Come rimuovere elementi scaduti da HashMap e aggiungere più elementi contemporaneamente – Java Timer, TimerTask e futures() – Esempio completo

Pubblicato: 2021-04-02

Hashmap, ArrayList, Static Map, Vettori, ecc. sono gli elementi del framework di raccolta Java più utilizzati. Ci sono un numero infinito di scenari che potresti utilizzare secondo le tue necessità.

Questo esempio è molto interessante Esempio Java. Eseguiremo l'operazione seguente su una singola HashMap().

Crea un oggetto crunchifyMap
Continua ad aggiungere elementi alla mappa ogni secondo con un tempo di scadenza impostato su 5 seconds
Check la presenza di elementi scaduti come la cache ogni secondo ed delete from map if expired
Dopo 5 secondi otterrai always same size e always 5 elements mentre aggiungi ed elimini elementi scaduti ogni secondo

Inoltre, se hai below questions di seguito, sei nel posto giusto:

Che cos'è l'esempio di mappa in scadenza passiva
Mappa simultanea con elemento scaduto
Esempio di mappa cache Java
Esempio Java TimerTask
Evictor: un esempio di mappa simultanea Java

Iniziamo:

Punto-1

Crea elemento timer crunchifyTimer
Pianifica l'attività specificata CrunchifyReminder() per l'esecuzione ripetuta con ritardo fisso pari a 1 secondo
In attività pianificata
- add elemento in crunchifyMap
- controlla l'elemento scaduto da crunchifyMap ed elimina

Punto-2

Durante l'operazione addElement()
- Stiamo associando current time per ogni elemento
Durante l'operazione crunchifyClearExipredElementsFromMap()
- Stiamo controllando l'ora corrente con l'ora dell'elemento
- Se la differenza di tempo è superiore a 5 secondi, elimina semplicemente l'elemento da crunchifyMap

Punto-3

Durante l'operazione di aggiunta e rimozione, stampa l'elemento sulla console Eclipse
Il 1° elemento aggiunto verrà rimosso il 1° e così via
Si prega di controllare l'output della console Eclipse per il risultato

Ecco un programma Java:

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package crunchify . com . tutorials ;

import java . util . ArrayList ;

import java . util . Date ;

import java . util . HashMap ;

import java . util . Iterator ;

import java . util . Map ;

import java . util . Map . Entry ;

import java . util . Timer ;

import java . util . TimerTask ;

/**

* @author Crunchify.com

* <p>

* - Keep adding element to Map every second which has expire time set to 5 seconds

* - Check for expired element every second and delete from map if expired

* - After 5 seconds you will get always same size as you are adding and deleting expired elements every second

public class CrunchifyCleanExipredMapElements {

Timer crunchifyTimer ;

private static final long EXPIRED_TIME_IN_SEC = 5L ;

private static final Map < Double , ArrayList < Date > > crunchifyMap = new HashMap < > ( ) ;

public static void main ( String [ ] args ) {

new CrunchifyCleanExipredMapElements ( 1 ) ;

crunchifyLog ( "Start Adding element every second\n\n" ) ;

}

public CrunchifyCleanExipredMapElements ( int seconds ) {

// Timer() - Creates a new timer. The associated thread does not run as a daemon.

crunchifyTimer = new Timer ( ) ;

// Schedules the specified task for repeated fixed-delay execution

// schedule(): Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay.

// Subsequent executions take place at approximately regular intervals separated by the specified period.

// In fixed-delay execution, each execution is scheduled relative to the actual execution time of the previous execution.

// If an execution is delayed for any reason (such as garbage collection or other background activity), subsequent executions will be delayed as well.

// In the long run, the frequency of execution will generally be slightly lower than the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).

crunchifyTimer . schedule ( new CrunchifyReminder ( ) , 0 , seconds * 1000L ) ;

}

// TimeTask: A task that can be scheduled for one-time or repeated execution by a Timer.

