I would like to convert an array to a Set in Java. There are some obvious ways of doing this (i.e. with a loop) but I would like something a bit neater, something like:
java.util.Arrays.asList(Object[] a);
Any ideas?
This question is related to
java
collections
arrays
set
Use CollectionUtils
or ArrayUtils
from stanford-postagger-3.0.jar
import static edu.stanford.nlp.util.ArrayUtils.asSet;
or
import static edu.stanford.nlp.util.CollectionUtils.asSet;
...
String [] array = {"1", "q"};
Set<String> trackIds = asSet(array);
In Eclipse Collections, the following will work:
Set<Integer> set1 = Sets.mutable.of(1, 2, 3, 4, 5);
Set<Integer> set2 = Sets.mutable.of(new Integer[]{1, 2, 3, 4, 5});
MutableSet<Integer> mutableSet = Sets.mutable.of(1, 2, 3, 4, 5);
ImmutableSet<Integer> immutableSet = Sets.immutable.of(1, 2, 3, 4, 5);
Set<Integer> unmodifiableSet = Sets.mutable.of(1, 2, 3, 4, 5).asUnmodifiable();
Set<Integer> synchronizedSet = Sets.mutable.of(1, 2, 3, 4, 5).asSynchronized();
ImmutableSet<Integer> immutableSet = Sets.mutable.of(1, 2, 3, 4, 5).toImmutable();
Note: I am a committer for Eclipse Collections
Set<T> b = new HashSet<>(Arrays.asList(requiredArray));
Quickly : you can do :
// Fixed-size list
List list = Arrays.asList(array);
// Growable list
list = new LinkedList(Arrays.asList(array));
// Duplicate elements are discarded
Set set = new HashSet(Arrays.asList(array));
and to reverse
// Create an array containing the elements in a list
Object[] objectArray = list.toArray();
MyClass[] array = (MyClass[])list.toArray(new MyClass[list.size()]);
// Create an array containing the elements in a set
objectArray = set.toArray();
array = (MyClass[])set.toArray(new MyClass[set.size()]);
We have the option of using Stream
as well. We can get stream in various ways:
Set<String> set = Stream.of("A", "B", "C", "D").collect(Collectors.toCollection(HashSet::new));
System.out.println(set);
String[] stringArray = {"A", "B", "C", "D"};
Set<String> strSet1 = Arrays.stream(stringArray).collect(Collectors.toSet());
System.out.println(strSet1);
// if you need HashSet then use below option.
Set<String> strSet2 = Arrays.stream(stringArray).collect(Collectors.toCollection(HashSet::new));
System.out.println(strSet2);
The source code of Collectors.toSet()
shows that elements are added one by one to a HashSet
but specification does not guarantee it will be a HashSet
.
"There are no guarantees on the type, mutability, serializability, or thread-safety of the Set returned."
So it is better to use the later option. The output is:
[A, B, C, D]
[A, B, C, D]
[A, B, C, D]
Java 9 introduced Set.of
static factory method which returns immutable set for the provided elements or the array.
@SafeVarargs
static <E> Set<E> of?(E... elements)
Check Immutable Set Static Factory Methods for details.
We can also get an immutable set in two ways:
Set.copyOf(Arrays.asList(array))
Arrays.stream(array).collect(Collectors.toUnmodifiableList());
The method Collectors.toUnmodifiableList()
internally makes use of Set.of
introduced in Java 9. Also check this answer of mine for more.
After you do Arrays.asList(array)
you can execute Set set = new HashSet(list);
Here is a sample method, you can write:
public <T> Set<T> GetSetFromArray(T[] array) {
return new HashSet<T>(Arrays.asList(array));
}
new HashSet<Object>(Arrays.asList(Object[] a));
But I think this would be more efficient:
final Set s = new HashSet<Object>();
for (Object o : a) { s.add(o); }
Set<T> mySet = new HashSet<T>();
Collections.addAll(mySet, myArray);
That's Collections.addAll(java.util.Collection, T...) from JDK 6.
Additionally: what if our array is full of primitives?
For JDK < 8, I would just write the obvious for
loop to do the wrap and add-to-set in one pass.
For JDK >= 8, an attractive option is something like:
Arrays.stream(intArray).boxed().collect(Collectors.toSet());
Varargs will work too!
Stream.of(T... values).collect(Collectors.toSet());
In Java 10:
String[] strs = {"A", "B"};
Set<String> set = Set.copyOf(Arrays.asList(strs));
Set.copyOf
returns an unmodifiable Set
containing the elements of the given Collection
.
The given Collection
must not be null
, and it must not contain any null
elements.
Java 8:
String[] strArray = {"eins", "zwei", "drei", "vier"};
Set<String> strSet = Arrays.stream(strArray).collect(Collectors.toSet());
System.out.println(strSet);
// [eins, vier, zwei, drei]
Sometime using some standard libraries helps a lot. Try to look at the Apache Commons Collections. In this case your problems is simply transformed to something like this
String[] keys = {"blah", "blahblah"}
Set<String> myEmptySet = new HashSet<String>();
CollectionUtils.addAll(pythonKeywordSet, keys);
And here is the CollectionsUtils javadoc
With Guava you can do:
T[] array = ...
Set<T> set = Sets.newHashSet(array);
For anyone solving for Android:
The asterisk *
is the spread
operator. It applies all elements in a collection individually, each passed in order to a vararg
method parameter. It is equivalent to:
val myArray = arrayOf("data", "foo")
val mySet = setOf(*myArray)
// Equivalent to
val mySet = setOf("data", "foo")
// Multiple spreads ["data", "foo", "bar", "data", "foo"]
val mySet = setOf(*myArray, "bar", *myArray)
Passing no parameters setOf()
results in an empty set.
In addition to setOf
, you can also use any of these for a specific hash type:
hashSetOf()
linkedSetOf()
mutableSetOf()
sortableSetOf()
This is how to define the collection item type explicitly.
setOf<String>()
hashSetOf<MyClass>()
I've written the below from the advice above - steal it... it's nice!
/**
* Handy conversion to set
*/
public class SetUtil {
/**
* Convert some items to a set
* @param items items
* @param <T> works on any type
* @return a hash set of the input items
*/
public static <T> Set<T> asSet(T ... items) {
return Stream.of(items).collect(Collectors.toSet());
}
}
private Map<Integer, Set<Integer>> nobreaks = new HashMap();
nobreaks.put(1, new HashSet(Arrays.asList(new int[]{2, 4, 5})));
System.out.println("expected size is 3: " +nobreaks.get(1).size());
the output is
expected size is 3: 1
change it to
nobreaks.put(1, new HashSet(Arrays.asList( 2, 4, 5 )));
the output is
expected size is 3: 3
There has been a lot of great answers already, but most of them won't work with array of primitives (like int[]
, long[]
, char[]
, byte[]
, etc.)
In Java 8 and above, you can box the array with:
Integer[] boxedArr = Arrays.stream(arr).boxed().toArray(Integer[]::new);
Then convert to set using stream:
Stream.of(boxedArr).collect(Collectors.toSet());
Source: Stackoverflow.com