySetBasedMap
        Returns:
        true if this map contains a mapping for the specified key, false otherwise.

      • get

        public Object get​(Object oKey)
        Returns the value to which this map maps the specified key.
        Specified by:
        get in interface Map
        Specified by:
        get in class AbstractKeyBasedMap
        Parameters:
        oKey - the key object
        Returns:
        the value to which this map maps the specified key, or null if the map contains no mapping for this key

      • isEmpty

        public boolean isEmpty()
        Returns true if this map contains no key-value mappings.
        Specified by:
        isEmpty in interface Map
        Overrides:
        isEmpty in class AbstractKeySetBasedMap
        Returns:
        true if this map contains no key-value mappings

      • put

        public Object put​(Object oKey,
                  Object oValue)
        Associates the specified value with the specified key in this map.
        Specified by:
        put in interface CacheMap
        Specified by:
        put in interface Map
        Overrides:
        put in class AbstractKeyBasedMap
        Parameters:
        oKey - key with which the specified value is to be associated
        oValue - value to be associated with the specified key
        Returns:
        previous value associated with specified key, or null if there was no mapping for key

      • putAll

        public void putAll​(Map map)
        Copies all of the mappings from the specified map to this map. The effect of this call is equivalent to that of calling AbstractKeyBasedMap.put(K, V) on this map once for each mapping in the passed map. The behavior of this operation is unspecified if the passed map is modified while the operation is in progress.
        Specified by:
        putAll in interface Map
        Overrides:
        putAll in class AbstractKeyBasedMap
        Parameters:
        map - the Map containing the key/value pairings to put into this Map

      • remove

        public Object remove​(Object oKey)
        Removes the mapping for this key from this map if present. Expensive: updates both the underlying cache and the local cache.
        Specified by:
        remove in interface Map
        Overrides:
        remove in class AbstractKeyBasedMap
        Parameters:
        oKey - key whose mapping is to be removed from the map
        Returns:
        previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values.

      • size

        public int size()
        Returns the number of key-value mappings in this map.
        Specified by:
        size in interface Map
        Overrides:
        size in class AbstractKeySetBasedMap
        Returns:
        the number of key-value mappings in this map

      • put

        public Object put​(Object oKey,
                  Object oValue,
                  long cMillis)
        Associates the specified value with the specified key in this cache. If the cache previously contained a mapping for this key, the old value is replaced. This variation of the CacheMap.put(Object oKey, Object oValue) method allows the caller to specify an expiry (or "time to live") for the cache entry.
        Specified by:
        put in interface CacheMap
        Parameters:
        oKey - key with which the specified value is to be associated
        oValue - value to be associated with the specified key
        cMillis - the number of milliseconds until the cache entry will expire, also referred to as the entry's "time to live"; pass CacheMap.EXPIRY_DEFAULT to use the cache's default time-to-live setting; pass CacheMap.EXPIRY_NEVER to indicate that the cache entry should never expire; this milliseconds value is not a date/time value, such as is returned from System.currentTimeMillis()
        Returns:
        previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values

      • getAll

        public Map getAll​(Collection colKeys)
        Get all the specified keys, if they are in the Map. For each key that is in the cache, that key and its corresponding value will be placed in the map that is returned by this method. The absence of a key in the returned map indicates that it was not in the cache, which may imply (for caches that can load behind the scenes) that the requested data could not be loaded.
        Specified by:
        getAll in interface CacheMap
        Overrides:
        getAll in class AbstractKeyBasedMap
        Parameters:
        colKeys - a collection of keys that may be in the named cache
        Returns:
        a Map of keys to values for the specified keys passed in colKeys

      • addMapListener

        public void addMapListener​(MapListener listener)
        Add a standard map listener that will receive all events (inserts, updates, deletes) that occur against the map, with the key, old-value and new-value included. This has the same result as the following call: 
           addMapListener(listener, (Filter) null, false);
        Specified by:
        addMapListener in interface ObservableMap
        Parameters:
        listener - the MapEvent listener to add

      • removeMapListener

        public void removeMapListener​(MapListener listener)
        Remove a standard map listener that previously signed up for all events. This has the same result as the following call: 
           removeMapListener(listener, (Filter) null);
        Specified by:
        removeMapListener in interface ObservableMap
        Parameters:
        listener - the listener to remove

      • addMapListener

        public void addMapListener​(MapListener listener,
                           Object oKey,
                           boolean fLite)
        Add a map listener for a specific key.

