Iterators

COMP122 Lectures 2021-04-17

Using for loops on lists can be dangerous as their size is not static.

We can separate the traversal logic from the underlying collection by using iterators.

Iterator Design Pattern

Iterators provide a way to access elements of an aggregate object sequentially without exposing its underling representation.

• Iterators are objects.
• Single-use
  • Represents one complete iteration across the collection.
• Can provide safe access to a collection.
• Are often created by the collection itself.

Example

We can filer a List<Bill> using an Iterator as follows:

// get an iterator from the collection
Iterator<Bill> iter = bills.iterator();

while (iter.hasNext()){
	Bill b = iter.next();	// access next entry
	if (b.is_paid()){
		iter.remove();	// remove it from the list
	}
}

Java Iterators

• Iterator<T> is a parameterised interface. You must specify the type of object stored in the collection.
• Iterators provide a safe remove() method that removes the last element returned by its next() method.
• This is different from the remove() method of the collection.

Alternative for Syntax

If you don’t need to write to the collection, so don’t need the iterator, then you can use the following syntax:

for (Bill b : bills){
	// System.out.println(b);
}

which is equivalent to:

Iterator<Bill> iter=bills.iterator();
while (iter.hasNext()){
	Bill b = iter.next();
	// System.out.println(b);
}