Reverse Order of All Elements of Java Vector - GeeksforGeeks

20 Sep, 2021
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: 
Using for-loop (Naive Approach)Using Collection.reverse() methodUsing listIterator() method
Using for-loop (Naive Approach)
Using Collection.reverse() method
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 classimport 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));        }    }}
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 order of elements of vector // Importing generic java input/output classesimport java.io.*; // Importing Vector and Collection class// from java.util packageimport java.util.Vector;import java.util.Collections; // Main Classpublic 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 passing vector object so as to reverse        Collections.reverse(v);         // Display vector element after reversing        System.out.println(            "After Reverse Order, Vector Contains : " + v);    }}
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 order of elements of vector// using listiterator // Importing Vector and ListIterator classes// of java.util packageimport java.util.Vector;import java.util.ListIterator; // Classpublic 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 message        System.out.println("After: ");               while (l_itr.hasPrevious())                     // Print vector elements after reversing            System.out.println(l_itr.previous());    }}
Before: [Geeks, for, Geeks, is, Best]
After: 
Best
is
Geeks
for
Geeks
 
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