HashSet and TreeSet Processing
Objective
In this lab, we will explore and demonstrate HashSet and TreeSet. We will utilize the built-in methods.
Learning Objective
By the end of this lab, learners will be able to explain and use HashSet and TreeSet.
HashSet Examples
Example One: Insert Elements to HashSet using addAll().
addAll() - Inserts all of the elements of the specified collection to the set. Create a new Java project and create a new Class named “exampleOne.” Write the code below in the class.
import java.util.HashSet;
public class exampleOne {
public static void main(String[] args) {
HashSet<Integer> evenNumber = new HashSet<>();
// Using add() method
evenNumber.add(2);
evenNumber.add(4);
evenNumber.add(6);
System.out.println("HashSet: " + evenNumber);
HashSet<Integer> numbers = new HashSet<>();
// Using addAll() method
numbers.addAll(evenNumber);
numbers.add(5);
System.out.println("New HashSet: " + numbers);
}
}
Output:
HashSet: [2, 4, 6]
New HashSet: [2, 4, 5, 6]
Example Two: Union of Sets.
To perform the union between two sets, we can use the addAll() method.
Create a new Class named “exampletwo,” and then write the code below in the class.
import java.util.HashSet;
public class exampletwo {
public static void main(String[] args) {
HashSet<Integer> evenNumbers = new HashSet<>();
evenNumbers.add(2);
evenNumbers.add(4);
System.out.println("HashSet1: " + evenNumbers);
HashSet<Integer> numbers = new HashSet<>();
numbers.add(1);
numbers.add(3);
System.out.println("HashSet2: " + numbers);
// Union of two set
numbers.addAll(evenNumbers);
System.out.println("Union is: " + numbers);
}
}
Output
HashSet1: [2, 4]
HashSet2: [1, 3]
Union is: [1, 2, 3, 4]
Example Three: Difference of Sets.
To calculate the difference between the two sets, we can use the removeAll() method: Create a new Class named “examplethree,” and then write the code below in the class.
import java.util.HashSet;
public class examplethree {
public static void main(String[] args) {
HashSet<Integer> primeNumbers = new HashSet<>();
primeNumbers.add(2);
primeNumbers.add(3);
primeNumbers.add(5);
System.out.println("HashSet1: " + primeNumbers);
HashSet<Integer> oddNumbers = new HashSet<>();
oddNumbers.add(1);
oddNumbers.add(3);
oddNumbers.add(5);
System.out.println("HashSet2: " + oddNumbers);
// Difference between HashSet1 and HashSet2
primeNumbers.removeAll(oddNumbers);
System.out.println("Difference : " + primeNumbers);
}
}
Output:
HashSet1: [2, 3, 5]
HashSet2: [1, 3, 5]
Difference: [2]
Example Four: Enhanced for() Loop
Create a new Class named “examplefour,” and then write the code below in the class.
import java.util.HashSet;
public class Examplefour {
public static void main(String args[]) {
// HashSet declaration
HashSet<String> hset = new HashSet<String>();
// Adding elements to the HashSet
hset.add("Apple");
hset.add("Mango");
hset.add("Grapes");
hset.add("Orange");
hset.add("Fig");
//Addition of duplicate elements
hset.add("Apple");
hset.add("Mango");
//Addition of null values
hset.add(null);
hset.add(null);
// Using advanced for loop
for (String str : hset) {
System.out.println(" ---> " + str);
}
}
}
Output:
---> null
---> Apple
---> Grapes
---> Fig
---> Mango
---> Orange
Remember: HashSet DOES NOT maintain any order, so this order might be changed if you run your code a second time.
TreeSet Examples
Example one: Iterate Through TreeSet.
To access the individual elements of TreeSet, we need to iterate through the TreeSet —— traverse through the TreeSet. We do this by declaring an Iterator for the TreeSet, and then use that Iterator to access each element. For this, we use the next() method of an iterator that returns the next element in the TreeSet.
