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.

1. 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]

3KB

GLAB - 303.11.5 - HashSet and TreeSet Processing.zip

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