        The listeners will receive MapEvent objects, but if fLite is passed as true, they might not contain the OldValue and NewValue properties.

        To unregister the MapListener, use the ObservableMap.removeMapListener(MapListener, Object) method.

        Specified by:
        addMapListener in interface ObservableMap
        Parameters:
        listener - the MapEvent listener to add
        oKey - the key that identifies the entry for which to raise events
        fLite - true to indicate that the MapEvent objects do not have to include the OldValue and NewValue property values in order to allow optimizations

      • removeMapListener

        public void removeMapListener​(MapListener listener,
                              Object oKey)
        Remove a map listener that previously signed up for events about a specific key.
        Specified by:
        removeMapListener in interface ObservableMap
        Parameters:
        listener - the listener to remove
        oKey - the key that identifies the entry for which to raise events

      • addMapListener

        public void addMapListener​(MapListener listener,
                           Filter filter,
                           boolean fLite)
        Add a map listener that receives events based on a filter evaluation.

        The listeners will receive MapEvent objects, but if fLite is passed as true, they might not contain the OldValue and NewValue properties.

        To unregister the MapListener, use the ObservableMap.removeMapListener(MapListener, Filter) method.

        Specified by:
        addMapListener in interface ObservableMap
        Parameters:
        listener - the MapEvent listener to add
        filter - a filter that will be passed MapEvent objects to select from; a MapEvent will be delivered to the listener only if the filter evaluates to true for that MapEvent (see MapEventFilter); null is equivalent to a filter that alway returns true
        fLite - true to indicate that the MapEvent objects do not have to include the OldValue and NewValue property values in order to allow optimizations

      • removeMapListener

        public void removeMapListener​(MapListener listener,
                              Filter filter)
        Remove a map listener that previously signed up for events based on a filter evaluation.
        Specified by:
        removeMapListener in interface ObservableMap
        Parameters:
        listener - the listener to remove
        filter - the filter that was passed into the corresponding addMapListener() call

      • removeBlind

        protected boolean removeBlind​(Object oKey)
        Removes the mapping for this key from this map if present. This method exists to allow sub-classes to optimize remove functionality for situations in which the original value is not required.
        Overrides:
        removeBlind in class AbstractKeyBasedMap
        Parameters:
        oKey - key whose mapping is to be removed from the map
        Returns:
        true iff the Map changed as the result of this operation

      • getInternalKeySet

        protected Set getInternalKeySet()
        Obtain a set of keys that are represented by this Map.

        The AbstractKeySetBasedMap only utilizes the internal key set as a read-only resource.

        Specified by:
        getInternalKeySet in class AbstractKeySetBasedMap
        Returns:
        an internal Set of keys that are contained by this Map

      • getInternal

        protected Object getInternal​(Object oKey,
                             boolean fStats,
                             Map mapResult)
        A combined implementation of get(java.lang.Object) and getAll(java.util.Collection) that eliminates duplicate (and very complex) code.
        Parameters:
        oKey - the key to access
        fStats - pass true to update statistics
        mapResult - pass a map into which the key and its value will be put iff the key is contained within the overflow map
        Returns:
        the value (which may be null) is returned from the entry if it exists, otherwise null is returned

      • putInternal

        protected Object putInternal​(Object oKey,
                             Object oValue,
                             boolean fStoreOnly,
                             long cMillis)
        A combined implementation of put(Object, Object) and "void put(Object, Object)" that eliminates duplicate (and muchos complex) code between the put(java.lang.Object, java.lang.Object) and putAll(java.util.Map) methods. Additionally implements the put-with-expiry option.
        Parameters:
        oKey - the key to store
        oValue - the value to store
        fStoreOnly - pass true to simply store the new value, or false to both store the new value and return the previous value
        cMillis - the time-to-live for the entry as defined by CacheMap.put(Object, Object, long)
        Returns:
        the previous value if fStoreOnly is false and the entry existed in the overflow map, otherwise undefined

      • removeInternal

        protected Object removeInternal​(Object oKey,
                                boolean fCheckRemovedOnly,
                                boolean fEviction)
        A combined implementation of remove() and removeBlind() that eliminates duplicate (and muchos complex) code
        Parameters:
        oKey - the key to remove
        fCheckRemovedOnly - pass true to only check for entry existence
        fEviction - pass true if the removal is an eviction and if the status is already owned by this thread
        Returns:
        if only checking for existence, then a Boolean value is returned, otherwise the value (which may be null) is returned from the entry if it exists, otherwise null is returned

      • getFrontMap

        public ObservableMap getFrontMap()
        Returns the front Map.

