How To Take Away All Elements From Arraylist Inwards 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 2 ways to reset an ArrayList inwards Java, yesteryear using clear() method or calling removeAll(). If your ArrayList is pocket-size plenty e.g. contains alone 10 or 100 elements together with hence y'all tin job whatever of these 2 methods without worrying also much, but, if y'all lead maintain a huge listing of lots of objects e.g. an ArrayList containing 10M entries, together with hence pick of clear() vs removeAll() tin brand a huge divergence inwards functioning of your Java application. Sometimes it's fifty-fifty meliorate to exercise a novel ArrayList instead of resetting the quondam one, specially if resetting takes a long time, but this also has a caveat, y'all necessitate to brand certain that quondam ArrayList is eligible for garbage collection, otherwise at that spot is a huge opportunity of java.lang.OutOfMemoryError: Java Heap Space.

Coming dorsum to clear() vs removeAll() method, y'all should ever job clear(), because it gives y'all O(n) performance, spell removeAll(Collection c) is worse, it gives O(n^2) performance, that's why y'all encounter huge divergence inwards fourth dimension taken yesteryear clearing a large ArrayList yesteryear these 2 methods.

Things volition hold upward obvious, when y'all volition run our event programme together with encounter the code of clear() together with removeAll() method from JDK API. By the way, if y'all are inwards doubt, job clear() method together with if non together with hence ever prefer clear over removeAll inwards Java.

Clear() vs RemoveAll(Collection c)

In guild to compare the functioning of both these methods, it's rattling of import to encounter their code. You tin cheque the origin code of the clear() method inwards java.util.ArrayList class, for convenience I lead maintain included it here. This code is from JDK version 1.7.0_40. If y'all desire to larn to a greater extent than virtually functioning touchstone together with tuning together with hence I strongly propose reading Java Performance The Definitive Guide By Scott Oaks. It covers Java vii together with roughly bits of Java 8 every bit well.

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

You tin encounter that this method loop over ArrayList together with assign nil to every chemical ingredient to brand them eligible for garbage collection, of course of educational activity if at that spot is no external reference. Similarly, y'all tin cheque the origin code of java.util.AbstractCollection cast to expect 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 ingredient returned yesteryear the iterator, inwards turn, to encounter if it's contained inwards the specified collection.  If it's acquaint the it is removed from this collection yesteryear using Iterator's withdraw method. Because of using contains() method, removeAll() functioning goes into the hit of O(n^2), which is an absolutely NO, specially if y'all are trying to reset a large ArrayList. Now let's encounter their functioning inwards activity to reset an ArrayList of simply 100K entries.

If y'all are interested to a greater extent than inwards Java Performance touchstone together with tuning together with hence I also propose y'all accept a expect at Java Performance The Definitive Guide By Scott Oaks, ane of the best majority on Java profiling.

Removing all elements from ArrayList alongside 100K Objects

I lead maintain initially tried to run this event alongside 10M elements but later waiting for to a greater extent than than one-half an lx minutes to allow removeAll() finish, I decided to cut down the release of objects to 100K, fifty-fifty together with hence the divergence 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 unlike inwards API, clear() method is meant to reset a Collection yesteryear removing all elements, spell removeAll(Collection c) alone 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, together with hence use clear(), spell if y'all desire to withdraw alone roughly elements, which are acquaint inwards roughly other Collection, e.g. listing of shut orders, together with hence use removeAll() method .

import java.util.ArrayList;  /**  * Java Program to withdraw all elements from listing inwards Java together with comparing  * functioning 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 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 y'all furnish sufficient retention to run this programme because it's uses 2 ArrayList to shop Integers, specially if y'all desire to compare the functioning of clear() together with removeAll() for List alongside 1M elements. You also necessitate Java vii to run this programme because I am using underscore alongside the numeric literal feature. If y'all don't lead maintain JDK vii together with hence simply withdraw underscores from SIZE constants, those are simply for improving readability.

That's all virtually how to reset an ArrayList inwards Java. We lead maintain non alone learned 2 ways to withdraw all elements from ArrayList but also learned virtually the divergence betwixt clear vs removeAll method. We lead maintain seen that removeAll performs poorly when the listing is large together with that's why y'all should ever prefer clear() over removeAll() inwards Java.

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

Further Learning
Java In-Depth: Become a Complete Java Engineer
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How to clone an ArrayList inwards Java? (solution)

How exercise y'all convert a Map to List inwards Java? (solution)

Java - Performance comparing of contains() vs binarySearch() (read more)

Java - How to initialize an ArrayList alongside values inwards Java? (example)

Java - The ArrayList Guide (tutorial)

The divergence betwixt an ArrayList together with a Vector inwards Java? (answer)

How to brand an ArrayList unmodifiable inwards Java? (solution)

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