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Iterable without element at index in Guava

If I have a FluentIterable, how do I get a new FluentIterable without the element at index i?

There's nothing built in for this, and no particularly great implementation either.
I'd honestly recommend using a mutable list and calling remove(int index). Fluent view chains really don't seem likely to be effective for this sort of use case.

Here's a way to do it without mutable lists:
FluentIterable.concat(myIterable.limit(i), myIterable.skip(i + 1));
And here's a slightly more performant way:
ImmutableList<String> list = ImmutableList.copyOf(myIterable);
FluentIterable.concat(
list.subList(0, i - 1),
list.subList(i + 1, list.size() - 1));
Note that #subList is backed by the original ImmutableList, so we only iterate through myIterable once here.

You can do this by applying a transform to associate each element with its index, then filter out the element with the index you don't want, and finally transform each element with its index back to just the element. e.g.:
fluentIterable
.transform(new WithIndexTransformer<>())
.filter(indexedValue -> indexedValue.getIndex() != i)
.transform(IndexedValue::getValue);
IndexedValue.java
public class IndexedValue<T> {
private final int index;
private final T value;
IndexedValue(int index, T value) {
this.index = index;
this.value = value;
}
public int getIndex() {
return index;
}
public T getValue() {
return value;
}
}
WithIndexTransformer.java
public class WithIndexTransformer<T> implements Function<T, IndexedValue<T>> {
private int index = 0;
#Override
public IndexedValue<T> apply(T input) {
return new IndexedValue<>(index++, input);
}
}

Collection that uses ListIterator and is a unique list

This has been annoying me in a project recently and my Google phoo is failing me at finding a suitable answer.
Is there a collection, that has access to the ListIterator but also only allows for unique values inside the collection?
Reasoning for this, I have a collection of items, that in this collection, there should only ever be one of each element. I also want to be able to traverse this collection in both directions whilst also being sorted or allow me to sort it using Collections.Sort();
I've not found anything suitable and had to write my own class using the following code:
public class UniqueArrayList<E> extends ArrayList<E> {
#Override
public boolean add(E element){
if (this.contains(element))
return false;
else
return super.add(element);
}
#Override
public void add(int index, E element){
if (this.contains(element))
return;
else
super.add(index, element);
}
#Override
public boolean addAll(Collection<? extends E> c){
if (new HashSet<E>(c).size() < c.size())
return false;
for(E element : c){
if (this.contains(c))
return false;
}
return super.addAll(c);
}
#Override
public boolean addAll(int index, Collection<? extends E> c) {
if (new HashSet<E>(c).size() < c.size())
return false;
for(E element : c){
if (this.contains(c))
return false;
}
return super.addAll(index, c);
}
#Override
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > this.size())
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
#Override
public ListIterator<E> listIterator() {
return new ListItr(0);
}
#Override
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size();
}
#SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size())
throw new NoSuchElementException();
Object[] elementData = UniqueArrayList.this.toArray();
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
UniqueArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}
public boolean hasPrevious() {
return cursor != 0;
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor - 1;
}
#SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = UniqueArrayList.this.toArray();
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
//Need to allow this for the collections sort to work!
//if (!UniqueArrayList.this.contains(e))
UniqueArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
UniqueArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}
}
However this is far from perfect, for one I can't override the ListIterator.set(); because Collections.sort(); uses it to move items in the list. If I try to prevent non unique items from being added to the list here, the sort never happens.
So, does anyone have a better method or know of another collection that abides by the rules that I would like? Or do I just need to live with this rather irritating issue?
[Edit]
This is the Collections.sort(); method:
public static <T extends Comparable<? super T>> void sort(List<T> list) {
Object[] a = list.toArray();
Arrays.sort(a);
ListIterator<T> i = list.listIterator();
for (int j=0; j<a.length; j++) {
i.next();
i.set((T)a[j]);
}
}
The reasoning they give for doing this is:
This implementation dumps the specified list into an array, sorts the
array, and iterates over the list resetting each element from the
corresponding position in the array. This avoids the n2
log(n) performance that would result from attempting to sort a linked
list in place.

When you need unique values you should try to switch to a Set.
You can use a TreeSet together with a Comparator instance to sort the entries. The descendingSet() method of TreeSet will give you the reverse order.
If you really need a ListIterator at some point you could create a temporary list from the set.

