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Trail: Collections
Lesson: Interfaces

## The List Interface

> A `List` is an ordered `Collection`(sometimes called a sequence). Lists may contain duplicate elements. In addition to the operations inherited from `Collection`, the `List` interface includes operations for:
> - Positional Access: manipulate elements based on their numerical position in the list.
> - Search: search for a specified object in the list and return its numerical position.
> - List Iteration: extend `Iterator` semantics to take advantage of the list's sequential nature.
> - Range-view: perform arbitrary range operations on the list.

> The `List` interface is shown below:

> >     public interface List extends Collection {
> >         // Positional Access
> >         Object get(int index);
> >         Object set(int index, Object element);            // Optional
> >         void add(int index, Object element);              // Optional
> >         Object remove(int index);                         // Optional
> >         abstract boolean addAll(int index, Collection c); // Optional

> >         // Search
> >         int indexOf(Object o);
> >         int lastIndexOf(Object o);

> >         // Iteration
> >         ListIterator listIterator();
> >         ListIterator listIterator(int index);

> >         // Range-view
> >         List subList(int from, int to);
> >     }

> The JDK contains two general-purpose `List` implementations. `ArrayList`, which is generally the best-performing implementation, and `LinkedList` which offers better performance under certain circumstances. Also, `Vector` has been retrofitted to implement `List`. For more information on implementations, see the Implementations lesson.

### Comparison to Vector

> If you've used `Vector`, you're already familiar with the general flavor of `List`. (Of course, `List` is an interface, while `Vector` is a concrete implementation.) `List` fixes several minor API deficiencies in `Vector`. For starters, commonly used `Vector` operations such as `elementAt` and `setElementAt` have been given much shorter names. When you consider that these two operations are the `List` analogue of square brackets for arrays, it becomes apparent that shorter names are highly desirable. Consider the following assignment statement:

>         a[i] = a[j].times(a[k]);

> The `Vector` equivalent is:

>         v.setElementAt(v.elementAt(j).times(v.elementAt(k)), i);

> The `List` equivalent is:

>         v.set(i, v.get(j).times(v.get(k)));

> You may already have noticed that the `set` method, which replaces `setElementAt`, reverses the order of the arguments so that they match the corresponding array operation. Consider this assignment statement:

>         beatle[5] = "Billy Preston";

> The `Vector` equivalent is:

>         beatle.setElementAt("Billy Preston", 5);

> The `List` equivalent is:

>         beatle.set(5, "Billy Preston");

> For consistency's sake, the `add(int, Object)` method, which replaces `insertElementAt(Object, int)`, also reverses the order of the arguments.

> The various range operations in `Vector` (`indexOf`, `lastIndexOf`(`setSize`) have been replaced by a single range-view operation (`subList`), which is far more powerful and consistent.

### Collection Operations

> The operations inherited from `Collection` all do about what you'd expect them to do, assuming you're already familiar with them from `Collection`. If you're not familiar with them, now would be a good time to read the lesson on the `Collection` interface. The `remove` operation always removes the first occurrence of the specified element from the list. The `add` and `addAll` operations always append the new element(s) to the end of the list. Thus, the following idiom concatenates one list to another:

> >     list1.addAll(list2);

> Here's a non-destructive form of this idiom, which produces a third `List` consisting of the second list appended to the first:

> >     List list3 = new ArrayList(list1);
> >     list3.addAll(list2);

> Note that the idiom, in its non-destructive form, takes advantage of `ArrayList`'s standard `Collection` constructor.

> Like the `Set` interface, `List` strengthens the requirements on the `equals` and `hashCode` methods so that two `List` objects can be compared for logical equality without regard to their implementation classes. Two `List` objects are equal if they contain the same elements in the same order.

### Positional Access and Search Operations

> The basic positional access operations (`get`, `set`, `add` and `remove`) behave just like their longer-named counterparts in `Vector` (`elementAt`, `setElementAt`, `insertElementAt` and `removeElementAt`) with one noteworthy exception. The `set` and `remove` operations return the old value that is being overwritten or removed; the `Vector` counterparts (`setElementAt` and `removeElementAt`) return nothing (`void`). The search operations `indexOf` and `lastIndexOf` behave exactly like the identically named operations in `Vector`.