        Warning: Direct modifications of the returned map may cause unpredictable behavior of the Overflow Map.

        Returns:
        the front Map

      • getBackMap

        public Map getBackMap()
        Returns the back Map.

        Warning: Direct modifications of the returned map may cause unpredictable behavior of the Overflow Map.

        Returns:
        the back Map

      • getExpiryDelay

        public int getExpiryDelay()
        Determine the "time to live" for each individual cache entry.
        Returns:
        the number of milliseconds that a cache entry value will live, or EXPIRY_NEVER if the entries never expire by default

      • setExpiryDelay

        public void setExpiryDelay​(int cMillis)
        Specify the "time to live" for cache entries. This does not affect the already-scheduled expiry of existing entries.
        Parameters:
        cMillis - the number of milliseconds to specify that cache entries subsequently added without an expiry will live, EXPIRY_DEFAULT to indicate that the OverflowMap's default expiry be used (which is to never automatically expire values), or EXPIRY_NEVER to specify that cache entries subsequently added without an expiry will never expire

      • getSize

        protected int getSize()
        Obtain the number of entries in the OverflowMap.
        Returns:
        the cached size of the OverflowMap

      • setSize

        protected void setSize​(int cItems)
        Update the number of entries in the OverflowMap.
        Parameters:
        cItems - the cached size of the OverflowMap

      • adjustSize

        protected void adjustSize​(int cItems)
        Adjust the number of entries in the OverflowMap.
        Parameters:
        cItems - the number of items to adjust the cached size of the OverflowMap by, for example +1 or -1

      • getCacheStatistics

        public CacheStatistics getCacheStatistics()
        Returns the CacheStatistics for this cache.
        Returns:
        a CacheStatistics object

      • getGate

        protected Gate getGate()
        Obtain the Gate for managing key-level and Collection-level operations against the Map, versus Map-level operations themselves.
        Returns:
        the Gate for the OverflowMap

      • getStatusMap

        protected Map getStatusMap()
        Obtain the Map of Status objects for entries managed by this Overflow Map. There will be a Status object for each entry in the front map, and a Status object for each key that has a current operation underway.
        Returns:
        the Map of Status objects

      • getExpiryArray

        protected LongArray getExpiryArray()
        Obtain the array of keys that have an expiration, indexed by expiration times.
        Returns:
        a sparse LongArray keyed by expiry time with a corresponding value of a set of keys that expire at that time

      • setExpiryArray

        protected void setExpiryArray​(LongArray laExpiry)
        Specify the array of keys that have an expiration, indexed by expiration times.
        Parameters:
        laExpiry - a LongArray to use to keep track of what expires when

      • getFrontMapListener

        protected MapListener getFrontMapListener()
        Get the MapListener for the front map.
        Returns:
        the MapListener for the front map

      • setFrontMapListener

        protected void setFrontMapListener​(MapListener listener)
        Specify the MapListener for the front map.

        The caller is required to manage all of the thread concurrency issues associated with modifying the listener.

        Parameters:
        listener - the MapListener for the front map

      • getBackMapListener

        protected MapListener getBackMapListener()
        Get the MapListener for the back map.
        Returns:
        the MapListener for the back map

      • setBackMapListener

        protected void setBackMapListener​(MapListener listener)
        Specify the MapListener for the back map.

        The caller is required to manage all of the thread concurrency issues associated with modifying the listener.

        Parameters:
        listener - the MapListener for the back map

      • getDeferredList

        protected List getDeferredList()
        Obtain the List of keys that may have deferred events.
        Returns:
        the List of keys of deferred-event Status objects

      • isNullValuesAllowed

        public boolean isNullValuesAllowed()
        Determine if null values are permitted. By default, they are not.
        Returns:
        true iff null values are permitted

      • setNullValuesAllowed

        public void setNullValuesAllowed​(boolean fAllowNulls)
        Specify whether null values are permitted.
        Parameters:
        fAllowNulls - pass true to allow null values; false to disallow

      • isExpiryEnabled

        public boolean isExpiryEnabled()
        Determine if this OverflowMap supports entry expiry.
        Returns:
        true iff the entry expiry feature is supported

      • setExpiryEnabled

        public void setExpiryEnabled​(boolean fEnableExpiry)
        Specify whether or not entry expiry is supported. Since there is a per-entry (memory) cost just to support the expiry feature, and since there is additional memory cost (and a small performance cost) to keep track of entries that are set to expire, this feature is made optional.

        Note that this feature must be enabled before populating the cache.