The following Java program demonstrates the use of the Iterator to iterate through TreeSet.
import java.util.TreeSet;
import java.util.Iterator;
public class TreesetExampleone {
public static void main(String[] args) {
TreeSet<Integer> num_Treeset = new TreeSet<>();
num_Treeset.add(20);
num_Treeset.add(5);
num_Treeset.add(15);
num_Treeset.add(25);
num_Treeset.add(10);
// Call iterator() method to define Iterator for TreeSet
Iterator<Integer> iter_set = num_Treeset.iterator();
System.out.print("TreeSet using Iterator: ");
// Access TreeSet elements using Iterator
while(iter_set.hasNext()) {
System.out.print(iter_set.next());
System.out.print(", ");
}
}
}
Output;
TreeSet using Iterator: 5, 10, 15, 20, 25,
Example two: Remove Elements
remove() - removes the specified element from the set. removeAll() - removes all of the elements from the set.
import java.util.TreeSet;
public class TreesetExampletwo {
public static void main(String[] args) {
TreeSet<Integer> numbers = new TreeSet<>();
numbers.add(2);
numbers.add(5);
numbers.add(6);
System.out.println("TreeSet: " + numbers);
// Using the remove() method
boolean value1 = numbers.remove(5);
System.out.println("Is 5 removed? " + value1);
// Using the removeAll() method
boolean value2 = numbers.removeAll(numbers);
System.out.println("Are all elements removed? " + value2);
}
}
Output:
TreeSet: [2, 5, 6]
Is 5 removed? true
Are all elements removed? true
Example three: Methods for Navigation.
Since the TreeSet class implements NavigableSet, it provides various methods to navigate over the elements of the TreeSet.
first() and last() Methods
first()- returns the first element of the set.last()- returns the last element of the set.
import java.util.TreeSet;
public class TreesetExamplethree {
public static void main(String[] args) {
TreeSet<Integer> numbers = new TreeSet<>();
numbers.add(2);
numbers.add(5);
numbers.add(6);
System.out.println("TreeSet: " + numbers);
// Using the first() method
int first = numbers.first();
System.out.println("First Number: " + first);
// Using the last() method
int last = numbers.last();
System.out.println("Last Number: " + last);
}
}
Output:
TreeSet: [5, 10, 15, 20, 25]
First Number: 5
Last Number: 25
Example four: sort the given TreeSet alphabetically in reverse order.
In this example, we will implement a Comparator class to sort the given TreeSet alphabetically in reverse order. By default, the TreeSet sorts data in ascending order. We can also sort TreeSet in a customized order by defining a new comparator class. In this comparator class, we need to override the ‘compare’ method to sort the elements of the TreeSet. This comparator object is then passed to the TreeSet constructor.
Create a class named “cities_Comparator” and add the code below:
import java.util.Comparator;
public class cities_Comparator implements Comparator<String> {
//override compare method to compare two elements of the TreeSet
@Override
public int compare(String cities_one, String cities_two) {
int value = cities_one.compareTo(cities_two);
// sort elements in reverse order
if (value > 0) {
return -1;
}
else if (value < 0) {
return 1;
}
else {
return 0;
}
}
}
Create a class named “TreeSetExampleCom” and add the code below:
import java.util.TreeSet;
public class TreeSetExampleCom {
public static void main(String[] args) {
// Create a TreeSet with user-defined comparator
TreeSet<String> cities = new TreeSet<>(new cities_Comparator());
//add elements to the comparator
cities.add("UAE");
cities.add("Mumbai");
cities.add("NewYork");
cities.add("Hyderabad");
cities.add("Karachi");
cities.add("Xanadu");
cities.add("Lahore");
cities.add("Zagazig");
cities.add("Yingkou");
//print the contents of TreeSet
System.out.println("TreeSet: " + cities);
}
}
Output:
TreeSet: [Zagazig, Yingkou, Xanadu, UAE, NewYork, Mumbai, Lahore, Karachi, Hyderabad]Last updated