As Thor Stan mentioned, a TreeSet gets you most of what you want. It ensures elements are unique, it keeps them sorted, and you can iterate it in either direction using iterator() or descendingIterator().
It's not entirely clear why you're asking for ListIterator though. Most things about a ListIterator are very positional: the notion of an index, or adding something at the current position, or setting the current element. These don't make sense for a sorted set.
One aspect of a ListIterator that you might be looking for, though, is the ability to reverse directions in the midst of iteration. You can't do this directly with a TreeSet iterator, since it offers access only via an ordinary Iterator instead of a ListIterator.
However, a TreeSet implements the NavigableSet interface, which lets you step through the elements in order, in either direction. The NavigableSet interface is a subinterface of SortedSet, which provides the first() and last() methods to get you started at one of the "ends" of the set. Once you have an element in the set, you can step in either direction using the lower(E) and higher(E) methods. Or, if you want to start somewhere in the middle, you can't start at a position by index, but you can start with a value (which needn't be a member of the set) and then call lower(E) or higher(E).
For example:
TreeSet<String> set = new TreeSet<>(
Arrays.asList("a", "z", "b", "y"));
String cur;
cur = set.first(); // a
cur = set.higher(cur); // b
cur = set.higher(cur); // y
cur = set.higher(cur); // z
cur = set.lower(cur); // y
cur = set.lower(cur); // b
cur = set.lower(cur); // a
cur = set.lower(cur); // null

Java considers List to allow non-unique items and Set to not. Sets obviously don't support ListIterators and therefore code that uses ListIterator can assume that the underlying collection is not a Set.
Java 8 doesn't use ListIterator for sorting anymore, but if you're stuck with Java 7 that doesn't really help you. Basically depending a lot on your context and usage, it might be useful to either use a Set like Thor Stan said in his response, creating a List on demand when needed.
Another option is to just provide your own ListIterator that accesses the list through a method that doesn't check for duplicates. This would have the advantage of not creating extraneous objects, but the Set option would most likely result in shorter code and is highly unlikely to be significant performance-wise.
There are probably other options too, but I can't think of any elegant ones.

This is a problem with no easy answers.
The problem is that you have broken the contract for add(int, E). This method must either add the element or throw an exception - it is not allowed to return without adding the element.
If you override set(int, E) so that sometimes it doesn't set the element, it would break the contract for that method too, and it would prevent Collections.sort() from working as you identified.
I would not recommend breaking these contracts - it may cause other methods that act on lists to behave in unpredictable ways.
Others have experienced these difficulties - see the java docs for SetUniqueList for example.
Another problem with your implementation is that it would be extremely slow because ArrayList.contains() is a linear search.
One solution would be to write a class that uses both an ArrayList and a HashSet, and write your own versions of add and set, rather than breaking the contracts for the usual versions. This is not tested, but you get the idea.
public final class MyList<E extends Comparable<? super E>> extends AbstractList<E> {
private final List<E> list = new ArrayList<>();
private final Set<E> set = new HashSet<>();
public E get(int i) {
return list.get(i);
}
public int size() {
return list.size();
}
public boolean tryAdd(E e) {
return set.add(e) && list.add(e);
}
public boolean tryAdd(int i, E e) {
if (set.add(e)) {
list.add(i, e);
return true;
}
return false;
}
public boolean trySet(int i, E e) {
return set.add(e) && set.remove(list.set(i, e));
}
public boolean remove(Object o) {
return set.remove(o) && list.remove(o);
}
public void sort() {
Collections.sort(list);
}
// One bonus of this approach is that contains() is now O(1)
public boolean contains(Object o) {
return set.contains(o);
}
// rest omitted.
}
Something like this would not break the contract for List. Note that with this version, add and set throw an UnsupportedOperationException, because this is the behaviour inherited from AbstractList. You would be able to sort by calling myList.sort(). Also, the listIterator method would work, but you would not be able to use it to set or add (although remove would work).
If you needed to add or set elements while iterating over this List, you would need to use an explicit index, rather than a ListIterator. Personally, I do not consider this a major problem. ListIterator is necessary for lists like LinkedList that do not have a constant time get method, but for ArrayList it's nice but not essential.

