Working with Iterators and Comparators
1. Using Iterator Interface
Method Detail hasNext()Check next element next()Return next, advance remove()Remove last returned (optional op) forEachRemaining(c)Java 8+
Warning: Modifying collection outside iterator throws ConcurrentModificationException.
2. Using ListIterator
Extra Methods Use hasPrevious / previousBidirectional add(e)Insert at cursor set(e)Replace last returned nextIndex / previousIndexCursor position
3. Using Enhanced for Loop
Aspect Detail Syntax for (T x : iterable) { ... }Works for Arrays + Iterable Limitations No index, cannot remove
4. Using forEach() Method
Example: forEach
list. forEach (System.out :: println); map. forEach ((k, v) -> System.out. println (k + "=" + v));
Source Signature IterableforEach(Consumer)MapforEach(BiConsumer)StreamforEach (no order) / forEachOrdered
5. Implementing Comparable Interface
Example: Comparable
public class Version implements Comparable < Version > { int major, minor, patch; @ Override public int compareTo (Version o ) { if (major != o.major) return Integer. compare (major, o.major); if (minor != o.minor) return Integer. compare (minor, o.minor); return Integer. compare (patch, o.patch); } }
Contract Detail Sign <0 / 0 / >0 Antisymmetric sgn(a.compareTo(b)) == -sgn(b.compareTo(a))Transitive a<b, b<c → a<c Consistency with equals Strongly recommended
6. Implementing Comparator Interface
Approach Detail Class class ByLen implements Comparator<String>Lambda (a,b) -> a.length() - b.length()Factory Comparator.comparingInt(String::length)
7. Creating Comparator with Lambda
Example: Lambda comparator
Comparator< String > byLength = (a, b) -> Integer. compare (a. length (), b. length ()); Comparator< User > byAge = Comparator. comparingInt (User :: age);
Tip Detail Avoid a - b for ints Overflow risk; use Integer.compare Null handling Use Comparator.nullsFirst/nullsLast
8. Chaining Comparators
Method Use .thenComparing(other)Tiebreaker .thenComparingInt(fn)Primitive optimization
9. Reversing Comparators
Method Behavior cmp.reversed()Reverses order Comparator.reverseOrder()Natural reverse
10. Using Comparator Factory Methods
Factory Description naturalOrder()Uses Comparable comparing(keyFn)By extracted key comparingInt/Long/DoubleAvoid boxing nullsFirst(cmp) / nullsLast(cmp)Null-safe
11. Understanding Natural Ordering vs Custom Ordering
Aspect Comparable Comparator Defined in Class itself External One per class Yes Many possible Method compareTocompareWhen to use Inherent ordering exists Multiple/contextual orderings