Trace the following program step by step, to find the program modification of th

Question

Trace the following program step by step, to find the program modification of the names array list when user enter ahmed. Write the output when the user enters ahemd and when enters salem.

import java.util.Scanner;

public class Student_ArrayList {

public static void main(String[] args) {

java.util.ArrayList<String> names = new java.util.ArrayList<String>();      

names.add("Eman");

names.add("Rwan");

names.add("Ahmed");

names.add("Kaled");

System.out.print("Names: ");

System.out.println(names);

Scanner in = new Scanner(System.in);

System.out.print("Enter a name: ");

String x = in.next();

boolean removed_flag= false;

for (int i = 0; i < names.size(); i++)

   {

      if (i < names.size()) {

            String pName = names.get(i);

            if (pName.equalsIgnoreCase(x)) {

                  names.remove(i);

                  removed_flag= true;

            }

      }

}

if (removed_flag==false) {

names.add(x);

}                            

System.out.print("Updated names: ");

System.out.println(names);

}          

}

note: make it as simple as possible this is my first time learning about java and i don't want the teacher to know that im getting an outside help, thank you.

Explanation / Answer

Firstly I have added comments to your program to help you understand it ... Please go through the program and comments before going through the trace.

//Class declaration

public class Student_ArrayList {

//main method declaration

public static void main(String[] args) {

//declaring and defining an ArrayList with string

java.util.ArrayList<String> names = new java.util.ArrayList<String>();

//adding names to list

names.add("Eman");

names.add("Rwan");

names.add("Ahmed");

names.add("Kaled");

//printing names

System.out.print("Names: ");

System.out.println(names);

//getting input from user

Scanner in = new Scanner(System.in);

System.out.print("Enter a name: ");

String x = in.next();

//flag to check if name is removed

boolean removed_flag = false;

//loop from 0 to size of Arraylist

//searching if name is there than removing it and setting the flag to true

for (int i = 0; i < names.size(); i++)

{

//this if block is not needed because

//this check if performed by for loop itself

if (i < names.size()) {

//getting each name from arraylist

String pName = names.get(i);

//comparing the name from list with entered name

// if equal then remove it an dset removed_flag to true

if (pName.equalsIgnoreCase(x)) {

names.remove(i);

removed_flag = true;

}

}

}

//If removed flag is false that means the name is not there.

// so it is added to the list

if (removed_flag == false) {

names.add(x);

}

//printing the updated list

System.out.print("Updated names: ");

System.out.println(names);

}

}

When user enter ahmed

Firstly the program prints the initial names list as below

Names: [Eman, Rwan, Ahmed, Kaled]

Now user is asked to enter the name , and when it enters ahmed, the loop is executed.

For i ranging from 0 to 3( names.size() is 4 and we are checking that i is less than size. So we will only iterate till i is 3),

for each iteration we get the name out of list.

for i = 0, pName = Eman, and x = ahmed

we compare pName and x with equalsIgnoreCase, it means Case insensitive comparision is done.. So eman will be compared to ahmed, as we move to next iteration

for i = 1, pName = Rwan and x = ahmed,

again not equal , so moving to next iteration

for i = 2, pName = Ahmed and x = ahmed,

As it is case insensitive comparision, so  (pName.equalsIgnoreCase(x)) comes out to be true, So Ahmed is removed from list. The new list becomes as follows: -

[Eman, Rwan, Kaled] and removed_flag is set to true.

for i = 3, pName = Kaled and x = ahmed,

again not equal , so moving to next iteration. But we are checking till i = 3 only, So for loop ends, Next we check if removed_flag is false, if it is false we need to add the element in the list. But it is true, so list is not changed.

So final list is [Eman, Rwan, Kaled]

Output when user enter ahmed

Names: [Eman, Rwan, Ahmed, Kaled]
Enter a name: ahmed
Updated names: [Eman, Rwan, Kaled]

Output when user enter ahemd

Names: [Eman, Rwan, Ahmed, Kaled]
Enter a name: ahemd
Updated names: [Eman, Rwan, Ahmed, Kaled, ahemd]

Output when user enter salem

Names: [Eman, Rwan, Ahmed, Kaled]
Enter a name: salem
Updated names: [Eman, Rwan, Ahmed, Kaled, salem]

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