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Reverse Order of All Elements of Java Vector
  • Last Updated : 17 Dec, 2020

Vector class Implements a dynamic array means it can shrink and expand its size as required just likely having the same operations like that in the arrays. Don’t confuse it with ArrayList as there is a thin line between vector and ArrayList, where the vector is synchronized rest the insertion order remains the same in both of them. Vector is present in java.util package and implements the list interface

Illustration: Reversing the order of elements in a vector

Input : [1, 2, 3, 4, 5]

Output: [5, 4, 3, 2, 1]

Input : [“GEEKS”, “FOR”, “geeks”]



Output: [“geeks”, “FOR”, “GEEKS”]

Methods: 

  1. Using for-loop (Naive Approach)
  2. Using Collection.reverse() method
  3. Using listIterator() method

Method 1: Using for loop to print the element of vector in reverse order.

Java




// Java program to print vector element
// in reverse using for loop
  
// Importing Vector, Collection & ListIterator classes
// and generic java input output classs
import java.io.*;
import java.util.Vector;
import java.util.Collections;
import java.util.ListIterator;
  
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating vector of integer
        Vector<Integer> v1 = new Vector<Integer>();
  
        // Adding element to vector
        // Custom inputs
        v1.add(1);
        v1.add(2);
        v1.add(3);
        v1.add(4);
        v1.add(5);
  
        // Display message
        System.out.println("Before reverse of vector : ");
  
        // Printing all elements of vector before reversing
        System.out.println(v1);
  
        // Display message
        System.out.println("After reverse of vector : ");
  
        // Iterating from last index of vector to 0
        // index = vector.size()-1 (last index)
        for (int i = v1.size() - 1; i >= 0; i--) {
  
            // Printing elements of vector after reversing
            System.out.println(v1.get(i));
        }
    }
}
Output
Before reverse of vector : 
[1, 2, 3, 4, 5]
After reverse of vector : 
5
4
3
2
1

Time complexity: O(n) of n time, where n is a number of elements in the vector.

Method 2: Collection.reverse() method as the name suggests is a method of Collection class.

Syntax:

Collections.reverse(vector) ;

Parameters: Vector object to be reversed



Return value: It returns the reverse of the vector element.

Java




// Java Program to reverse orer of elements of vector
  
// Importing generic java input/output clases
import java.io.*;
  
// Importing Vector and Collection class 
// from java.util package
import java.util.Vector;
import java.util.Collections;
  
// Main Class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Create a Vector object
        Vector<String> v = new Vector<String>();
  
        // Add elements to Vector
        v.add("GFG");
        v.add("EarlierGreen");
        v.add("NowBlack");
  
        // Display vector element before reversing
        System.out.println(
            "Before Reverse Order, Vector Contains : " + v);
  
        // reverse() method to reverse vector element
        // by passsing vector object so as to reverse
        Collections.reverse(v);
  
        // Display vector element after reversing
        System.out.println(
            "After Reverse Order, Vector Contains : " + v);
    }
}
Output
Before Reverse Order, Vector Contains : [GFG, EarlierGreen, NowBlack]
After Reverse Order, Vector Contains : [NowBlack, EarlierGreen, GFG]

Method 3: Using listIterator() method

Syntax:

public ListIterator listIterator()

Parameters: This method accepts no input arguments.

Return Value: This method returns a ListIterator object which can be used to traverse the Vector object. This object can be used to traverse the Vector object. It is bidirectional, so both forward and backward traversal is possible, using next() and previous() respectively.

Return Type: A ListIterator 

Java




// Java Program to reverse orer of elements of vector
// using listiterator
  
// Importing Vector and ListIterator classes
// of java.util package 
import java.util.Vector;
import java.util.ListIterator;
  
// Class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Create(Declare) empty vector
        Vector<String> v1 = new Vector<String>();
  
        // Add elements to vector using add() method
        v1.add("Geeks");
        v1.add("for");
        v1.add("Geeks");
        v1.add("is");
        v1.add("Best");
  
        // Print message
        System.out.print("Before: ");
  
        // Printing all elements of Vector before reversing
        System.out.println(v1);
  
        // Declare list iterator
        ListIterator<String> l_itr
            = v1.listIterator(v1.size());
  
        // Iistiterator to reverse the vector element using
        // hashPrevious() method
        
      // Print messsage
        System.out.println("After: ");
        
        while (l_itr.hasPrevious())
            
          // Print vector elements after reversing
            System.out.println(l_itr.previous());
    }
}
Output
Before: [Geeks, for, Geeks, is, Best]
After: 
Best
is
Geeks
for
Geeks

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