Does Java Instantiate a New Iterator Every Time the iterator() Method is Called?

Does Java create a new instance of iterator() each time it's called?

In Java, the behavior of the iterator() method varies depending on the collection type used. Understanding when a new instance of an iterator is created can help optimize the handling of collections and ensure correct iteration over elements.

List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
Iterator<String> it1 = list.iterator();  // New instance created
Iterator<String> it2 = list.iterator();  // Another new instance created
while (it1.hasNext()) {
    System.out.println(it1.next());  // Output: A, B
}  
while (it2.hasNext()) {
    System.out.println(it2.next());  // Output: A, B (separate iteration) 
}

Causes

• The iterator() method is designed to return a new iterator instance for certain collection types such as ArrayList and HashSet.
• Other collections, like LinkedList, may also create a new iterator each time the method is invoked.

Solutions

• To avoid unnecessary overhead, you can store the length of the collection in a variable if you are iterating multiple times.
• For collections that do not change size, consider reusing the same iterator instance when possible.

Mistake: Assuming that an iterator is a singleton for the collection.

Solution: Remember that calling iterator() produces a new instance; iterate through one instance at a time.

Mistake: Neglecting to check if the collection was modified during iteration.

Solution: Always ensure the collection is not changed while an iterator is being used to avoid ConcurrentModificationException.