> The `addAll` operation inserts all of the elements of the specified `Collection` starting at the specified position. The elements are inserted in the order they are returned by the specified `Collection`'s `iterator`. This call is the positional access analogue of `Collection`'s `addAll` operation.

> Here's a little function to swap two indexed values in a `List`. It should look familiar from Programming 101 (assuming you stayed awake):

> >     private static void swap(List a, int i, int j) {
> >         Object tmp = a.get(i);
> >         a.set(i, a.get(j));
> >         a.set(j, tmp);
> >     }

> Of course there's one big difference. This is a polymorphic algorithm: it swaps two elements in any `List`, regardless of its implementation type. "Big deal," you say, "What's it good for?" Funny you should ask. Take a look at this:

> >     public static void shuffle(List list, Random rnd) {
> >         for (int i=list.size(); i>1; i--)
> >             swap(list, i-1, rnd.nextInt(i));
> >     }

> This algorithm (which is included in the JDK's `Collections` class) randomly permutes the specified `List` using the specified source of randomness. It's a bit subtle: It runs up the list from the bottom, repeatedly swapping a randomly selected element into the current position. Unlike most naive attempts at shuffling, it's fair (all permutations occur with equal likelihood, assuming an unbiased source of randomness) and fast (requiring exactly `list.size()-1` iterations). The following short program uses this algorithm to print the words in its argument list in random order:

> >     import java.util.*;

> >     public class Shuffle {
> >         public static void main(String args[]) {
> >             List l = new ArrayList();
> >             for (int i=0; i<args.length; i++)
> >                 l.add(args[i]);
> >             Collections.shuffle(l, new Random());
> >             System.out.println(l);
> >         }
> >     }

> In fact, we can make this program even shorter and faster. The `Arrays` class has a static factory method called `asList` that allows an array to be viewed as a `List`. This method does not copy the array; changes in the `List` write through to the array, and vice-versa. The resulting `List` is not a general-purpose `List` implementation, in that it doesn't implement the (optional) `add` and `remove` operations: arrays are not resizable. Taking advantage of `Arrays.asList` and calling an alternate form of `shuffle` that uses a default source of randomness, you get the following `tiny program`, whose behavior is identical to the previous program:

> >     import java.util.*;

> >     public class Shuffle {
> >         public static void main(String args[]) {
> >             List l = Arrays.asList(args);
> >             Collections.shuffle(l);
> >             System.out.println(l);
> >         }
> >     }

### Iterators

> As you'd expect, the `Iterator` returned by `List`'s `iterator` operation returns the elements of the list in proper sequence. Additionally, `List` provides a richer iterator, called a `ListIterator`, that allows you to traverse the list in either direction, modify the list during iteration, and obtain the current position of the iterator. The `ListIterator` interface is summarized below (including the three methods it inherits from `Iterator`):

> >     public interface ListIterator extends Iterator {
> >         boolean hasNext();
> >         Object next();

> >         boolean hasPrevious();
> >         Object previous();

> >         int nextIndex();
> >         int previousIndex();

> >         void remove();          // Optional
> >         void set(Object o);     // Optional
> >         void add(Object o);     // Optional
> >     }

> The three methods that `ListIterator` inherits from `Iterator` (`hasNext`, `next`, and `remove`) are intended to do exactly the same thing in both interfaces. The `hasPrevious` and `previous` operations are exact analogues of `hasNext` and `next`. The former operations refer to the element before the (implicit) cursor, whereas the latter refer to the element after the cursor. The `previous` operation moves the cursor backwards whereas `next` moves it forwards.

> Here's the standard idiom for iterating backwards through a list:

> >     for (ListIterator i=l.listIterator(l.size()); i.hasPrevious(); ) {
> >         Foo f = (Foo) i.previous();
> >         ...
> >     }

> Note the argument to `listIterator` in the above idiom. The `List` interface has two forms of the `listIterator` method. The form with no arguments returns a `ListIterator` positioned at the beginning of the list, and the form with an `int` argument returns a `ListIterator` positioned at the specified index. The index refers to the element that would be returned by an initial call to `next`. If the index value of `n` is used, then the initial call to `next` would return null, since it would point just past the end of the list. An initial call to `previous` would return the element whose index was `index-1`. In a list of length `n`, there are `n+1` valid values for `index`, from `0` to `n`, inclusive.

