# Interfaces

Domains:

## The Collection Interface

Collection represents a group of objects known as its elements. The Collection interface is used to pass around collections of objects where maximum generality is desired. For example, by convention all general-purpose collection implementations have a constructor that takes a Collection argument. This constructor, known as a conversion constructor, initializes the new collection to contain all of the elements in the specified collection, whatever the given collection's subinterface or implementation type. In other words, it allows you to convert the collection's type.

Suppose, for example, that you have a Collection<String> c, which may be a List, a Set, or another kind of Collection. This idiom creates a new ArrayList (an implementation of the Listinterface), initially containing all the elements in c.

		List<String> list = new ArrayList<String>(c);

Or — if you are using JDK 7 or later — you can use the diamond operator:
		List<String> list = new ArrayList<>(c);


The Collection interface contains methods that perform basic operations, such as int size()boolean isEmpty()boolean contains(Object element)boolean add(E element)boolean remove(Object element), and Iterator<E> iterator().

It also contains methods that operate on entire collections, such as boolean containsAll(Collection<?> c)boolean addAll(Collection<? extends E> c)boolean removeAll(Collection<?> c)boolean retainAll(Collection<?> c), and  void clear().

Additional methods for array operations (such as Object[] toArray() and <T> T[] toArray(T[] a) exist as well.

In JDK 8 and later, the Collection interface also exposes methods Stream<E> stream() and Stream<E> parallelStream(), for obtaining sequential or parallel streams from the underlying collection. (See the lesson entitled Aggregate Operations for more information about using streams.)

The Collection interface does about what you'd expect given that a Collection represents a group of objects. It has methods that tell you how many elements are in the collection (sizeisEmpty), methods that check whether a given object is in the collection (contains), methods that add and remove an element from the collection (addremove), and methods that provide an iterator over the collection (iterator).

The add method is defined generally enough so that it makes sense for collections that allow duplicates as well as those that don't. It guarantees that the Collection will contain the specified element after the call completes, and returns true if the Collection changes as a result of the call. Similarly, the remove method is designed to remove a single instance of the specified element from the Collection, assuming that it contains the element to start with, and to return true if the Collection was modified as a result.

### Traversing Collections

There are three ways to traverse collections: (1) using aggregate operations (2) with the for-each construct and (3) by using Iterators.

### Aggregate Operations

In JDK 8 and later, the preferred method of iterating over a collection is to obtain a stream and perform aggregate operations on it. Aggregate operations are often used in conjunction with lambda expressions to make programming more expressive, using less lines of code. The following code sequentially iterates through a collection of shapes and prints out the red objects:

		myShapesCollection.stream()
.filter(e -> e.getColor() == Color.RED)
.forEach(e -> System.out.println(e.getName()));


Likewise, you could easily request a parallel stream, which might make sense if the collection is large enough and your computer has enough cores:

		myShapesCollection.parallelStream()
.filter(e -> e.getColor() == Color.RED)
.forEach(e -> System.out.println(e.getName()));


There are many different ways to collect data with this API. For example, you might want to convert the elements of a Collection to String objects, then join them, separated by commas:

	String joined = elements.stream()
.map(Object::toString)
.collect(Collectors.joining(", "));


Or perhaps sum the salaries of all employees:

		int total = employees.stream()
.collect(Collectors.summingInt(Employee::getSalary)));


These are but a few examples of what you can do with streams and aggregate operations. For more information and examples, see the lesson entitled Aggregate Operations.

The Collections framework has always provided a number of so-called "bulk operations" as part of its API. These include methods that operate on entire collections, such as containsAlladdAllremoveAll, etc. Do not confuse those methods with the aggregate operations that were introduced in JDK 8. The key difference between the new aggregate operations and the existing bulk operations (containsAlladdAll, etc.) is that the old versions are all mutative, meaning that they all modify the underlying collection. In contrast, the new aggregate operations do not modify the underlying collection. When using the new aggregate operations and lambda expressions, you must take care to avoid mutation so as not to introduce problems in the future, should your code be run later from a parallel stream.

