Many times nosotros desire to reset an ArrayList for the reusing purpose, past times resetting nosotros hateful clearing it or removing all elements. There are ii ways to reset an ArrayList inwards Java, past times using clear() method or calling removeAll(). If your ArrayList is small-scale plenty e.g. contains entirely 10 or 100 elements thus yous tin utilization whatever of these ii methods without worrying likewise much, but, if yous possess got a huge listing of lots of objects e.g. an ArrayList containing 10M entries, thus selection of clear() vs removeAll() tin brand a huge departure inwards surgical physical care for of your Java application. Sometimes it's fifty-fifty ameliorate to exercise a novel ArrayList instead of resetting the former one, specially if resetting takes a long time, but this also has a caveat, yous postulate to brand certain that former ArrayList is eligible for garbage collection, otherwise at that topographic point is a huge peril of java.lang.OutOfMemoryError: Java Heap Space.
Coming dorsum to clear() vs removeAll() method, yous should ever utilization clear(), because it gives yous O(n) performance, piece removeAll(Collection c) is worse, it gives O(n^2) performance, that's why yous run into huge departure inwards fourth dimension taken past times clearing a large ArrayList past times these ii methods.
Things volition hold out obvious, when yous volition run our event plan together with run into the code of clear() together with removeAll() method from JDK API. By the way, if yous are inwards doubt, utilization clear() method together with if non thus ever prefer clear over removeAll inwards Java.
You tin run into that this method loop over ArrayList together with assign zilch to every chemical cistron to brand them eligible for garbage collection, of course of pedagogy if at that topographic point is no external reference. Similarly, yous tin cheque the root code of java.util.AbstractCollection course of pedagogy to await at how removeAll(Collection c) method works, hither is snippet:
This implementation iterate over the collection, checking each chemical cistron returned past times the iterator, inwards turn, to run into if it's contained inwards the specified collection. If it's introduce the it is removed from this collection past times using Iterator's take method. Because of using contains() method, removeAll() surgical physical care for goes into the hit of O(n^2), which is an absolutely NO, specially if yous are trying to reset a large ArrayList. Now let's run into their surgical physical care for inwards activeness to reset an ArrayList of simply 100K entries.
If yous are interested to a greater extent than inwards Java Performance measuring together with tuning thus I also advise yous possess got a await at Java Performance The Definitive Guide By Scott Oaks, i of the best mass on Java profiling.
Actually, the role of clear() together with removeAll(Collection c) are dissimilar inwards API, clear() method is meant to reset a Collection past times removing all elements, piece removeAll(Collection c) entirely removes elements which are introduce inwards supplied collection. This method is non designed to remove all elements from a Collection.
So, if your intention is to delete all elements from a Collection, thus use clear(), piece if yous desire to take entirely approximately elements, which are introduce inwards approximately other Collection, e.g. listing of unopen orders, thus use removeAll() method .
Make certain yous render sufficient retentiveness to run this plan because it's uses ii ArrayList to shop Integers, specially if yous desire to compare the surgical physical care for of clear() together with removeAll() for List alongside 1M elements. You also postulate Java vii to run this plan because I am using underscore alongside the numeric literal feature. If yous don't possess got JDK vii thus simply take underscores from SIZE constants, those are simply for improving readability.
That's all virtually how to reset an ArrayList inwards Java. We possess got non entirely learned ii ways to take all elements from ArrayList but also learned virtually the departure betwixt clear vs removeAll method. We possess got seen that removeAll performs poorly when the listing is large together with that's why yous should ever prefer clear() over removeAll() inwards Java.
By the way, if clearing ArrayList is taking pregnant time, consider using novel ArrayList, equally Java is pretty fast inwards creating objects.
Further Learning
Java In-Depth: Become a Complete Java Engineer
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Coming dorsum to clear() vs removeAll() method, yous should ever utilization clear(), because it gives yous O(n) performance, piece removeAll(Collection c) is worse, it gives O(n^2) performance, that's why yous run into huge departure inwards fourth dimension taken past times clearing a large ArrayList past times these ii methods.
Things volition hold out obvious, when yous volition run our event plan together with run into the code of clear() together with removeAll() method from JDK API. By the way, if yous are inwards doubt, utilization clear() method together with if non thus ever prefer clear over removeAll inwards Java.
