Array list elements in constructor not equal to those in the function

Question

The array list request in the constructor is equal to the array list passed to this class but I try to use it in the function calcHead the elements in the array disappears.

import java.io.*;
import java.util.*;

public class C_SCAN {
    private ArrayList<Integer> requests = new ArrayList<Integer>();

    C_SCAN(ArrayList<Integer> r){
        this.requests = r;
        System.out.println(requests.size());

    }

    public int calcHead(int start , int end) throws IOException {
        BufferedWriter writer = new BufferedWriter(new FileWriter("sequence.txt", true));
        writer.write(" \nC_SCAN : ");
        int THM = 0;
        System.out.println(requests.size());
        requests.add(start);
        sortReq(requests);
        int current = requests.indexOf(start);
        for(int i = current; i > 0; i--) {
            THM += Math.abs(requests.get(i) - requests.get(i - 1));
            writer.write(String.valueOf(requests.get(i)));
        }
        THM += Math.abs(requests.get(0));
        THM += end;
        THM += end - requests.get(requests.size() -1);
        for(int i = requests.size() - 1; i > current + 1; i--) {
            THM += Math.abs(requests.get(i) - requests.get(i - 1));
            writer.write(String.valueOf(requests.get(i)));
        }
        writer.write("\ntotal head movement : " + String.valueOf(THM));
        writer.close();
        return THM;
    }

    public void sortReq(ArrayList<Integer> requests) {
        for(int i = 0; i < requests.size(); i++) {
            for(int j = 0; j < requests.size(); j++) {
                if(requests.get(j) > requests.get(i)) {
                    Collections.swap(requests, i, j);
                }
            }
        }
    }

}

Answer 1

I can assure you that the constructor list is the same reference as the one in the method

Given that you mention list elements are "gone", you seem to be moving elements around in the list as you're iterating over it (twice) and potentially overwriting them

Debug by printing the list before and after your sort method and I think you'll see an issue

The correct iteration procedure looks like this (start j = i+1) so that the indices never match and you don't compare same elements more than once

public void sortReq(List<Integer> requests) {
    for(int i = 0; i < requests.size(); i++) {
        for(int j = i + 1; j < requests.size(); j++) {

Please don't use bubble sort... Collections.sort() does the right thing, so no need to implement your own method

Also, your field initial value is never used, so the class can start like

public class C_SCAN {
    private final List<Integer> requests;

    C_SCAN(List<Integer> r){
        this.requests = r;
        System.out.println(requests.size());

    } 

Answer 2

The problem is in the function named 'sortReq'

public void sortReq(ArrayList requests) { //....}

The argument requests to this function has the same name as that of the data member of the class.

How to resolve the issue?

Either changing the name of the argument to sortReq function, Or removing the argument to the function may help resolve the issue.

An important point to note:

Methods of a class can directly access the data members of the same class.

Therefore, the name of a Data member need not be passed as an argument to the method of the same class.