### for-each Construct

The for-each construct allows you to concisely traverse a collection or array using a for loop — see The for Statement. The following code uses the for-each construct to print out each element of a collection on a separate line.

		for (Object o : collection)
System.out.println(o);


### Iterator

An Iterator is an object that enables you to traverse through a collection and to remove elements from the collection selectively, if desired. You get an Iterator for a collection by calling its iteratormethod. The following is the Iterator interface.

		public interface Iterator<E> {
boolean hasNext();
E next();
void remove(); //optional
}


The hasNext method returns true if the iteration has more elements, and the next method returns the next element in the iteration. The remove method removes the last element that was returned by next from the underlying Collection. The remove method may be called only once per call to next and throws an exception if this rule is violated.

Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Use Iterator instead of the for-each construct when you need to:

• Remove the current element. The for-each construct hides the iterator, so you cannot call remove. Therefore, the for-each construct is not usable for filtering.
• Iterate over multiple collections in parallel.

The following method shows you how to use an Iterator to filter an arbitrary Collection — that is, traverse the collection removing specific elements.

		static void filter(Collection<?> c) {
for (Iterator<?> it = c.iterator(); it.hasNext(); )
if (!cond(it.next()))
it.remove();
}


This simple piece of code is polymorphic, which means that it works for any Collection regardless of implementation. This example demonstrates how easy it is to write a polymorphic algorithm using the Java Collections Framework.

### Collection Interface Bulk Operations

Bulk operations perform an operation on an entire Collection. You could implement these shorthand operations using the basic operations, though in most cases such implementations would be less efficient. The following are the bulk operations:

• containsAll — returns true if the target Collection contains all of the elements in the specified Collection.
• addAll — adds all of the elements in the specified Collection to the target Collection.
• removeAll — removes from the target Collection all of its elements that are also contained in the specified Collection.
• retainAll — removes from the target Collection all its elements that are not also contained in the specified Collection. That is, it retains only those elements in the target Collection that are also contained in the specified Collection.
• clear — removes all elements from the Collection.

The addAllremoveAll, and retainAll methods all return true if the target Collection was modified in the process of executing the operation.

As a simple example of the power of bulk operations, consider the following idiom to remove all instances of a specified element, e, from a Collectionc.

		c.removeAll(Collections.singleton(e));


More specifically, suppose you want to remove all of the null elements from a Collection.

		c.removeAll(Collections.singleton(null));


This idiom uses Collections.singleton, which is a static factory method that returns an immutable Set containing only the specified element.

### Collection Interface Array Operations

The toArray methods are provided as a bridge between collections and older APIs that expect arrays on input. The array operations allow the contents of a Collection to be translated into an array. The simple form with no arguments creates a new array of Object. The more complex form allows the caller to provide an array or to choose the runtime type of the output array.

For example, suppose that c is a Collection. The following snippet dumps the contents of c into a newly allocated array of Object whose length is identical to the number of elements in c.

		Object[] a = c.toArray();


Suppose that c is known to contain only strings (perhaps because c is of type Collection<String>). The following snippet dumps the contents of c into a newly allocated array of String whose length is identical to the number of elements in c.

		String[] a = c.toArray(new String[0]);

## The Set Interface

Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited. Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ. Two Set instances are equal if they contain the same elements.

The Java platform contains three general-purpose Set implementations: HashSetTreeSet, and LinkedHashSetHashSet, which stores its elements in a hash table, is the best-performing implementation; however it makes no guarantees concerning the order of iteration. TreeSet, which stores its elements in a red-black tree, orders its elements based on their values; it is substantially slower than HashSetLinkedHashSet, which is implemented as a hash table with a linked list running through it, orders its elements based on the order in which they were inserted into the set (insertion-order). LinkedHashSet spares its clients from the unspecified, generally chaotic ordering provided by HashSet at a cost that is only slightly higher.

Here's a simple but useful Set idiom. Suppose you have a Collectionc, and you want to create another Collection containing the same elements but with all duplicates eliminated. The following one-liner does the trick.

		Collection<Type> noDups = new HashSet<Type>(c);


It works by creating a Set (which, by definition, cannot contain duplicates), initially containing all the elements in c. It uses the standard conversion constructor described in the The Collection Interface section.

