How To Take All Elements From Arraylist Inward Coffee - Clear Vs Removeall

Many times nosotros desire to reset an ArrayList for the reusing purpose, yesteryear resetting nosotros hateful clearing it or removing all elements. There are ii ways to reset an ArrayList inwards Java, yesteryear using clear() method or calling removeAll(). If your ArrayList is modest plenty e.g. contains solely 10 or 100 elements thence you lot tin terminate utilization whatever of these ii methods without worrying also much, but, if you lot possess got a huge listing of lots of objects e.g. an ArrayList containing 10M entries, thence alternative of clear() vs removeAll() tin terminate brand a huge deviation inwards functioning of your Java application. Sometimes it's fifty-fifty improve to practise a novel ArrayList instead of resetting the one-time one, peculiarly if resetting takes a long time, but this also has a caveat, you lot necessitate to brand certain that one-time ArrayList is eligible for garbage collection, otherwise at that spot is a huge run a endangerment of java.lang.OutOfMemoryError: Java Heap Space.

Coming dorsum to clear() vs removeAll() method, you lot should ever utilization clear(), because it gives you lot O(n) performance, spell removeAll(Collection c) is worse, it gives O(n^2) performance, that's why you lot run into huge deviation inwards fourth dimension taken yesteryear clearing a large ArrayList yesteryear these ii methods.

Things volition hold upwards obvious, when you lot volition run our instance programme too run into the code of clear() too removeAll() method from JDK API. By the way, if you lot are inwards doubt, utilization clear() method too if non thence ever prefer clear over removeAll inwards Java.

Clear() vs RemoveAll(Collection c)

In guild to compare the functioning of both these methods, it's real of import to run into their code. You tin terminate depository fiscal establishment stand upwards for the rootage 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 you lot desire to acquire to a greater extent than nearly functioning measuring too tuning thence I strongly propose reading Java Performance The Definitive Guide By Scott Oaks. It covers Java seven too around bits of Java 8 equally well.

   /**      * Removes all of the elements from this list.  The listing volition      * hold upwards empty afterwards this telephone phone returns.      */     public void clear() {         modCount++;         // clear to allow GC practise its work         for (int i = 0; i < size; i++)             elementData[i] = null;         size = 0;     }

You tin terminate run into that this method loop over ArrayList too assign nix to every chemical cistron to brand them eligible for garbage collection, of class if at that spot is no external reference. Similarly, you lot tin terminate depository fiscal establishment stand upwards for the rootage code of java.util.AbstractCollection class 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 yesteryear the iterator, inwards turn, to run into if it's contained inwards the specified collection.  If it's acquaint the it is removed from this collection yesteryear using Iterator's take method. Because of using contains() method, removeAll() functioning goes into the hit of O(n^2), which is an absolutely NO, peculiarly if you lot are trying to reset a large ArrayList. Now let's run into their functioning inwards activeness to reset an ArrayList of but 100K entries.

If you lot are interested to a greater extent than inwards Java Performance measuring too tuning thence I also propose you lot accept a await at Java Performance The Definitive Guide By Scott Oaks, ane of the best mass on Java profiling.

Removing all elements from ArrayList amongst 100K Objects

I possess got initially tried to run this instance amongst 10M elements but afterwards waiting for to a greater extent than than one-half an hr to allow removeAll() finish, I decided to cut down the publish of objects to 100K, fifty-fifty thence the deviation 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 operate of clear() too removeAll(Collection c) are unlike inwards API, clear() method is meant to reset a Collection yesteryear removing all elements, spell removeAll(Collection c) solely removes elements which are acquaint 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, thence use clear(), spell if you lot desire to take solely around elements, which are acquaint inwards around other Collection, e.g. listing of unopen orders, thence use removeAll() method .

import java.util.ArrayList;  /**  * Java Program to take all elements from listing inwards Java too comparing  * functioning of clearn() too 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 too removeAll         ArrayList numbers = new ArrayList(SIZE);         ArrayList integers = new ArrayList(SIZE);                  // Initialize ArrayList amongst 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 yesteryear 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 yesteryear removeAll to reset ArrayList of 1M elements (ns): " + time);     } }  Output: Time taken yesteryear clear to empty ArrayList of 100000 elements (ns): 889619 Time taken yesteryear removeAll to reset ArrayList of 100000 elements (ns): 36633112126

Make certain you lot furnish sufficient retention to run this programme because it's uses ii ArrayList to shop Integers, peculiarly if you lot desire to compare the functioning of clear() too removeAll() for List amongst 1M elements. You also necessitate Java seven to run this programme because I am using underscore amongst the numeric literal feature. If you lot don't possess got JDK seven thence but take underscores from SIZE constants, those are but for improving readability.

That's all nearly how to reset an ArrayList inwards Java. We possess got non solely learned ii ways to take all elements from ArrayList but also learned nearly the deviation betwixt clear vs removeAll method. We possess got seen that removeAll performs poorly when the listing is large too that's why you lot should ever prefer clear() over removeAll() inwards Java.

By the way, if clearing ArrayList is taking meaning time, consider using novel ArrayList, equally Java is pretty fast inwards creating objects.

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