Iterators

Iterable

• It basically promises that the collection can be iterated

public interface Iterable<T> {
    Iterator<T> iterator();
    void forEach(Consumer<? super T> action);
    Spliterator<T> spliterator();
}

• All Collections are Iterable

public interface Collection<E> extends Iterable<E> {
    // ...
}

Iterator

• methods
  • hasNext(): boolean
  • next(): E
  • remove(): void

ListIterator

• Same as Iterator but provides bidirectional iteration capabilities
• It is used in List Collections only
• methods
  • hasNext(), next()
  • hasPrevious(), previous()
  • remove(): void
  • add(E): void
    • Insert new element
  • set(E): void
    • Replaces last element with the given element

Spliterator

• Splittable Iterator
• It is same as Iterator but it can split and parallelize the iteration
• They are used internally in terminal operators in streams:
  • streams are lazily evaluated
  • terminal operators causes the start of iteration and execution

How for-each works

• https://stackoverflow.com/questions/85190/in-detail-how-does-the-for-each-loop-work-in-java/33232565#33232565
• It has two forms
  • Iterable
  • Array
• Iterable

List<String> someList = new ArrayList<>();
someList.add("Apple");
someList.add("Banana");
 
for (String item : someList) {
    // ...
}
 
// same as
for (Iterator<String> i = someList.iterator(); i.hasNext();) {
    String item = i.next();
    // ...
}

• Array

String[] someArray = new String[2]
someArray[0] = "Apple";
someArray[1] = "Banana";
 
for(String item : someArray) {
    // ...
}
 
// same as
for (int i = 0; i < someArray.length; i++) {
    String item = someArray[i];
    // ...
}

Fail-fast vs Fail-safe

• Also See Iterator
• https://www.baeldung.com/java-fail-safe-vs-fail-fast-iterator
• Fail Fast
  • These throw ConcurrentModificationException if the collection is modified during iteration
  • Collections maintain internal counter modCount to track modifications, which is compared with initial value, if mismatch happens exception is thrown
  • During iteration if iterator.remove() is called, then it is safe
  • If you remove using Collection method like list.remove(2) then it throws exception
  • Examples: Iterators for Collections like ArrayList, HashMap etc.

iterator = numbers.iterator();
while (iterator.hasNext()) {
    if (iterator.next() == 40) {
        iterator.remove(); // safe
        numbers.remove(2); // exception
    }
}

• Fail Safe
  • These iterators make a copy of collection and then iterate
  • It is not guaranteed to return updated collection after iteration
  • Examples: Iterators for Concurrent Collections like ConcurrentHashMap, CopyOnWriteArrayList etc.

Java Cursor

• It is used to iterate, traverse, or retrieve a Collection or Stream object’s elements one by one.
• Types:
  • Enumeration
  • Iterator
  • ListIterator
• Enumeration
  • Legacy class
• Iterator
  • Provides forward moving sequential iteration
• ListIterator
  • extends Iterator
  • Provides Bidirectional iteration capabilities