Or, if using JDK 8 or later, you could easily collect into a Set using aggregate operations:

		c.stream()
.collect(Collectors.toSet()); // no duplicates


Here's a slightly longer example that accumulates a Collection of names into a TreeSet:

		Set<String> set = people.stream()
.map(Person::getName)
.collect(Collectors.toCollection(TreeSet::new));


And the following is a minor variant of the first idiom that preserves the order of the original collection while removing duplicate elements:

		Collection<Type> noDups = new LinkedHashSet<Type>(c);


The following is a generic method that encapsulates the preceding idiom, returning a Set of the same generic type as the one passed.

		public static <E> Set<E> removeDups(Collection<E> c) {
}


### Set Interface Basic Operations

The size operation returns the number of elements in the Set (its cardinality). The isEmpty method does exactly what you think it would. The add method adds the specified element to the Set if it is not already present and returns a boolean indicating whether the element was added. Similarly, the remove method removes the specified element from the Set if it is present and returns a boolean indicating whether the element was present. The iterator method returns an Iterator over the Set.

The following program prints out all distinct words in its argument list. Two versions of this program are provided. The first uses JDK 8 aggregate operations. The second uses the for-each construct.

Using JDK 8 Aggregate Operations:

		import java.util.*;
import java.util.stream.*;

public class FindDups {
public static void main(String[] args) {
Set<String> distinctWords = Arrays.asList(args).stream()
.collect(Collectors.toSet());
System.out.println(distinctWords.size()+
" distinct words: " +
distinctWords);
}
}


Using the for-each Construct:

		import java.util.*;

public class FindDups {
public static void main(String[] args) {
Set<String> s = new HashSet<String>();
for (String a : args)
System.out.println(s.size() + " distinct words: " + s);
}
}



Now run either version of the program.

		java FindDups i came i saw i left


The following output is produced:

		4 distinct words: [left, came, saw, i]


Note that the code always refers to the Collection by its interface type (Set) rather than by its implementation type. This is a strongly recommended programming practice because it gives you the flexibility to change implementations merely by changing the constructor. If either of the variables used to store a collection or the parameters used to pass it around are declared to be of the Collection's implementation type rather than its interface type, all such variables and parameters must be changed in order to change its implementation type.

Furthermore, there's no guarantee that the resulting program will work. If the program uses any nonstandard operations present in the original implementation type but not in the new one, the program will fail. Referring to collections only by their interface prevents you from using any nonstandard operations.

The implementation type of the Set in the preceding example is HashSet, which makes no guarantees as to the order of the elements in the Set. If you want the program to print the word list in alphabetical order, merely change the Set's implementation type from HashSet to TreeSet. Making this trivial one-line change causes the command line in the previous example to generate the following output.

		java FindDups i came i saw i left

4 distinct words: [came, i, left, saw]


### Set Interface Bulk Operations

Bulk operations are particularly well suited to Sets; when applied, they perform standard set-algebraic operations. Suppose s1 and s2 are sets. Here's what bulk operations do:

• s1.containsAll(s2) — returns true if s2 is a subset of s1. (s2 is a subset of s1 if set s1 contains all of the elements in s2.)
• s1.addAll(s2) — transforms s1 into the union of s1 and s2. (The union of two sets is the set containing all of the elements contained in either set.)
• s1.retainAll(s2) — transforms s1 into the intersection of s1 and s2. (The intersection of two sets is the set containing only the elements common to both sets.)
• s1.removeAll(s2) — transforms s1 into the (asymmetric) set difference of s1 and s2. (For example, the set difference of s1 minus s2 is the set containing all of the elements found in s1 but not in s2.)

To calculate the union, intersection, or set difference of two sets nondestructively (without modifying either set), the caller must copy one set before calling the appropriate bulk operation. The following are the resulting idioms.

		Set<Type> union = new HashSet<Type>(s1);

Set<Type> intersection = new HashSet<Type>(s1);
intersection.retainAll(s2);

Set<Type> difference = new HashSet<Type>(s1);
difference.removeAll(s2);


The implementation type of the result Set in the preceding idioms is HashSet, which is, as already mentioned, the best all-around Set implementation in the Java platform. However, any general-purpose Set implementation could be substituted.

Let's revisit the FindDups program. Suppose you want to know which words in the argument list occur only once and which occur more than once, but you do not want any duplicates printed out repeatedly. This effect can be achieved by generating two sets — one containing every word in the argument list and the other containing only the duplicates. The words that occur only once are the set difference of these two sets, which we know how to compute. Here's how the resulting program looks.

		import java.util.*;

public class FindDups2 {
public static void main(String[] args) {
Set<String> uniques = new HashSet<String>();
Set<String> dups    = new HashSet<String>();

for (String a : args)

// Destructive set-difference
uniques.removeAll(dups);

System.out.println("Unique words:    " + uniques);
System.out.println("Duplicate words: " + dups);
}
}


When run with the same argument list used earlier (i came i saw i left), the program yields the following output.