How to create a Java Iterator for 2D array/list

I was recently asked about the question that how to create a Java Iterator for 2D Array, specifically how to implement:
public class PersonIterator implements Iterator<Person>{
private List<List<Person>> list;
public PersonIterator(List<List<Person>> list){
this.list = list;
}
#Override
public boolean hasNext() {
}
#Override
public Person next() {
}
}
1D array is pretty straightforward by using a index to track the position, any idea about how to do it for 2D lists.

In the 1D case you need to keep one index to know where you left, right?
Well, in the 2D case you need two indices: one to know in which sub-list you were working, and other one to know at what element inside that sub-list you left.

Something like this? (Note: untested)
public class PersonIterator implements Iterator<Person>{
// This keeps track of the outer set of lists, the lists of lists
private Iterator<List<Person>> iterator;
// This tracks the inner set of lists, the lists of persons we're going through
private Iterator<Person> curIterator;
public PersonIterator(List<List<Person>> list){
// Set the outer one
this.iterator = list.iterator();
// And set the inner one based on whether or not we can
if (this.iterator.hasNext()) {
this.curIterator = iterator.next();
} else {
this.curIterator = null;
}
}
#Override
public boolean hasNext() {
// If the current iterator is valid then we obviously have another one
if (curIterator != null && curIterator.hasNext()) {
return true;
// Otherwise we need to safely get the iterator for the next list to iterate.
} else if (iterator.hasNext()) {
// We load a new iterator here
curIterator = iterator.next();
// and retry peeking to see if the new curIterator has any elements to iterate.
return hasNext();
// Otherwise we're out of lists.
} else {
return false;
}
}
#Override
public Person next() {
// Return the current value off the inner iterator if we can
if (curIterator != null && curIterator.hasNext()) {
return curIterator.next();
// Otherwise try to iterate along the next list and retry getting the next one.
// This won't infinitely loop at the end since next() at the end of the outer
// iterator should result in an NoSuchElementException.
} else {
curIterator = iterator.next();
return next();
}
}
}

Iterator over multiple SortedSet objects

In Java, I have several SortedSet instances. I would like to iterate over the elements from all these sets. One simple option is to create a new SortedSet, such as TreeSet x, deep-copy the contents of all the individual sets y_1, ..., y_n into it using x.addAll(y_i), and then iterate over x.
But is there a way to avoid deep copy? Couldn't I just create a view of type SortedSet which would somehow encapsulate the iterators of all the inner sets, but behave as a single set?

