Solution to Issues in ArrayList with the Thread

    

     ArrayList   are  not  Synchronized ,  so  we  have  to consider this  issue when  we  deal with  threads.  Some  things  we can do  are  use Collections.synchronizedList() for thread safety.

• List<String> synchronizedList = Collections.synchronizedList(new ArrayList<>());

          This means that only one thread can modify the list at a time, preventing race conditions  and ensuring data consistency in a multi threaded environment.

• Collections.synchronizedList()

   is a static factory method in Java that returns a thread-safe (synchronized) wrapper for a specified List. It is part of the java.util.Collections class and ensures that all access to the underlying list is properly synchronized, which is crucial in a multi-threaded environment where multiple threads might access and modify the list simultaneously. 

  How it works

  The method works by wrapping the given non-thread-safe list (such as an ArrayList or LinkedList) inside a new object. All public methods of this wrapper object (like add(), remove(), get(), etc.) are synchronized on an internal lock, ensuring that only one thread can access the list's methods at any given time. 

  Syntax

  java

  List<String> threadSafeList = Collections.synchronizedList(new ArrayList<>());

  Important considerations

  Manual synchronization for iteration

  Although individual methods are synchronized, compound operations like iteration are not automatically protected. If one thread is iterating over the list and another thread modifies it, a ConcurrentModificationException can occur. To prevent this, you must manually synchronize the list when iterating over it. 

  Example of proper iteration:

  java

  List<String> syncList = Collections.synchronizedList(new ArrayList<>());
  
  // ... add elements in a thread-safe way ...
  
  synchronized (syncList) {
      for (String item : syncList) {
          System.out.println(item);
      }
  }
  

  
  

  Performance overhead

  Using synchronizedList() comes with a performance cost. Since every method call acquires a lock on the entire list, it can become a bottleneck in scenarios with high contention (many threads frequently accessing the list). For applications that need better scalability, more specialized collections from the java.util.concurrent package may be a better choice. 

           import java.util.concurrent.CopyOnWriteArrayList; 

  CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
  

   

   

  synchronizedList vs. CopyOnWriteArrayList

  CopyOnWriteArrayList is a concurrent collection often compared to Collections.synchronizedList(). The main differences lie in how they handle reads and writes. 

  Characteristic	Collections.synchronizedList()	CopyOnWriteArrayList
  Thread Safety	Achieves thread safety by locking the entire list for every read and write operation.	Achieves thread safety for writes by creating a new copy of the underlying array, so reads can proceed without a lock.
  Iterator Behavior	Its iterator is "fail-fast" and will throw a ConcurrentModificationException if the list is modified during iteration without external synchronization.	Its iterator is "fail-safe" and operates on a snapshot of the list. It will not throw a ConcurrentModificationException and will not reflect changes made after the iterator was created.
  Performance	Better performance when there are more write operations than reads, as writing does not involve creating a new copy.	Better performance in scenarios with many more read operations than writes, as read operations are non-blocking.
  Memory Usage	Uses less memory, as new copies are not made on every write.	Uses more memory, as every write operation creates and copies a new internal array.

  When to use Collections.synchronizedList()

  • For simple use cases where the number of threads accessing the list is low and synchronization is straightforward.
  • When you need to make an existing, non-thread-safe list implementation thread-safe with minimal code changes.
  • When the number of write operations is significantly higher than the number of read operations.
  • When you require a "fail-fast" iterator to detect concurrent modifications during iteration. 

To  address  that  issue  and  to iterate over the synchronized list.

Crucially, iteration over synchronized collections requires external synchronization.

This is because the synchronization provided by synchronizedList() only covers individual operations,

not compound operations like iterating through the entire list.

synchronized (synchronizedList) {
Iterator<String> iterator = synchronizedList.iterator();
while (iterator.hasNext()) {
String fruit = iterator.next();
System.out.println("Fruit: " + fruit);
}
}

References  used  are  AI Overvie  .