		Unique words:    [left, saw, came]
Duplicate words: [i]


A less common set-algebraic operation is the symmetric set difference — the set of elements contained in either of two specified sets but not in both. The following code calculates the symmetric set difference of two sets nondestructively.

		Set<Type> symmetricDiff = new HashSet<Type>(s1);
Set<Type> tmp = new HashSet<Type>(s1);
tmp.retainAll(s2);
symmetricDiff.removeAll(tmp);


### Set Interface Array Operations

The array operations don't do anything special for Sets beyond what they do for any other Collection. These operations are described in The Collection Interface section.

## The List Interface

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 the following:

• Positional access — manipulates elements based on their numerical position in the list. This includes methods such as getsetaddaddAll, and remove.
• Search — searches for a specified object in the list and returns its numerical position. Search methods include indexOf and lastIndexOf.
• Iteration — extends Iterator semantics to take advantage of the list's sequential nature. The listIterator methods provide this behavior.
• Range-view — The sublist method performs arbitrary range operations on the list.

The Java platform contains two general-purpose List implementations. ArrayList, which is usually the better-performing implementation, and LinkedList which offers better performance under certain circumstances.

### Collection Operations

The operations inherited from Collection all do about what you'd expect them to do, assuming you're already familiar with them. If you're not familiar with them from Collection, now would be a good time to read The Collection Interface section. 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 nondestructive form of this idiom, which produces a third List consisting of the second list appended to the first.

			List<Type> list3 = new ArrayList<Type>(list1);


Note that the idiom, in its nondestructive form, takes advantage of ArrayList's standard conversion constructor.

And here's an example (JDK 8 and later) that aggregates some names into a List:

			List<String> list = people.stream()
.map(Person::getName)
.collect(Collectors.toList());


Like the Set interfaceList 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 are getsetadd and remove. (The set and remove operations return the old value that is being overwritten or removed.) Other operations (indexOf and lastIndexOf) return the first or last index of the specified element in the list.

The addAll operation inserts all 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 analog of Collection's addAll operation.

Here's a little method to swap two indexed values in a List.

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


This is a polymorphic algorithm: It swaps two elements in any List, regardless of its implementation type. Here's another polymorphic algorithm that uses the preceding swap method.

			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 Java platform'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 swaps). The following 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<String> list = new ArrayList<String>();
for (String a : args)
Collections.shuffle(list, new Random());
System.out.println(list);
}
}


In fact, this program can be made even shorter and faster. The Arrays class has a static factory method called asList, which 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, because it doesn't implement the (optional) add and remove operations: Arrays are not resizable. Taking advantage of Arrays.asList and calling the library version of shuffle, which 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<String> list = Arrays.asList(args);
Collections.shuffle(list);
System.out.println(list);
}
}


### Iterators

As you'd expect, the Iterator returned by List's iterator operation returns the elements of the list in proper sequence. List also provides a richer iterator, called a ListIterator, which allows you to traverse the list in either direction, modify the list during iteration, and obtain the current position of the iterator

The three methods that ListIterator inherits from Iterator (hasNextnext, and remove) do exactly the same thing in both interfaces. The hasPrevious and the 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 backward, whereas next moves it forward.

Here's the standard idiom for iterating backward through a list.

			for (ListIterator<Type> it = list.listIterator(list.size()); it.hasPrevious(); ) {
Type t = it.previous();
...
}


Note the argument to listIterator in the preceding 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; 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. 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 following figure shows the five possible cursor positions in a list containing four elements.

Calls to next and previous can be intermixed, but you have to be a bit careful. The first call to previous 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.

