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Java : HashSet with Examples

January 18, 2025 June 23, 2022

HashSet (Java SE 23 & JDK 23) in Java with Examples.
You will find code samples for most of the HashSet<E> methods.


Summary

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.

HashSet (Java SE 23 & JDK 23)

Class diagram

final var set = new HashSet<String>();
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

System.out.println(set.contains("a")); // true
System.out.println(set.contains("b")); // true
System.out.println(set.contains("X")); // false

Constructors

HashSet ()

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

HashSet (int initialCapacity)

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>(10000000);
System.out.println(set); // []

final var startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {
    set.add(String.valueOf(i));
}

final var endTime = System.nanoTime();

// 0.051620 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);

final var set = new HashSet<String>(1);
System.out.println(set); // []

final var startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {
    set.add(String.valueOf(i));
}

final var endTime = System.nanoTime();

// 0.083717 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);

HashSet (int initialCapacity, float loadFactor)

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>(16, 0.05f);
System.out.println(set); // []

final var startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {
    set.add(String.valueOf(i));
}

final var endTime = System.nanoTime();

// 0.130706 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);

final var set = new HashSet<String>(16, 0.75f);
System.out.println(set); // []

final var startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {
    set.add(String.valueOf(i));
}

final var endTime = System.nanoTime();

// 0.101407 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);

HashSet (Collection<? extends E> c)

Constructs a new set containing the elements in the specified collection.

HashSet (Java SE 23 & JDK 23) 
final var c = List.of("a", "b", "c");

final var set = new HashSet<>(c);
System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

Methods

boolean add (E e)

Adds the specified element to this set if it is not already present.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
System.out.println(set); // []

System.out.println(set.add("a")); // true
System.out.println(set); // [a]

System.out.println(set.add("b")); // true
System.out.println(set); // [a, b]

System.out.println(set.add("c")); // true
System.out.println(set); // [a, b, c]

System.out.println(set.add("a")); // false
System.out.println(set); // [a, b, c]

void clear ()

Removes all of the elements from this set.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
System.out.println(set); // []

set.add("a");
System.out.println(set); // [a]

set.add("b");
System.out.println(set); // [a, b]

set.clear();
System.out.println(set); // []

Object clone ()

Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]

final var cloned = set.clone();
System.out.println(cloned); // [a, b, c]
System.out.println(cloned.getClass()); // class java.util.HashSet

boolean contains (Object o)

Returns true if this set contains the specified element.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

System.out.println(set.contains("")); // false
System.out.println(set.contains("a")); // true
System.out.println(set.contains("b")); // true
System.out.println(set.contains("c")); // true

System.out.println(set.contains("X")); // false
System.out.println(set.contains("abc")); // false

boolean isEmpty ()

Returns true if this set contains no elements.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.isEmpty()); // false

set.clear();
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

Iterator<E> iterator ()

Returns an iterator over the elements in this set.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
Collections.addAll(set, "aaa", "bbb", "ccc");
System.out.println(set); // [aaa, ccc, bbb]

set.iterator().forEachRemaining(System.out::println);

// Result
// ↓
//aaa
//ccc
//bbb

static <T> HashSet<T> newHashSet (int numElements)

Creates a new, empty HashSet suitable for the expected number of elements.

HashSet (Java SE 23 & JDK 23) 
final var set = HashSet.<String>newHashSet(3);
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

boolean remove (Object o)

Removes the specified element from this set if it is present.

HashSet (Java SE 23 & JDK 23) 
final var src = Set.of("a", "b", "c");

{
    final var set = new HashSet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("")); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new HashSet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("a")); // true
    System.out.println(set); // [b, c]
}
{
    final var set = new HashSet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("b")); // true
    System.out.println(set); // [a, c]
}
{
    final var set = new HashSet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("X")); // false
    System.out.println(set); // [a, b, c]
}

int size ()

Returns the number of elements in this set (its cardinality).

HashSet (Java SE 23 & JDK 23) 
final var set1 = new HashSet<String>();
System.out.println(set1); // []
System.out.println(set1.size()); // 0

final var set2 = new HashSet<String>();
Collections.addAll(set2, "a", "b", "c");
System.out.println(set2); // [a, b, c]
System.out.println(set2.size()); // 3

Spliterator<E> spliterator ()

Creates a late-binding and fail-fast Spliterator over the elements in this set.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
Collections.addAll(set, "aaa", "bbb", "ccc");
System.out.println(set); // [aaa, ccc, bbb]

final var spliterator = set.spliterator();
spliterator.forEachRemaining(System.out::println);

// Result
// ↓
//aaa
//ccc
//bbb

Object[] toArray ()

Returns an array containing all of the elements in this collection.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

final Object[] array = set.toArray();
System.out.println(Arrays.toString(array)); // [a, b, c]

<T> T[] toArray (T[] a)

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

HashSet (Java SE 23 & JDK 23) 
final var set = new HashSet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

final String[] ret = set.toArray(new String[0]);
System.out.println(Arrays.toString(ret)); // [a, b, c]

final var set = new HashSet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

{
    final String[] array = new String[3];
    System.out.println(Arrays.toString(array)); // [null, null, null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [a, b, c]
    System.out.println(Arrays.toString(ret)); // [a, b, c]
}
{
    final String[] array = new String[5];
    System.out.println(Arrays.toString(array)); // [null, null, null, null, null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [a, b, c, null, null]
    System.out.println(Arrays.toString(ret)); // [a, b, c, null, null]
}
{
    final String[] array = new String[1];
    System.out.println(Arrays.toString(array)); // [null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [null]
    System.out.println(Arrays.toString(ret)); // [a, b, c]
}

Methods declared in AbstractSet

equals, hashCode, removeAll

Please see the link below.

Methods declared in AbstractCollection

addAll, containsAll, retainAll, toArray, toArray, toString

Please see the link below.

Methods declared in Collection

parallelStream, removeIf, stream, toArray

Please see the link below.

Methods declared in Iterable

forEach

Please see the link below.

Methods declared in Set

addAll, containsAll, equals, hashCode, removeAll, retainAll

Please see the link below.


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