> Intuitively speaking, the cursor is always between two elements, the one that would be returned by a call to `previous` and the one that would be returned by a call to `next`. The `n+1` valid `index` values correspond to the `n+1` "gaps" between elements, from the gap before the first element to the gap after the last one. The diagram below shows the five possible cursor positions in a list containing four elements.

>                Element(0)   Element(1)   Element(2)   Element(3)
>              ^            ^            ^            ^            ^
>       Index: 0            1            2            3            4

> Calls to `next` and `previous` can be intermixed, but you have to be a bit careful. The first call to `previous` after a sequence of calls to `next` returns the same element as the last call to `next`. Similarly, the first call to `next` after a sequence of calls to `previous` returns the same element as the last call to `previous`. This will become obvious if you stare at the foregoing text long enough. If it doesn't, go get yourself a steaming hot mug of Java, and try again.

> It should come as no surprise that the `nextIndex` method returns the index of the element that would be returned by a subsequent call to `next`, and `previousIndex` returns the index of the element that would be returned by a subsequent call to `previous`. These calls are typically used for one of two purposes: To report the position where something was found, or to record the position of the `ListIterator` so that another `ListIterator` with identical position can be created.

> It should also come as no surprise that the number returned by `nextIndex` is always one greater than the number returned by `previousIndex`. This implies the behavior of the two boundary cases: a call to `previousIndex` when the cursor is before the initial element returns `-1`, and a call to `nextIndex` when the cursor is after the final element returns `list.size()+1`. To make all of this concrete, here's a possible implementation of `List.indexOf`:

> >     public int indexOf(Object o) {
> >         for (ListIterator i = listIterator(); i.hasNext(); )
> >             if (o==null ? i.next()==null : o.equals(i.next()))
> >                 return i.previousIndex();

> >         return -1; // Object not found
> >     }

> Note that the `indexOf` method returns `i.previousIndex()` though it is traversing the list in the forward direction. This is because `i.nextIndex()` would return the index of the element that we are about to examine, and we want to return the index of the element that we just examined.

> The `Iterator` interface provides the `remove` operation to remove from the `Collection` the last element returned by `next`. The `ListIterator` interface provides two additional operations to modify the list: `set` and `add`. The `set` method "overwrites" the last element returned by `next` or `previous` with the specified element. It is demonstrated by the following polymorphic algorithm to replace all occurrences of one specified value with another:

> >     public static void replace(List l, Object val, Object newVal) {
> >         for (ListIterator i = l.listIterator(); i.hasNext(); )
> >             if (val==null ? i.next()==null : val.equals(i.next()))
> >                 i.set(newVal);
> >     }

> The only bit of trickiness in this example is the equality test between `val` and `i.next`. We have to special-case an `val` value of `null` in order to prevent a `NullPointerException`.

> The `add` method inserts a new element into the list, immediately before the current cursor position. This method is illustrated in the following polymorphic algorithm to replace all occurrences of a specified value with the sequence of values contained in the specified list:

> >     public static void replace(List l, Object val, List newVals) {
> >         for (ListIterator i = l.listIterator(); i.hasNext(); ) {
> >             if (val==null ? i.next()==null : val.equals(i.next())) {
> >                 i.remove();
> >                 for (Iterator j = newVals.iterator(); j.hasNext(); )
> >                     i.add(j.next());
> >             }
> >         }
> >     }

### Range-view Operation

> The range-view operation, `subList(int fromIndex, int toIndex)`, returns a `List` view of the portion of this list whose indices range from `fromIndex`, inclusive, to `toIndex`, exclusive. This half-open range mirrors the typical `for`-loop:

> >     for (int i=fromIndex; i<toIndex; i++) {
> >         ...
> >     }

> As the term view implies, the returned `List` is backed by the `List` on which `subList` was called, so changes in the former `List` are reflected in the latter.

