The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
A
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: HashSet
, TreeSet
, and LinkedHashSet
.
HashSet
, 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 HashSet
.
LinkedHashSet
, 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 Collection
, c
, 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) { return new LinkedHashSet<E>(c); }
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) s.add(a); 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]
Bulk operations are particularly well suited to Set
s; 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); union.addAll(s2); 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) if (!uniques.add(a)) dups.add(a); // 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); symmetricDiff.addAll(s2); Set<Type> tmp = new HashSet<Type>(s1); tmp.retainAll(s2); symmetricDiff.removeAll(tmp);
The array operations don't do anything special for Set
s beyond what they do for any other Collection
. These operations are described in
The Collection Interface section.