Summary - Iterator

Combine Iterator and Iterable to write a custom collection.

Summary - Iterator

This group covered four tools for walking collections: Iterator basic one-way traversal, ListIterator bidirectional and List-specific, Iterable the interface enabling for-each, Spliterator suited for parallel traversal.

iterator.remove() safely performs conditional removal; removal inside for-each is forbidden. Any class implementing Iterable can be walked with for-each. Spliterator underpins the Stream API and enables parallel processing through trySplit.

Exercise Mixed traversal exercise

Combine three tools. Declare List<String> names = new ArrayList<>();; add "Ayşe", "Alexander", "Alex". Walk with Iterator<String> it = names.iterator();; if s.length() > 5 call it.remove();. After the loop print the remaining names with for (String n : names) System.out.println(n);.

Show example solution

import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Ayşe");
        names.add("Alexander");
        names.add("Alex");
        Iterator<String> it = names.iterator();
        while (it.hasNext()) {
            String s = it.next();
            if (s.length() > 5) it.remove();
        }
        for (String n : names) {
            System.out.println(n);
        }
    }
}

Test yourself

Match the traversal tool

Iterator

One-way basic
ListIterator

Bidirectional, List-only
Iterable

for-each permission
Spliterator

Parallel traversal
Which interface enables the for-each loop?
- Iterable
- Iterator
- Comparator
- Comparable
Show answer

Iterable — Classes implementing Iterable can be traversed with for-each.
Match the method to its role

hasNext()

Has next?
next()

Advance and return
remove()

Delete last read
trySplit()

Split in two

Next lesson: Lambda basics

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