I'd prefer an existing, tested solution, rather than writing my own.
I'm not aware of any existing solution to accomplish this task, so I took the time to write one for you. I'm sure there's room for improvement on it, so take it as a guideline and nothing else.
As Sandor points out in his answer, there are some limitations that must be imposed or assumed. One such limitation is that every SortedSet must be sorted relative to the same order, otherwise there's no point in comparing their elements without creating a new set (representing the union of every individual set).
Here follows my code example which, as you'll notice, is relatively more complex than just creating a new set and adding all elements to it.
import java.util.*;
final class MultiSortedSetView<E> implements Iterable<E> {
private final List<SortedSet<E>> sets = new ArrayList<>();
private final Comparator<? super E> comparator;
MultiSortedSetView() {
comparator = null;
}
MultiSortedSetView(final Comparator<? super E> comp) {
comparator = comp;
}
#Override
public Iterator<E> iterator() {
return new MultiSortedSetIterator<E>(sets, comparator);
}
MultiSortedSetView<E> add(final SortedSet<E> set) {
// You may remove this `if` if you already know
// every set uses the same comparator.
if (comparator != set.comparator()) {
throw new IllegalArgumentException("Different Comparator!");
}
sets.add(set);
return this;
}
#Override
public boolean equals(final Object o) {
if (this == o) { return true; }
if (!(o instanceof MultiSortedSetView)) { return false; }
final MultiSortedSetView<?> n = (MultiSortedSetView<?>) o;
return sets.equals(n.sets) &&
(comparator == n.comparator ||
(comparator != null ? comparator.equals(n.comparator) :
n.comparator.equals(comparator)));
}
#Override
public int hashCode() {
int hash = comparator == null ? 0 : comparator.hashCode();
return 37 * hash + sets.hashCode();
}
#Override
public String toString() {
return sets.toString();
}
private final static class MultiSortedSetIterator<E>
implements Iterator<E> {
private final List<Iterator<E>> iterators;
private final PriorityQueue<Element<E>> queue;
private MultiSortedSetIterator(final List<SortedSet<E>> sets,
final Comparator<? super E> comparator) {
final int n = sets.size();
queue = new PriorityQueue<Element<E>>(n,
new ElementComparator<E>(comparator));
iterators = new ArrayList<Iterator<E>>(n);
for (final SortedSet<E> s: sets) {
iterators.add(s.iterator());
}
prepareQueue();
}
#Override
public E next() {
final Element<E> e = queue.poll();
if (e == null) {
throw new NoSuchElementException();
}
if (!insertFromIterator(e.iterator)) {
iterators.remove(e.iterator);
}
return e.element;
}
#Override
public boolean hasNext() {
return !queue.isEmpty();
}
private void prepareQueue() {
final Iterator<Iterator<E>> iterator = iterators.iterator();
while (iterator.hasNext()) {
if (!insertFromIterator(iterator.next())) {
iterator.remove();
}
}
}
private boolean insertFromIterator(final Iterator<E> i) {
while (i.hasNext()) {
final Element<E> e = new Element<>(i.next(), i);
if (!queue.contains(e)) {
queue.add(e);
return true;
}
}
return false;
}
private static final class Element<E> {
final E element;
final Iterator<E> iterator;
Element(final E e, final Iterator<E> i) {
element = e;
iterator = i;
}
#Override
public boolean equals(final Object o) {
if (o == this) { return true; }
if (!(o instanceof Element)) { return false; }
final Element<?> e = (Element<?>) o;
return element.equals(e.element);
}
}
private static final class ElementComparator<E>
implements Comparator<Element<E>> {
final Comparator<? super E> comparator;
ElementComparator(final Comparator<? super E> comp) {
comparator = comp;
}
#Override
#SuppressWarnings("unchecked")
public int compare(final Element<E> e1, final Element<E> e2) {
if (comparator != null) {
return comparator.compare(e1.element, e2.element);
}
return ((Comparable<? super E>) e1.element)
.compareTo(e2.element);
}
}
}
}
The inner workings of this class are simple to grasp. The view keeps a list of sorted sets, the ones you want to iterate over. It also needs the comparator that will be used to compare elements (null to use their natural ordering). You can only add (distinct) sets to the view.
The rest of the magic happens in the Iterator of this view. This iterator keeps a PriorityQueue of the elements that will be returned from next() and a list of iterators from the individual sets.
This queue will have, at all times, at most one element per set, and it discards repeating elements. The iterator also discards empty and used up iterators. In short, it guarantees that you will traverse every element exactly once (as in a set).
Here's an example on how to use this class.
SortedSet<Integer> s1 = new TreeSet<>();
SortedSet<Integer> s2 = new TreeSet<>();
SortedSet<Integer> s3 = new TreeSet<>();
SortedSet<Integer> s4 = new TreeSet<>();
// ...
MultiSortedSetView<Integer> v =
new MultiSortedSetView<Integer>()
.add(s1)
.add(s2)
.add(s3)
.add(s4);
for (final Integer i: v) {
System.out.println(i);
}

I do not think that is possible unless it is some special case, which would require custom implementation.
For example take the following two comparators:
public class Comparator1 implements Comparator<Long> {
#Override
public int compare(Long o1, Long o2) {
return o1.compareTo(o2);
}
}
public class Comparator2 implements Comparator<Long> {
#Override
public int compare(Long o1, Long o2) {
return -o1.compareTo(o2);
}
}
and the following code:
TreeSet<Long> set1 = new TreeSet<Long>(new Comparator1());
TreeSet<Long> set2 = new TreeSet<Long>(new Comparator2());
set1.addAll(Arrays.asList(new Long[] {1L, 3L, 5L}));
set2.addAll(Arrays.asList(new Long[] {2L, 4L, 6L}));
System.out.println(Joiner.on(",").join(set1.descendingIterator()));
System.out.println(Joiner.on(",").join(set2.descendingIterator()));
This will result in:
5,3,1
2,4,6
and is useless for any Comparator operating on the head element of the given Iterators.
This makes it impossible to create such a general solution. It is only possible if all sets are sorted using the same Comparator, however that cannot be guaranteed and ensured by any implementation which accept SortedSet objects, given multiple SortedSet instances (e.g. anything that would accept SortedSet<Long> instances, would accept both TreeSet objects).
A little bit more formal approach:
Given y_1,..,y_n are all sorted sets, if:
the intersect of these sets are an empty set
and there is an ordering of the sets where for every y_i, y_(i+1) set it is true that y_i[x] <= y_(i+1)[1] where x is the last element of the y_i sorted set, and <= means a comparative function
then the sets y_1,..,y_n can be read after each other as a SortedSet.
Now if any of the following conditions are not met:
if the first condition is not met, then the definition of a Set is not fulfilled, so it can not be a Set until a deep copy merge is completed and the duplicated elements are removed (See Set javadoc, first paragraph:
sets contain no pair of elements e1 and e2 such that e1.equals(e2)
the second condition can only be ensured using exactly the same comparator <= function
The first condition is the more important, because being a SortedSet implies being a Set, and if the definition of being a Set cannot be fulfilled, then the stronger conditions of a SortedSet definitely cannot be fulfilled.
There is a possibility that an implementation can exists which mimics the working of a SortedSet, but it will definitely not be a SortedSet.

