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HashSet
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| Java HashSet |
Introduction
Previously, we have seen in our module on Java the arrays and ArrayLists. Both Arrays and ArrayLists can contain an element greater than one.
Now in this unit, we shall see the HashSet that holds unique items.
1. HashSet
There is also Java built-in class, HashSet, much like the set in Python, which gives the functionalism of a set.
A HashSet is an unordered collection with unique elements that should not be repeated.
A HashSet class is from package java.util.
HashSet contains only objects like String, Integer, etc., just like ArrayList.
Declaration of HashSet
The declaration of HashSet works with the following syntax:
Syntax
HashSet<Type> hset = new HashSet<>();
Here,
HashSet is the class name
hset is the name of the variable
Type is the data type of the hset
Now let us create a HashSet called players, with type String.
Code
import java. util.HashSet.
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
}
}
This code shows that we have created an empty HashSet which has assigned to the players variable.
2. Add Stuff to HashSet
Add element in HashSet using the add() function which is exactly like in Python for adding a single element to the HashSet.
If the element is not already in the collection, it is added to the collection and true is returned. Otherwise , it does not add it to the set and returns false.
HashSet behaves pretty much the same as an ArrayList in that automatically resizes whenever an element is added or removed.
Syntax
hset.add(element);
Where,
hset is the variable name
element is the element to be added
Example 1:
Code
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
System.out.println(players);
}
}
Output:
[Rohit, Virat, Sachin]
It can print a HashSet similar to an ArrayList.
Example 2:
Code
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
boolean isAdded = players.add("Rohit");
System.out.println(isAdded);
System.out.println(players);
isAdded = players.add("Rohit");
System.out.println(isAdded);
System.out.println(players);
}
}
Output:
true
[Rohit]
false
[Rohit]
This is because initially, it treats the Hashset as empty. Hence, one call to add() added the string Rohit to the HashSet and returned true; next time, even though the string was added to HashSet at the first time, this time it returned false because already the given string in add was in collection.
3. Removing Elements from HashSet
Two methods may remove an element from HashSet:
remove()
clear()
remove()
The remove() method removes an element from the HashSet.
It removes the element and returns true if the HashSet contains the specified element, otherwise it returns false.
Syntax
hset.remove(element);
Here element is a value to be removed from the HashSet.
Code
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
System.out.println("Before Removing: " + players);
boolean isRemoved1 = players.remove("Virat");
boolean isRemoved2 = players.remove("Kane");
System.out.println(isRemoved1);
System.out.println(isRemoved2);
System.out.println("After Removing: " + players);
}
}
Output:
Before Removing: [Rohit, Virat, Sachin]
true
false
After Removing: [Rohit, Sachin]
In the above code, Virat is present in the players HashSet and Kane is not present. Hence, isRemoved1 contains true, isRemoved2 contains false, and Virat is removed from the HashSet.
clear()
The clear() method removes all the elements from a HashSet.
It only removes the elements from the HashSet and does not delete the HashSet.
Syntax
hset.clear();
Code:
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
System.out.println("Before Clearing: " + players);
players.clear();
System.out.println("After Clearing: " + players);
}
}
Output:
Before Clearing: [Rohit, Virat, Sachin]
After Clearing: []
4. Searching in a HashSet.
The contains() method checks whether some element is a part of the given HashSet. If the element is found, then it returns true; otherwise, false is returned.
Syntax
hset.contains(element);
Here, element is a value to be searched in HashSet.
Code
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
boolean isPresent1 = players.contains("Virat");
boolean isPresent2 = players.contains("Kane");
System.out.println(isPresent1);
System.out.println(isPresent2);
}
}
Output:
true
false
5. Size of HashSet
The method size() is used to find the size of a HashSet.
Syntax
hset.size();
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
System.out.println(players);
System.out.println(players.size());
}
}
Output:
[Rohit, Virat, Sachin]
3
6. Iterating Through a HashSet
You can also iterate through a HashSet by using the for-each loop, just like iterating through an Array or ArrayList.
Code
import java.util.HashSet;
class Main {
public static void main(String[] args) {
HashSet<String> players = new HashSet<>();
players.add("Rahul");
players.add("Rohit");
players.add("Virat");
players.add("Sachin");
for (String name : players) {
System.out.println(name);
}
}
}
Output:
Rohit
Rahul
Virat
Sachin
Summary
HashSet is an unordered collection of elements. HashSet elements are unique.
HashSet Methods:
add(): adds a single element to a HashSet.
remove(): removes an element from a given HashSet. It returns true if a HashSet contains the specified element, otherwise false.
clear(): removes all the elements from a HashSet.
contains(element): checks if an element is present in a given HashSet. It returns true if the element is found, otherwise false.
size(): finds the size of a HashSet.
The for-each loop can be used to iterate over a HashSet.
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