Clear() vs RemoveAll(Collection c)
In fellowship to compare the surgical physical care for of both these methods, it's real of import to run into their code. You tin cheque the root code of the clear() method inwards java.util.ArrayList class, for convenience I possess got included it here. This code is from JDK version 1.7.0_40. If yous desire to larn to a greater extent than virtually surgical physical care for measuring together with tuning thus I strongly advise reading Java Performance The Definitive Guide By Scott Oaks. It covers Java vii together with approximately bits of Java 8 equally well./** * Removes all of the elements from this list. The listing volition * hold out empty subsequently this telephone outcry upward returns. */ public void clear() { modCount++; // clear to permit GC exercise its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }
You tin run into that this method loop over ArrayList together with assign zilch to every chemical cistron to brand them eligible for garbage collection, of course of pedagogy if at that topographic point is no external reference. Similarly, yous tin cheque the root code of java.util.AbstractCollection course of pedagogy to await at how removeAll(Collection c) method works, hither is snippet:
public boolean removeAll(Collection c) { boolean modified = false; Iterator it = iterator(); while (it.hasNext()) { if (c.contains(it.next())) { it.remove(); modified = true; } } return modified; }
This implementation iterate over the collection, checking each chemical cistron returned past times the iterator, inwards turn, to run into if it's contained inwards the specified collection. If it's introduce the it is removed from this collection past times using Iterator's take method. Because of using contains() method, removeAll() surgical physical care for goes into the hit of O(n^2), which is an absolutely NO, specially if yous are trying to reset a large ArrayList. Now let's run into their surgical physical care for inwards activeness to reset an ArrayList of simply 100K entries.
If yous are interested to a greater extent than inwards Java Performance measuring together with tuning thus I also advise yous possess got a await at Java Performance The Definitive Guide By Scott Oaks, i of the best mass on Java profiling.
Removing all elements from ArrayList alongside 100K Objects
I possess got initially tried to run this event alongside 10M elements but subsequently waiting for to a greater extent than than one-half an hr to permit removeAll() finish, I decided to trim back the reveal of objects to 100K, fifty-fifty thus the departure betwixt the fourth dimension taken by clear() vs removeAll() is quite significant. The removeAll(Collection c) are taking 10000 times to a greater extent than fourth dimension than clear to reset.Actually, the role of clear() together with removeAll(Collection c) are dissimilar inwards API, clear() method is meant to reset a Collection past times removing all elements, piece removeAll(Collection c) entirely removes elements which are introduce inwards supplied collection. This method is non designed to remove all elements from a Collection.
So, if your intention is to delete all elements from a Collection, thus use clear(), piece if yous desire to take entirely approximately elements, which are introduce inwards approximately other Collection, e.g. listing of unopen orders, thus use removeAll() method .
import java.util.ArrayList; /** * Java Program to take all elements from listing inwards Java together with comparing * surgical physical care for of clearn() together with removeAll() method. * * @author Javin Paul */ public class ArrayListResetTest { private static final int SIZE = 100_000; public static void main(String args[]) { // Two ArrayList for clear together with removeAll ArrayList numbers = new ArrayList(SIZE); ArrayList integers = new ArrayList(SIZE); // Initialize ArrayList alongside 10M integers for (int i = 0; i < SIZE; i++) { numbers.add(new Integer(i)); integers.add(new Integer(i)); } // Empty ArrayList using clear method long startTime = System.nanoTime(); numbers.clear(); long elapsed = System.nanoTime() - startTime; System.out.println("Time taken past times clear to empty ArrayList of 1M elements (ns): " + elapsed); // Reset ArrayList using removeAll method startTime = System.nanoTime(); integers.removeAll(integers); long fourth dimension = System.nanoTime() - startTime; System.out.println("Time taken past times removeAll to reset ArrayList of 1M elements (ns): " + time); } } Output: Time taken past times clear to empty ArrayList of 100000 elements (ns): 889619 Time taken past times removeAll to reset ArrayList of 100000 elements (ns): 36633112126
Make certain yous render sufficient retentiveness to run this plan because it's uses ii ArrayList to shop Integers, specially if yous desire to compare the surgical physical care for of clear() together with removeAll() for List alongside 1M elements. You also postulate Java vii to run this plan because I am using underscore alongside the numeric literal feature. If yous don't possess got JDK vii thus simply take underscores from SIZE constants, those are simply for improving readability.
That's all virtually how to reset an ArrayList inwards Java. We possess got non entirely learned ii ways to take all elements from ArrayList but also learned virtually the departure betwixt clear vs removeAll method. We possess got seen that removeAll performs poorly when the listing is large together with that's why yous should ever prefer clear() over removeAll() inwards Java.
By the way, if clearing ArrayList is taking pregnant time, consider using novel ArrayList, equally Java is pretty fast inwards creating objects.
Further Learning
Java In-Depth: Become a Complete Java Engineer
read more)