// A timer task is not reusable. Once a task has been scheduled for execution on a Timer or cancelled,

// subsequent attempts to schedule it for execution will throw IllegalStateException.

class CrunchifyReminder extends TimerTask {

public void run ( ) {

// We are checking for expired element from map every second

crunchifyClearExpiredElementsFromMap ( ) ;

// We are adding element every second

addElement ( ) ;

}

public void addElement ( ) {

crunchifyAddElementToMap ( Math . random ( ) ) ;

}

// Check for element's expired time. If element is > 5 seconds old then remove it

private static void crunchifyClearExpiredElementsFromMap ( ) {

// Date() - Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.

Date currentTime = new Date ( ) ;

Date actualExpiredTime = new Date ( ) ;

// if element time stamp and current time stamp difference is 5 second then delete element

actualExpiredTime . setTime ( currentTime . getTime ( ) - EXPIRED_TIME_IN_SEC * 1000L ) ;

// size() - Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

System . out . println ( "crunchifyMap size:" + CrunchifyCleanExipredMapElements . crunchifyMap . size ( ) ) ;

Iterator < Entry < Double , ArrayList < Date > > > crunchifyIterator = CrunchifyCleanExipredMapElements . crunchifyMap . entrySet ( ) . iterator ( ) ;

// hasNext() - Returns true if the iteration has more elements. (In other words, returns true if next would return an element rather than throwing an exception.)

while ( crunchifyIterator . hasNext ( ) ) {

// next() - Returns the next element in the iteration.

Entry < Double , ArrayList < Date > > entry = crunchifyIterator . next ( ) ;

// getValue() - Returns the value corresponding to this entry.

// If the mapping has been removed from the backing map (by the iterator's remove operation), the results of this call are undefined.

ArrayList < Date > crunchifyElement = entry . getValue ( ) ;

while ( crunchifyElement . size ( ) > 0

&& crunchifyElement.get(0).compareTo(actualExpiredTime) < 0) {

crunchifyLog("----------- Expired Element Deleted: " + entry.getKey());

crunchifyElement . remove ( 0 ) ;

}

if ( crunchifyElement . size ( ) == 0 ) {

// remove() - Removes from the underlying collection the last element returned by this iterator (optional operation).

// This method can be called only once per call to next.

crunchifyIterator . remove ( ) ;

}

// Adding new element to map with current timestamp

private static void crunchifyAddElementToMap ( Double digit ) {

ArrayList < Date > crunchifyList = new ArrayList < > ( ) ;

CrunchifyCleanExipredMapElements . crunchifyMap . put ( digit , crunchifyList ) ;

// add() - Appends the specified element to the end of this list.

crunchifyList . add ( new Date ( ) ) ;

crunchifyLog ( "+++++++++++ Add Element:" + digit + "\n" ) ;

}

private static void crunchifyLog ( String string ) {

System . out . println ( string ) ;

}

Uscita console Eclipse:

Basta eseguire sopra il programma e dovresti vedere il risultato come di seguito.

Come rimuovere elementi scaduti da HashMap e aggiungere più elementi contemporaneamente - Java Timer, TimerTask e futures() - Risultato console

Start Adding element every second

crunchifyMap size : 0

+++++++++++ Add Element : 0.6970683199424504

crunchifyMap size : 1

+++++++++++ Add Element : 0.8103548990859972

crunchifyMap size : 2

+++++++++++ Add Element : 0.849973176897403

crunchifyMap size : 3

+++++++++++ Add Element : 0.2770459184269851

crunchifyMap size : 4

+++++++++++ Add Element : 0.9136028332849369

crunchifyMap size : 5

----------- Expired Element Deleted : 0.6970683199424504

+++++++++++ Add Element : 0.6630362698520832

crunchifyMap size : 5

----------- Expired Element Deleted : 0.8103548990859972

+++++++++++ Add Element : 0.38906826918659854

crunchifyMap size : 5

----------- Expired Element Deleted : 0.849973176897403

+++++++++++ Add Element : 0.3997935729434138

crunchifyMap size : 5

----------- Expired Element Deleted : 0.2770459184269851

+++++++++++ Add Element : 0.20373441843273887

Process finished with exit code 130 ( interrupted by signal 2 : SIGINT )

Fammi sapere se riscontri problemi con l'esecuzione sopra il programma Java.