> This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a `List` can be used as a range operation by passing a `subList` view instead of a whole `List`. For example, the following idiom removes a range of elements from a list:

> >     list.subList(fromIndex, toIndex).clear();

> Similar idioms may be constructed to search for an element in a range:

> >     int i = list.subList(fromIndex, toIndex).indexOf(o);
> >     int j = list.subList(fromIndex, toIndex).lastIndexOf(o);

> Note that the above idioms return the index of the found element in the `subList`, not the index in the backing `List`.

> Any polymorphic algorithm that operates on a `List` (like the `replace` and `shuffle` examples, above) works with the `List` returned by `subList`.

> Here's a polymorphic algorithm whose implementation uses `subList` to deal a hand from a deck. That is to say, it returns a new `List` (the "hand") containing the specified number of elements taken from the end of the specified `List` (the "deck"). The elements returned in the hand are removed from the deck.

> >     public static List dealHand(List deck, int n) {
> >         int deckSize = deck.size();
> >         List handView = deck.subList(deckSize-n, deckSize);
> >         List hand = new ArrayList(handView);
> >         handView.clear();
> >         return hand;
> >     }

> The literal-minded might say that this program deals from the bottom of the deck, but I prefer to think that the computer is holding the deck upside down. At any rate, for many common `List` implementations, like `ArrayList`, the performance of removing elements from the end of the list is substantially better than that of removing elements from the beginning.

> Here's `a program` using the `dealHand` method in combination with `Collections.shuffle` to generate hands from a normal 52-card deck. The program takes two command line arguments: the number of hands to deal and the number of cards in each hand.

> >     import java.util.*;

> >     class Deal {
> >         public static void main(String args[]) {
> >             int numHands = Integer.parseInt(args[0]);
> >             int cardsPerHand = Integer.parseInt(args[1]);

> >            // Make a normal 52-card deck
> >             String[] suit = new String[] {"spades", "hearts",
> >                                           "diamonds", "clubs"};
> >             String[] rank = new String[]
> >                 {"ace","2","3","4","5","6","7","8",
> >                  "9","10","jack","queen","king"};
> >             List deck = new ArrayList();
> >             for (int i=0; i<suit.length; i++)
> >                 for (int j=0; j<rank.length; j++)
> >                     deck.add(rank[j] + " of " + suit[i]);

> >             Collections.shuffle(deck);

> >             for (int i=0; i<numHands; i++)
> >                 System.out.println(dealHand(deck, cardsPerHand));
> >         }
> >     }

> Let's run the program:

> >     % java Deal 4 5

> >     [8 of hearts, jack of spades, 3 of spades, 4 of spades, king of diamonds]
> >     [4 of diamonds, ace of clubs, 6 of clubs, jack of hearts, queen of hearts]
> >     [7 of spades, 5 of spades, 2 of diamonds, queen of diamonds, 9 of clubs]
> >     [8 of spades, 6 of diamonds, ace of spades, 3 of hearts, ace of hearts]

> While the `subList` operation is extremely powerful, some care must be exercised when using it. The semantics of the `List` returned by `subList` become undefined if elements are added to or removed from the backing `List` in any way other than via the returned `List`. Thus, it's highly recommended that you use the `List` returned by `subList` only as a transient object, to perform one or a sequence of range operations on the backing `List`. The longer you use the `subList` object, the greater the probability that you'll compromise it by modifying the backing `List` directly (or through another `subList` object).

### Algorithms

> Most of the polymorphic algorithms in the `Collections` class apply specifically to `List`. Having all of these algorithms at your disposal makes it very easy to manipulate lists. Here's a summary of these algorithms, which are described in more detail in the Algorithms lesson.
> - `sort(List)`: Sorts a List using a merge sort algorithm, which provides a fast, stable sort. (A stable sort is one that does not reorder equal elements.)
> - `shuffle(List)`: Randomly permutes the elements in a `List`. (Shown above.)
> - `reverse(List)`: Reverses the order of the elements in a `List`.
> - `fill(List, Object)`: Overwrites every element in a `List` with the specified value.
> - `copy(List dest, List src)`: Copies the source `List` into the destination List.
> - `binarySearch(List, Object)`: Searches for an element in an ordered `List` using the binary search algorithm.

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