com.google.common.collect.Sets#union from Guava will do the trick. It returns an unmodifiable view of the union of two sets. You may iterate over it. Returned set will not be sorted. You may then create new sorted set from returned set (new TreeSet() or com.google.common.collect.ImmutableSortedSet. I see no API to create view of given set as sorted set.

If your concern is a deep-copy on the objects passed to the TreeSet#addAll method, you shouldn't be. The javadoc does not indicate it's a deep-copy (and it certainly would say so if it was)...and the OpenJDK implementation doesn't show this either. No copies - simply additional references to the existing object.
Since the deep-copy isn't an issue, I think worrying about this, unless you've identified this as a specific performance problem, falls into the premature optimization category.

Any implementation of Ordered Set in Java?

If anybody is familiar with Objective-C there is a collection called NSOrderedSet that acts as Set and its items can be accessed as an Array's ones.
Is there anything like this in Java?
I've heard there is a collection called LinkedHashMap, but I haven't found anything like it for a set.

Take a look at LinkedHashSet class
From Java doc:
Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.).

Every Set has an iterator(). A normal HashSet's iterator is quite random, a TreeSet does it by sort order, a LinkedHashSet iterator iterates by insert order.
You can't replace an element in a LinkedHashSet, however. You can remove one and add another, but the new element will not be in the place of the original. In a LinkedHashMap, you can replace a value for an existing key, and then the values will still be in the original order.
Also, you can't insert at a certain position.
Maybe you'd better use an ArrayList with an explicit check to avoid inserting duplicates.

Take a look at the Java standard API doc. Right next to LinkedHashMap, there is a LinkedHashSet. But note that the order in those is the insertion order, not the natural order of the elements. And you can only iterate in that order, not do random access (except by counting iteration steps).
There is also an interface SortedSet implemented by TreeSet and ConcurrentSkipListSet. Both allow iteration in the natural order of their elements or a Comparator, but not random access or insertion order.
For a data structure that has both efficient access by index and can efficiently implement the set criterium, you'd need a skip list, but there is no implementation with that functionality in the Java Standard API, though I am certain it's easy to find one on the internet.

TreeSet is ordered.
http://docs.oracle.com/javase/6/docs/api/java/util/TreeSet.html

Try using java.util.TreeSet that implements SortedSet.
To quote the doc:
"The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used"
Note that add, remove and contains has a time cost log(n).
If you want to access the content of the set as an Array, you can convert it doing:
YourType[] array = someSet.toArray(new YourType[yourSet.size()]);
This array will be sorted with the same criteria as the TreeSet (natural or by a comparator), and in many cases this will have a advantage instead of doing a Arrays.sort()

treeset is an ordered set, but you can't access via an items index, just iterate through or go to beginning/end.

If we are talking about inexpensive implementation of the skip-list, I wonder in term of big O, what the cost of this operation is:
YourType[] array = someSet.toArray(new YourType[yourSet.size()]);
I mean it is always get stuck into a whole array creation, so it is O(n):
java.util.Arrays#copyOf

You might also get some utility out of a Bidirectional Map like the BiMap from Google Guava
With a BiMap, you can pretty efficiently map an Integer (for random index access) to any other object type. BiMaps are one-to-one, so any given integer has, at most, one element associated with it, and any element has one associated integer. It's cleverly underpinned by two HashTable instances, so it uses almost double the memory, but it's a lot more efficient than a custom List as far as processing because contains() (which gets called when an item is added to check if it already exists) is a constant-time and parallel-friendly operation like HashSet's, while List's implementation is a LOT slower.