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 either 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: (1) a call to previousIndex when the cursor is before the initial element returns -1 and (2) a call to nextIndex when the cursor is after the final element returns list.size(). To make all this concrete, the following is a possible implementation of List.indexOf.

			public int indexOf(E e) {
for (ListIterator<E> it = listIterator(); it.hasNext(); )
if (e == null ? it.next() == null : e.equals(it.next()))
return it.previousIndex();
return -1;
}


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

The Iterator interface provides the remove operation to remove the last element returned by next from the Collection. For ListIterator, this operation removes the last element returned by nextor previous. 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. The following polymorphic algorithm uses set to replace all occurrences of one specified value with another.

			public static <E> void replace(List<E> list, E val, E newVal) {
for (ListIterator<E> it = list.listIterator(); it.hasNext(); )
if (val == null ? it.next() == null : val.equals(it.next()))
it.set(newVal);
}


The only bit of trickiness in this example is the equality test between val and it.next. You need to special-case a val value of null 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 <E>
void replace(List<E> list, E val, List<? extends E> newVals) {
for (ListIterator<E> it = list.listIterator(); it.hasNext(); ){
if (val == null ? it.next() == null : val.equals(it.next())) {
it.remove();
for (E e : newVals)
}
}
}


### 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 up by the List on which subList was called, so changes in the former 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 can 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 preceding 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, such as the replace and shuffle examples, 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, 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 <E> List<E> dealHand(List<E> deck, int n) {
int deckSize = deck.size();
List<E> handView = deck.subList(deckSize - n, deckSize);
List<E> hand = new ArrayList<E>(handView);
handView.clear();
return hand;
}


Note that this algorithm removes the hand from the end of the deck. For many common List implementations, such as ArrayList, the performance of removing elements from the end of the list is substantially better than that of removing elements from the beginning.

The following is a program that uses the dealHand method in combination with Collections.shuffle to generate hands from a normal 52-card deck. The program takes two command-line arguments: (1) the number of hands to deal and (2) the number of cards in each hand.

			import java.util.*;

public class Deal {
public static void main(String[] args) {
if (args.length < 2) {
System.out.println("Usage: Deal hands cards");
return;
}
int numHands = Integer.parseInt(args[0]);
int cardsPerHand = Integer.parseInt(args[1]);

// Make a normal 52-card deck.
String[] suit = new String[] {
"diamonds", "clubs"
};
String[] rank = new String[] {
"ace", "2", "3", "4",
"5", "6", "7", "8", "9", "10",
"jack", "queen", "king"
};

List<String> deck = new ArrayList<String>();
for (int i = 0; i < suit.length; i++)
for (int j = 0; j < rank.length; j++)
deck.add(rank[j] + " of " + suit[i]);

// Shuffle the deck.
Collections.shuffle(deck);

if (numHands * cardsPerHand > deck.size()) {
System.out.println("Not enough cards.");
return;
}

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

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


Running the program produces output like the following.

			% java Deal 4 5

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]


Although 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 instance, the greater the probability that you'll compromise it by modifying the backing List directly or through another subListobject. Note that it is legal to modify a sublist of a sublist and to continue using the original sublist (though not concurrently).

### List Algorithms

Most polymorphic algorithms in the Collections class apply specifically to List. Having all 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 section.

• sort — 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 — randomly permutes the elements in a List.
• reverse — reverses the order of the elements in a List.
• rotate — rotates all the elements in a List by a specified distance.
• swap — swaps the elements at specified positions in a List.
• replaceAll — replaces all occurrences of one specified value with another.
• fill — overwrites every element in a List with the specified value.
• copy — copies the source List into the destination List.
• binarySearch — searches for an element in an ordered List using the binary search algorithm.
• indexOfSubList — returns the index of the first sublist of one List that is equal to another.
• lastIndexOfSubList — returns the index of the last sublist of one List that is equal to another.

## The Queue Interface

Queue is a collection for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, removal, and inspection operations. The Queue interface follows.

				public interface Queue<E> extends Collection<E> {
E element();
boolean offer(E e);
E peek();
E poll();
E remove();
}


Each Queue method exists in two forms: (1) one throws an exception if the operation fails, and (2) the other returns a special value if the operation fails (either null or false, depending on the operation). The regular structure of the interface is illustrated in the following table.

Queue Interface Structure
Type of Operation Throws exception Returns special value
Insert add(e) offer(e)
Remove remove() poll()
Examine element() peek()

Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to their values — see the Object Ordering section for details). Whatever ordering is used, the head of the queue is the element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.

It is possible for a Queue implementation to restrict the number of elements that it holds; such queues are known as bounded. Some Queue implementations in java.util.concurrent are bounded, but the implementations in java.util are not.

The add method, which Queue inherits from Collection, inserts an element unless it would violate the queue's capacity restrictions, in which case it throws IllegalStateException. The offermethod, which is intended solely for use on bounded queues, differs from add only in that it indicates failure to insert an element by returning false.

