Introduction:
CopyOnWriteArrayList in Java is a thread-safe implementation of a List interface. It belongs to the java.util.concurrent package and is an enhanced version of ArrayList implementation.
As the name suggests, CopyOnWriteArrayList creates a cloned internal copy of the underlying ArrayList for each add() or set() operations. Due to this additional overhead cost, we should ideally use a CopyOnWriteArrayList only when we have very frequent read operations and not many insertions or updations.
Constructing a CopyOnWriteArrayList:
We can use one of the following constructors to create a CopyOnWriteArrayList:
- CopyOnWriteArrayList(): creates an empty list
- CopyOnWriteArrayList(Collection c): creates a list initialized with all the elements in c
- CopyOnWriteArrayList(Object[] obj): creates a list holding a copy of given array obj
Let’s see a few examples:
Java
CopyOnWriteArrayList<String> emptyList
= new CopyOnWriteArrayList<>(); //[]
list.add("A");
CopyOnWriteArrayList<String> listUsingCollection =
new CopyOnWriteArrayList<>(list); //["A"]
CopyOnWriteArrayList<String> listUsingArr =
new CopyOnWriteArrayList<>(new String[] {"A", "B"}); //["A", "B"]
Just like an ArrayList, it provides implementations for all the methods of the List interface. We can read more about ArrayList here.
Insertion And Iteration Over a CopyOnWriteArrayList:
As we know, Iterator of ArrayList is fail-fast i.e. it will throw a ConcurrentModificationException when it detects any modification in the list once the iterator is instantiated.
The CopyOnWriteArrayList has a fail-safe iterator and doesn’t throw an exception even when we modify the list during its iteration. This is because the iterator is operating over a cloned internal copy of the original list.
However, any modifications made later to the list won’t be visible to the already instantiated iterator.
Let’s try that out-
Let’s say we have an instance of CopyOnWriteArrayList which currently holds two elements:
Java
CopyOnWriteArrayList<String> list =
new CopyOnWriteArrayList<>(new String[] {"A", "B"});
Now let’s get an Iterator instance for us to be able to iterate over its elements:
Java
Iterator itr = list.iterator();
After we have created an Iterator instance, let’s now add one more element to our list:
Java
list.add("C"); //doesn't throw an exception
Now, let’s iterator through the elements of our list using itr, we created previously:
Java
while(itr.hasNext()) {
System.out.print(itr.next() + " ");
}
The above code will print:
A B
As expected, itr doesn’t reflect the current state of the list and only prints “A” and “B” as its elements. However, if we create a new instance of an iterator, it will then represent the exact current state of the list.
Removal And Iteration Over a CopyOnWriteArrayList:
The CopyOnWriteArrayList’s Iterator doesn’t support remove() operation. Any attempt to do so will result in an UnsupportedOperationException:
Java
@Test(expected = UnsupportedOperationException.class)
public void iterationAndRemoval() {
CopyOnWriteArrayList<String> list
= new CopyOnWriteArrayList<>(new String[]{"A", "B"});
Iterator<String> itr = list.iterator();
while (itr.hasNext()) {
itr.remove();
}
}
ArrayList vs CopyOnWriteArrayList:
Let’s review the differences between the Java ArrayList and CopyOnWriteArrayList classes:
| ArrayList | CopyOnWriteArrayList |
|---|---|
| Belongs to java.util package and is not thread-safe | Is a thread-safe implementation and is present in the java.util.concurrent packages |
| Has a fail-fast Iterator which throws a ConcurrentModificationException when it detects any modification once the iteration starts | Has a fail-safe iterator as the iterator holds a cloned copy of the list |
| Iterator supports remove() operation | Its Iterator doesn’t supports remove() and throws UnsupportedOperationException for any attempt to do so |
Conclusion:
In this tutorial, we learned about a thread-safe implementation of Java List – the CopyOnWriteArrayList. We also covered the difference between the ArrayList and CopyOnWriteArrayList.
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Published on Java Code Geeks with permission by Shubhra Srivastava, partner at our JCG program. See the original article here: CopyOnWriteArrayList In Java Opinions expressed by Java Code Geeks contributors are their own. |
