CopyOnWriteArrayList difference with normal ArrayList

CopyOnWriteArrayList was introduced in JDK5, Which will be used to avoid the java.util.ConcurrentModificationException while modifying List by multiple threads or inside loop.

below sample will show the difference.

	import java.util.ArrayList;
	import java.util.Iterator;
	import java.util.List;
	import java.util.concurrent.CopyOnWriteArrayList;
	/*
	* Difference between ArrayList and CopyOnWriteArrayList
	*/
	public class CopyOnWriteArrayListTest {
	public static void main(String[] args) {
	CopyOnWriteArrayList copyOnWriteArrayList = new CopyOnWriteArrayList();
	copyOnWriteArrayList.add("copyOnWriteArrayList_item");
	copyOnWriteArrayList.add("copyOnWriteArrayList_item1");
	Iterator iCopyIterator = copyOnWriteArrayList.iterator();
	while (iCopyIterator.hasNext()) {
	//System.out.println(iCopyIterator.next());
	iCopyIterator.next();
	copyOnWriteArrayList.add("copyOnWriteArrayList_item2");
	}
	System.out.println("|---- After modification of copyOnWriteArrayList ----| ");
	Iterator i2 = copyOnWriteArrayList.iterator();
	while (i2.hasNext()) {
	System.out.println(i2.next());
	}
	/*
	* If List modified in the loop iteration it will throw
	* ConcurrentModificationException or if multiple threads try to modify
	* the List in loop iteration will throw the exception for normal arrayList
	*/
	List list = new ArrayList();
	list.add("test");
	list.add("test1");
	Iterator listIterator = list.iterator();
	while (listIterator.hasNext()) {
	System.out.println(listIterator.next());
	list.add("test2");
	}
	}
	}

view raw java.util.ConcurrentModificationException.java hosted with ❤ by GitHub

Find Number of duplicates present in a ArrayList

To find the number of occurrences of duplicate integer present in List, first we will create temp List which will have no duplicates and then use collection frequecy() function to get the occurrence of duplicate number in original list.

	public static void main(String[] args) {
	List<Integer> list = new ArrayList<Integer>();
	list.add(67);
	list.add(32);
	list.add(6);
	list.add(4);
	list.add(67);
	list.add(3);
	list.add(6);
	list.add(2);
	list.add(6);
	list.add(2);
	list.add(67);
	list.add(6);
	list.add(4);
	Set<Integer> filterList = new HashSet<Integer>(list);
	Iterator<Integer> iterator = filterList.iterator();
	while(iterator.hasNext()){
	int number = iterator.next();
	int occurrence = Collections.frequency(list, number);
	System.out.println("| Number \t"+number+"\t| occures : \t"+occurrence+" Times |");
	System.out.println(" ------------------------------------------------");
	}
	}

view raw sample.java hosted with ❤ by GitHub