Sort Java Vector in Descending Order using Comparator

Vectors implement the List interface and are used to create dynamic arrays. The array whose size is not fixed and can grow as per our needs is called as a dynamic array. The Comparator is an interface available in ‘java.util’ package.

Sorting means rearranging the elements of a given list or array in ascending or descending order. In this article, we will create a vector and then try to sort its elements in descending order using a comparator.

Program to Sort Java Vector in Descending Order

Comparator

As the name suggests it is used to compare something. In Java, the Comparator is an interface that is used to sort custom objects. We can write our own logic inside its inbuilt method named ‘compare()’ to sort the specified objects. This method takes two objects as arguments and then returns an integer value. By this integer value, Comparator decides which object is greater.

Syntax

Comparator< TypeOfComparator > nameOfComparator = new Comparator< TypeOfComparator >() {
   compare( type object1, type object1 ) {
      // logic for comparison
   }
};

The nameOfComparator passed for sorting operation to methods like ‘Collection.sort()’.

Collections.sort() method

The class ‘Collections’ of the Collection Interface provides a static method named ‘Collections.sort()’ that can sort elements of specified collections like ArrayList or LinkedList. It is available in ‘java.util’ package.

Syntax

Collections.sort( nameOfcollection, ComparatorObject );

Collections.reverseOrder()

It returns the comparator in reverse order.

Example 1

In the following example, we will define a vector named ‘vectlist’ and store a few objects in it by using the ‘add()’ method. Then, use the Comparator object and ‘Collection.sort()’ method to sort the vector in descending order.

import java.util.*;
public class VectClass {
   public static void main(String args[]) {
      // Creation of vector 
      Vector<Integer> vectList = new Vector<>();
      // Adding elements in the vector
      vectList.add(97);
      vectList.add(93);
      vectList.add(95);
      vectList.add(99);
      vectList.add(82);
      vectList.add(88);
      System.out.println("Elements of the unsorted list: ");
      // loop to iterate through elements
      for(int i = 0; i < vectList.size(); i++ ) {
         // to print the elements of the vector
         System.out.print(vectList.get(i) + " "); 
      }
      System.out.println();
      // Using comparator interface for sorting
      Comparator comp = Collections.reverseOrder();
      Collections.sort(vectList, comp);
      System.out.println("Elements of the newly sorted list: ");
      // loop to iterate through elements
      for(int i = 0; i < vectList.size(); i++ ) {
         // to print the elements of the new vector
		   System.out.print(vectList.get(i) + " "); 
      }
   }
}

Output

Note: VectClass.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Elements of the unsorted list: 
97 93 95 99 82 88 
Elements of the newly sorted list: 
99 97 95 93 88 82 

Example 2

In this example, first, we will create a Comparator and inside it, we define our logic in ‘compare()’ method to sort the vector objects in descending order. The logic here states that take two objects at the same time and compare them using the if-else block. If first object is greater than second return -1 otherwise 1. Then, we pass the object of comparator to ‘Collection.sort()’ for sorting operation.

import java.util.*;
public class VectClass {
   public static void main(String args[]) {
      // Using comparator interface for sorting
      Comparator<Integer> comp = new Comparator<Integer>() {
         // logic to sort in descending order
         public int compare(Integer i, Integer j) {
            if(i < j) {
               return 1;
            } else {
               return -1;
            }
         }
      };
      // Creation of vector 
      Vector<Integer> vectList = new Vector<>();
      // Adding elements in the vector
      vectList.add(97);
      vectList.add(93);
      vectList.add(95);
      vectList.add(99);
      vectList.add(82);
      vectList.add(88);
      System.out.println("Elements of the unsorted list: ");
      // loop to iterate through elements
      for(int i = 0; i < vectList.size(); i++ ) {
         // to print the elements of the vector
		   System.out.print(vectList.get(i) + " "); 
      }
      System.out.println();
      Collections.sort(vectList, comp); // sort using comparator
      System.out.println("Elements of the newly sorted list: ");
      // loop to iterate through elements
      for(int i = 0; i < vectList.size(); i++ ) {
         // to print the elements of the new vector
         System.out.print(vectList.get(i) + " "); 
      }
   }
}

Output

Elements of the unsorted list: 
97 93 95 99 82 88 
Elements of the newly sorted list: 
99 97 95 93 88 82 

Conclusion

This article has explained the implementation of the Comparator Interface and also we discovered the use of a few inbuilt methods such as ‘compareTo()’, ‘Collection.sort()’ and ‘Collections.reverseOrder()’.