        Parameters:
        fEnableExpiry - pass true to enable entry expiry, false to disable it

      • isFrontPutBlind

        public boolean isFrontPutBlind()
        Determine if the front Map is more efficiently updated using putAll. (The use of putAll instead of put is called put-blind because it does not return a value.)
        Returns:
        true iff the use of the put method should be avoided when updating the front Map, and putAll should be used instead

      • setFrontPutBlind

        public void setFrontPutBlind​(boolean fUseFrontPutAll)
        Specify whether the front Map is more efficiently updated using putAll.
        Parameters:
        fUseFrontPutAll - pass true to allow null values; false to disallow

      • beginKeyProcess

        protected OverflowMap.Status beginKeyProcess​(Object oKey)
        Block until a key is available for processing, and return the Status object for that key. The caller is then expected to perform its processing, during which time events against the Map Entry may occur. Note that those events can occur on threads other than the current thread; when this current thread can no longer handle events from other threads, it must call OverflowMap.Status.closeProcessing() and then perform its final adjustments to the Status data structure, handling any events that were queued in the meantime on the Status. After completing the processing during this "quiet period" in which all other threads are prevented from accessing this entry or handling events for this entry, then the caller must call endKeyProcess(java.lang.Object, com.tangosol.net.cache.OverflowMap.Status) passing the Status object returned from this method.
        Parameters:
        oKey - the key to process
        Returns:
        the Status object for the specified key

      • beginMapProcess

        protected void beginMapProcess()
        Block until this thread has exclusive access to perform operations against the OverflowMap.

      • endMapProcess

        protected void endMapProcess()
        Release exclusive access for the OverflowMap.

      • prepareStatus

        protected boolean prepareStatus​(Object oKey,
                                OverflowMap.Status status)
        Validate the status and process any deferred events or pending expiry.
        Parameters:
        oKey - the entry key
        status - the Status object for the entry
        Returns:
        true iff the Status is in the processing state and oready to be used

      • onFrontEvent

        protected void onFrontEvent​(MapEvent evt)
        Either handle an event by turning it over to another thread that is processing the key specified by the event, or take responsibility on this thread for deferring the event and registering its immediate side-effects.
        Parameters:
        evt - the event

      • onBackEvent

        protected void onBackEvent​(MapEvent evtBack)
        Handle an event occuring against the "back" map.
        Parameters:
        evtBack - an event from the back map

      • mergeEvents

        protected static MapEvent mergeEvents​(MapEvent evtOld,
                                      MapEvent evtNew)
        Merge two events that have been raised in sequence from a given map.
        Parameters:
        evtOld - the first event
        evtNew - the second event
        Returns:
        the merged event

      • getLatestOldValue

        protected static Object getLatestOldValue​(MapEvent evt)
        Obtain the latest old value from the passed event. The latest old value is the value that must be retained upon the eviction of data from the front map.
        Parameters:
        evt - an event (which may be a HistoricCacheEvent)
        Returns:
        the latest old value from the event

      • processEvent

        protected void processEvent​(OverflowMap.Status status,
                            MapEvent evt)
        Process an arbitrary event that has occurred against either the front or back map.
        Parameters:
        status - the Status object representing the Overflow Map's entry against which the event was raised
        evt - the MapEvent to process

      • processFrontEvent

        protected void processFrontEvent​(OverflowMap.Status status,
                                 MapEvent evtFront)
        Process an event. The Status must already be owned by this thread and in the processing or committing state.
        Parameters:
        status - the status
        evtFront - the event to process; may be null

      • processBackEvent

        protected void processBackEvent​(OverflowMap.Status status,
                                MapEvent evtBack)
        Process an event. The Status must already be owned by this thread and in the processing or committing state.
        Parameters:
        status - the status
        evtBack - the event to process; may be null

      • processDeferredEvents

        protected void processDeferredEvents​(boolean fProcessAll)
        Process deferred events, if there are any.
        Parameters:
        fProcessAll - pass true to process all pending events, false to process just a fair portion of them

      • closeStatus

        protected void closeStatus​(OverflowMap.Status status)
        Helper method to close a Status, processing any pending events as part of the process.
        Parameters:
        status - the Status object to close

      • releaseClosedStatus

        protected void releaseClosedStatus​(Object oKey)
        Helper method to encapsulate the release of a Status object on which closeProcessing() has already been called.
        Parameters:
        oKey - the entry key

      • getMapListenerSupport

        protected MapListenerSupport getMapListenerSupport()
        Accessor for the MapListenerSupport for sub-classes.
        Returns:
        the MapListenerSupport, or null if there are no listeners

