Question

Activity 1. Suppose you are implementing a utility class, ArrayListUtil, in which you provide some utility methods to apply on ArrayLists with various types. Provide a static method that reverses the elements of a generic ArrayList. Provide another static method that returns the reverse of a generic ArrayList, without modifying the original list.

Answer 1

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Collections;
// Program to reverse an generic ArrayList in Java
class ArrayListUtil
{
// static method to reverse generic arrayList without modifying the original list
public static <T> List<T> arrayListReverse(List<T> lst) {
ArrayList reversed= new ArrayList(); // decleare a new ArrayList reversed
for (int i=lst.size()-1;i>=0;i--){
T t = lst.get(i);
reversed.add(t); //copy the list elements in reverse order into new ArrayList reversed
}
return reversed; // return ArrayList reversed
}
// static method to reverse generic arrayList
public static <T> void ListReverse(List<T> lst) {
Collections.reverse(lst); // use the reverse method to reverse the elements of original list
  }
   public static void main(String[] args)
   {
       List<Double> list = Arrays.asList(1.2, 2.5, 3.6, 4.0, 5.5); // double array list
//calling of method arrayListReverse to reverse generic arrayList without modifying the original list
List<Double> reverse = arrayListReverse(list);
       System.out.println(reverse); // print new list
       ListReverse(list); // calling of method ListReverse
System.out.println(list); //print the original list

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5); // integer array list
//calling of method arrayListReverse to reverse generic arrayList without modifying the original list
List<Integer> reverse = arrayListReverse(list);
       System.out.println(reverse); // print new list
       ListReverse(list); // calling of method ListReverse
System.out.println(list); //print the original list
   }
}