The remove and poll methods both remove and return the head of the queue. Exactly which element gets removed is a function of the queue's ordering policy. The remove and poll methods differ in their behavior only when the queue is empty. Under these circumstances, remove throws NoSuchElementException, while poll returns null.

The element and peek methods return, but do not remove, the head of the queue. They differ from one another in precisely the same fashion as remove and poll: If the queue is empty, element throws NoSuchElementException, while peek returns null.

Queue implementations generally do not allow insertion of null elements. The LinkedList implementation, which was retrofitted to implement Queue, is an exception. For historical reasons, it permits null elements, but you should refrain from taking advantage of this, because null is used as a special return value by the poll and peek methods.

Queue implementations generally do not define element-based versions of the equals and hashCode methods but instead inherit the identity-based versions from Object.

The Queue interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the interface java.util.concurrent.BlockingQueue, which extends Queue.

In the following example program, a queue is used to implement a countdown timer. The queue is preloaded with all the integer values from a number specified on the command line to zero, in descending order. Then, the values are removed from the queue and printed at one-second intervals. The program is artificial in that it would be more natural to do the same thing without using a queue, but it illustrates the use of a queue to store elements prior to subsequent processing.

				import java.util.*;

public class Countdown {
public static void main(String[] args) throws InterruptedException {
int time = Integer.parseInt(args[0]);

for (int i = time; i >= 0; i--)

while (!queue.isEmpty()) {
System.out.println(queue.remove());
}
}
}


In the following example, a priority queue is used to sort a collection of elements. Again this program is artificial in that there is no reason to use it in favor of the sort method provided in Collections, but it illustrates the behavior of priority queues.

				static <E> List<E> heapSort(Collection<E> c) {
Queue<E> queue = new PriorityQueue<E>(c);
List<E> result = new ArrayList<E>();

while (!queue.isEmpty())

return result;
}


## The Deque Interface

Usually pronounced as deck, a deque is a double-ended-queue. A double-ended-queue is a linear collection of elements that supports the insertion and removal of elements at both end points. The Dequeinterface is a richer abstract data type than both Stack and Queue because it implements both stacks and queues at the same time. The Deque interface, defines methods to access the elements at both ends of the Deque instance. Methods are provided to insert, remove, and examine the elements. Predefined classes like ArrayDeque and LinkedList implement the Deque interface

Note that the Deque interface can be used both as last-in-first-out stacks and first-in-first-out queues. The methods given in the Deque interface are divided into three parts:

### Insert

The addfirst and offerFirst methods insert elements at the beginning of the Deque instance. The methods addLast and offerLast insert elements at the end of the Deque instance. When the capacity of the Deque instance is restricted, the preferred methods are offerFirst and offerLast because addFirst might fail to throw an exception if it is full.

### Remove

The removeFirst and pollFirst methods remove elements from the beginning of the Deque instance. The removeLast and pollLast methods remove elements from the end. The methods pollFirst and pollLast return null if the Deque is empty whereas the methods removeFirst and removeLast throw an exception if the Deque instance is empty.

### Retrieve

The methods getFirst and peekFirst retrieve the first element of the Deque instance. These methods dont remove the value from the Deque instance. Similarly, the methods getLast and peekLastretrieve the last element. The methods getFirst and getLast throw an exception if the deque instance is empty whereas the methods peekFirst and peekLast return NULL.

The 12 methods for insertion, removal and retieval of Deque elements are summarized in the following table:

Deque Methods
Type of Operation First Element (Beginning of the Deque instance) Last Element (End of the Deque instance)
Insert addFirst(e)
offerFirst(e)
addLast(e)
offerLast(e)
Remove removeFirst()
pollFirst()
removeLast()
pollLast()
Examine getFirst()
peekFirst()
getLast()
peekLast()

In addition to these basic methods to insert,remove and examine a Deque instance, the Deque interface also has some more predefined methods. One of these is removeFirstOccurence, this method removes the first occurence of the specified element if it exists in the Deque instance. If the element does not exist then the Deque instance remains unchanged. Another similar method is removeLastOccurence; this method removes the last occurence of the specified element in the Deque instance. The return type of these methods is boolean, and they return true if the element exists in the Deque instance.

## The Map Interface

Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. It models the mathematical function abstraction. The Map interface includes methods for basic operation

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