I had a similar problem. I didn't quite need an ordered set but more a list with a fast indexOf/contains. As I didn't find anything out there I implemented one myself. Here's the code, it implements both Set and List, though not all bulk list operations are as fast as the ArrayList versions.
disclaimer: not tested
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Set;
import java.util.Collection;
import java.util.Comparator;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import static java.util.Objects.requireNonNull;
/**
* An ArrayList that keeps an index of its content so that contains()/indexOf() are fast. Duplicate entries are
* ignored as most other java Set's do.
*/
public class IndexedArraySet<E> extends ArrayList<E> implements Set<E> {
public IndexedArraySet() { super(); }
public IndexedArraySet(Iterable<E> c) {
super();
addAll(c);
}
private HashMap<E, Integer> indexMap = new HashMap<>();
private void reindex() {
indexMap.clear();
int idx = 0;
for (E item: this) {
addToIndex(item, idx++);
}
}
private E addToIndex(E e, int idx) {
indexMap.putIfAbsent(requireNonNull(e), idx);
return e;
}
#Override
public boolean add(E e) {
if(indexMap.putIfAbsent(requireNonNull(e), size()) != null) return false;
super.add(e);
return true;
}
#Override
public boolean addAll(Collection<? extends E> c) {
return addAll((Iterable<? extends E>) c);
}
public boolean addAll(Iterable<? extends E> c) {
boolean rv = false;
for (E item: c) {
rv |= add(item);
}
return rv;
}
#Override
public boolean contains(Object e) {
return indexMap.containsKey(e);
}
#Override
public int indexOf(Object e) {
if (e == null) return -1;
Integer i = indexMap.get(e);
return (i == null) ? -1 : i;
}
#Override
public int lastIndexOf(Object e) {
return indexOf(e);
}
#Override #SuppressWarnings("unchecked")
public Object clone() {
IndexedArraySet clone = (IndexedArraySet) super.clone();
clone.indexMap = (HashMap) indexMap.clone();
return clone;
}
#Override
public void add(int idx, E e) {
if(indexMap.putIfAbsent(requireNonNull(e), -1) != null) return;
super.add(idx, e);
reindex();
}
#Override
public boolean remove(Object e) {
boolean rv;
try { rv = super.remove(e); }
finally { reindex(); }
return rv;
}
#Override
public void clear() {
super.clear();
indexMap.clear();
}
#Override
public boolean addAll(int idx, Collection<? extends E> c) {
boolean rv;
try {
for(E item : c) {
// check uniqueness
addToIndex(item, -1);
}
rv = super.addAll(idx, c);
} finally {
reindex();
}
return rv;
}
#Override
public boolean removeAll(Collection<?> c) {
boolean rv;
try { rv = super.removeAll(c); }
finally { reindex(); }
return rv;
}
#Override
public boolean retainAll(Collection<?> c) {
boolean rv;
try { rv = super.retainAll(c); }
finally { reindex(); }
return rv;
}
#Override
public boolean removeIf(Predicate<? super E> filter) {
boolean rv;
try { rv = super.removeIf(filter); }
finally { reindex(); }
return rv;
}
#Override
public void replaceAll(final UnaryOperator<E> operator) {
indexMap.clear();
try {
int duplicates = 0;
for (int i = 0; i < size(); i++) {
E newval = requireNonNull(operator.apply(this.get(i)));
if(indexMap.putIfAbsent(newval, i-duplicates) == null) {
super.set(i-duplicates, newval);
} else {
duplicates++;
}
}
removeRange(size()-duplicates, size());
} catch (Exception ex) {
// If there's an exception the indexMap will be inconsistent
reindex();
throw ex;
}
}
#Override
public void sort(Comparator<? super E> c) {
try { super.sort(c); }
finally { reindex(); }
}
}

IndexedTreeSet from the indexed-tree-map project provides this functionality (ordered/sorted set with list-like access by index).

My own MapTreeAVL (whose implementations are under the mtAvl package next to that interface) provides both a method to obtain a SortedSet and a way to have index-based access (some so-called Optimizations such as MinMaxIndexIteration might be preferred in that case).
(Note: since my repo is open you can directly download the desired file, but I advise you to check if [and will be] some other classes in the same superpackage dataStructure are required/used/imported)