      • hasListeners

        protected boolean hasListeners()
        Determine if the OverflowMap has any listeners at all.
        Returns:
        true iff this OverflowMap has at least one MapListener

      • dispatchEvent

        protected void dispatchEvent​(OverflowMap.Status status,
                             int nId,
                             Object oKey,
                             Object oValueOld,
                             Object oValueNew,
                             boolean fSynthetic)
        Dispatch an event containing the passed event information.
        Parameters:
        status - the Status for the entry
        nId - this event's id
        oKey - the key into the map
        oValueOld - the old value
        oValueNew - the new value
        fSynthetic - true iff the event is caused by the cache internal processing such as eviction or loading

      • dispatchEvent

        protected void dispatchEvent​(OverflowMap.Status status,
                             MapEvent evt)
        Dispatch the passed event.
        Parameters:
        status - the Status for the entry
        evt - a MapEvent object

      • isSynthetic

        protected static boolean isSynthetic​(MapEvent evt)
        Helper method to determine if an event is synthetic.
        Parameters:
        evt - a Map Event
        Returns:
        true if the event is a synthetic cache event

      • registerExpiry

        protected void registerExpiry​(Object oKey,
                              long ldtExpire)
        Register an expiry for the specified key.
        Parameters:
        oKey - the key of the entry to expire
        ldtExpire - the time to expire the entry

      • unregisterExpiry

        protected void unregisterExpiry​(Object oKey,
                                long ldtExpire)
        Unregister the specified key expiry that is registered to expire at the specified time.
        Parameters:
        oKey - the key that is set to expire
        ldtExpire - the time that the key is set to expire

      • verifyNoNulls

        protected static void verifyNoNulls​(Collection collection,
                                    String sAssert)
        Check the passed collection for nulls, and fail if it contains any.
        Parameters:
        collection - a Collection
        sAssert - the human-readable description of the error if any nulls are found in the passed Collection

      • putOne

        protected static void putOne​(Map map,
                             Object oKey,
                             Object oValue,
                             boolean fPutBlind)
        Helper to put a value into a map using either the put or putAll method.
        Parameters:
        map - the Map to put into
        oKey - the key to put
        oValue - the value to put
        fPutBlind - true to use the putBlind optimization

      • evict

        public void evict()
        Flush items that have expired.
        Since:
        Coherence 3.2

      • evict

        protected static void evict​(Map map)
        If the passed Map supports it, evict its expired data.
        Parameters:
        map - a Map that may or may not support the requesting of an eviction of expired data
        Since:
        Coherence 3.2

      • warnMissingEvent

        protected void warnMissingEvent​(Object oKey,
                                int nId,
                                boolean fFront)
        An expected event did not get raised. Try to report it to the log.
        Parameters:
        oKey - the key for which an event is missing
        nId - the event type that is missing
        fFront - true if the event was expected to come from the front map, false if from the back map

      • warnUnfathomable

        protected void warnUnfathomable​(MapEvent evt)
        Something totally inexplicable has occurred. Try to report it to the log.
        Parameters:
        evt - the event that cannot be explained

      • warnEventSequence

        protected static void warnEventSequence​(MapEvent evtOld,
                                        MapEvent evtNew)
        Issue a one-time warning that events are being received in an order than cannot be explained by normal operation according to the contracts expected by this OverflowMap.
        Parameters:
        evtOld - the previous (potentially amalgamated) event
        evtNew - the new event

      • instantiateEntrySet

        protected Set instantiateEntrySet()
        Factory pattern: Create a Set that represents the entries in the Map.
        Overrides:
        instantiateEntrySet in class AbstractKeySetBasedMap
        Returns:
        a new instance of Set that represents the entries in the Map

      • instantiateInternalKeySet

        protected Set instantiateInternalKeySet()
        Factory pattern: Create a read-only Set of keys in the Overflow Map
        Returns:
        a new instance of Set that represents the keys in the Map

      • instantiateStatus

        protected OverflowMap.Status instantiateStatus()
        Factory method: Instantiate a Status object.
        Returns:
        a new Status object

      • instantiateFrontMapListener

        protected MapListener instantiateFrontMapListener()
        Factory pattern: Front Map Listener.
        Returns:
        a new instance of the listener for the front map

      • instantiateBackMapListener

        protected MapListener instantiateBackMapListener()
        Factory pattern: Back Map Listener.
        Returns:
        a new instance of the listener for the back map