Java HashSet
In a HashSet, every item is unique which is known as collection. It is found in the java.util packages. This collection uses a hash table for storage. It inherits the AbstractSet class and implements Set interface. A list can contain duplicate elements whereas Set contains unique elements only.
The importantfeatures about Java HashSet class are:
- It stores the elements by using a mechanism called hashing.
- It contains unique elements only.
- It allows null value.
- It class is none synchronized.
- It doesn’t maintain the insertion order. Here, elements are inserted on the basis of their hashcode.
- It is the best approach for search operations.
Java HashSet Declaration:
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable
Constructors of Java HashSet
- HashSet():It is used to construct a default HashSet.
- HashSet(int capacity):It is used to initialize the capacity of the hash set to the given integer value capacity. The capacity grows automatically as elements are added to the HashSet.
- HashSet(int capacity, float loadFactor):It is used to initialize the capacity of the hash set to the given integer value capacity and the specified load factor.
- HashSet(Collection<? extends E> c):It is used to initialize the hash set by using the elements of the collection c.
Methods of Java HashSet class
- add(E e):It is used to add the specified element to this set if it is not already present.
- clear():It is used to remove all of the elements from the set.
- object clone(): It is used to return a shallow copy of this HashSet instance: the elements themselves are not cloned.
- contains(Object o):It is used to return true if this set contains the specified element.
- isEmpty():It is used to return true if this set contains no elements.
- iterator():It is used to return an iterator over the elements in this set.
- remove(Object o):It is used to remove the specified element from this set if it is present.
- int size():It is used to return the number of elements in the set.
Example
//Create a HashSet object called letters that will store strings:
import java.util.HashSet; // Import the HashSet class
HashSet<String> lettes = new HashSet<String>();
Add Items: The HashSet class has many useful methods. For example, to add items to it, use the add() method:
Example
// Import the HashSet class
import java.util.HashSet;
public class Main {
public static void main(String[] args) {
HashSet<String> lettes = new HashSet<String>();
lettes.add("A");
lettes.add("B");
lettes.add("C");
lettes.add("B");
lettes.add("E");
System.out.println(lettes);
}
}
OUTPUT:
A B C E
In the example above, even though Bis added twice it only appears once in the set because every item in a set has to be unique.
- Check If an Item Exists: In order to check whether an item exists in a HashSet, use the contains() method:
Example
lettes.contains(“B”);
- Remove an Item: In order to remove an item, use the remove() method:
Example
lettes.remove(“A”);
To remove all items, use the clear() method:
Example
lettes.clear();
HashSet Size
In order to find out how many items there are, use the size method:
Example
lettes.size();
HashSet Loop: Loop through the items of an HashSet with a for-each loop:
Example
for (String i : letters) {
System.out.println(i);
}
Other Types:
In order To use other types, such as int, you must specify an equivalent wrapper class: Integer. For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:
Example
Use a HashSet that stores Integer objects:
import java.util.HashSet;
public class ExMain {
public static void main(String[] args) {
// Create a HashSet object called numbers
HashSet<Integer> numbers = new HashSet<Integer>();
// Add values to the set
numbers.add(4);
numbers.add(7);
numbers.add(8);
// Show which numbers between 1 and 10 are in the set
for(int i = 1; i <= 10; i++) {
if(numbers.contains(i)) {
System.out.println(i + " was found!");
} else {
System.out.println(i + " was not found!");
}
}
}
}
OUTPUT
1 was not found!
2 was not found!
3 was not found!
4 was found!
5 was not found!
6 was not found!
7 was found!
8 was found!
9 was not found!
10 was not found!