How to Add Duplicate Objects to an ArrayList in a Multithreaded Environment?

How can I add duplicate objects (objects that are considered equal based on equals and hashCode) to an ArrayList in a multithreaded environment in Java?

ArrayList<MyObject> list = new ArrayList<>();

MyObject obj1 = new MyObject("example");
MyObject obj2 = new MyObject("example"); // Overridden equals and hashCode

list.add(obj1); // Add first object
list.add(obj2); // Add duplicate object based on equals and hashCode

In Java, an ArrayList allows the storage of duplicate elements. However, correctly managing duplicates, especially in a multithreaded environment, requires understanding the implications of the equals and hashCode methods. Here's how to do it safely.

// Thread-safe way to add elements to ArrayList using synchronized
synchronized(list) {
    list.add(obj2);
} // This ensures that no other thread can modify list at the same time.

Causes

• The ArrayList uses equals and hashCode methods to determine object uniqueness, but it can store multiple references to the same object in memory.
• In a multithreaded application, concurrent modifications to the ArrayList may lead to data inconsistency if not handled properly.

Solutions

• Use synchronized blocks or methods to ensure thread safety while adding duplicate objects to the ArrayList.
• Consider using collections from the java.util.concurrent package, such as CopyOnWriteArrayList, which allows safe iteration and modification.

Mistake: Assuming ArrayList is inherently thread-safe when adding elements.

Solution: Use thread-safe collections like CopyOnWriteArrayList or synchronize the ArrayList.

Mistake: Not overriding equals and hashCode in custom objects.

Solution: Always override equals and hashCode when working with custom objects in collections to avoid unexpected behavior.