Arraylist 给出奇怪的结果

Question

so I am trying to solve a problem (scale problem) but the result is not what I expected and in particular, I cannot understand why it's giving me these numbers. Inline if(rep.indexOf(i) == 1) I am trying to check if the ArrayList value is true to add the weight to the left but if its fault adds it to the right side. I tried using the value TRUE or FALSE for the argument but it gives an error so I just put the value 1 for testing to figure out the result numbers. The expected result should be 20 as I have 5 true values being added to the left side. R=0, L=1+2+3+4+10=20, F=|20-0|=20

import java.util.*;
import java.lang.Math;
class Untitled {
    public static void main(String[] args) {
        ArrayList<Double> weight = new ArrayList<Double>( );
        ArrayList<Boolean> rep = new ArrayList<Boolean>( );
        rep.add(true); rep.add(true); rep.add(true); rep.add(true);rep.add(true);
        weight.add(1.0); weight.add(2.0); weight.add(3.0); weight.add(4.0); weight.add(10.0);
        
        System.out.println(ScalesFitness(rep,weight));
        
    }
    public static double ScalesFitness(ArrayList<Boolean> rep, ArrayList<Double> weights) {
        System.out.print(rep);
        System.out.println();
        
        
        double left = 0, right = 0, scalesolution =0;
                        
                    for(int i=0; i<rep.size(); i++)
            {
                if(rep.indexOf(i) == 1)
                {
                    left = left + weights.indexOf(i);
                    System.out.println("value of right side"+rep.indexOf(i));
                    
                }
                else {
                    right = right + weights.indexOf(i);
                    System.out.println("value of right side"+rep.indexOf(i));
                }
            }
        //}
        scalesolution = Math.abs(left-right);
        return scalesolution;
    }
    
}

The results:

[true, true, true, true, true]
value of right side-1
value of right side-1
value of right side-1
value of right side-1
value of right side-1
5.0

Thanks in advance

Answer 1

rep.indexOf(i) 

This doesn't retrieve the element of the position marked by i : it will return the position of the element passed as argument, if exists. If it doesn't exist, it will return -1.

From the docs :

public int indexOf(Object o)

Returns the index of the first occurrence of the specified element in this list, >or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null? get(i)==null: o.equals(get(i))), or -1 if there is no such index.

---

As there's no element in the arraylist that holds any of the values offered by i , you will just receive -1 as result from that call.

What you probably need is to call this other method :

rep.get(i)

public E get(int index)

Returns the element at the specified position in this list.