Java HashSet
Java HashSet
A HashSet is a collection of items where every item is unique, and it is found in the java.util
package:
Example
Create a HashSet object called cars that will store strings:
import java.util.HashSet; // Import the HashSet class
HashSet<String> cars = 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> cars = new HashSet<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("BMW");
cars.add("Mazda");
System.out.println(cars);
}
}
Note: In the example above, even though BMW is added twice it only appears once in the set because every item in a set has to be unique.
Check If an Item Exists
To check whether an item exists in a HashSet, use the contains() method:
Remove an Item
To remove an item, use the remove() method:
To remove all items, use the clear() method:
HashSet Size
To find out how many items there are, use the size method:
Loop Through a HashSet
Loop through the items of an HashSet with a for-each loop:
Other Types
Items in an HashSet are actually objects. In the examples above, we created
items
(objects) of type "String". Remember that a String in Java is an object (not a primitive type). 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 Main {
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 in the set.");
} else {
System.out.println(i + " was not found in the set.");
}
}
}
}
