ArrayList size shrinking by 1 upon each method call

Question

Quick question. I am currently making a synthetic division calculator and as a part of that I need to test whether or not a P/Q (a double) will work as a factor in an equation (given to the method as a list of integers).

Here is my code in the main class:

public static void main(String[] args)
{
    List<Integer> coeffs = new ArrayList<>();
    coeffs.add(1);
    coeffs.add(-6);
    coeffs.add(11);
    coeffs.add(-6);
    System.out.println(coeffs); // Returns [1, -6, 11, -6]
    System.out.println(doesPqWork(coeffs, 1)); // Returns 'true' as intended
    System.out.println(coeffs); // Returns [-6, 11, -6]
    System.out.println(doesPqWork(coeffs, -2)); // Returns 'false' not as intended
    System.out.println(coeffs); // Returns [11, -6]
    System.out.println(doesPqWork(coeffs, 3)); // Returns 'false' not as intended
    System.out.println(coeffs); // Returns [-6]
}

and here is the P/Q method itself:

private static boolean doesPqWork(List<Integer> coefficients, double PQ)
{
    List<Double> results = new ArrayList<>();
    double lastResult = coefficients.get(0);
    coefficients.remove(0);
    for(int coeff : coefficients) // 2, 1
    {
        double multiplied = PQ * lastResult;
        lastResult = coeff + multiplied;
        results.add(lastResult);
    }
    return results.get(results.size() - 1) == 0;
}

Please ignore how this method works to calculate it, I am only wondering why on earth the "coeffs" ArrayList is shrinking every time. I have been coding in Java for two years now and I have never come across this issue.

I thought that it was because when I call "coefficients.remove(0)" it is for some reason linking back to the "coeffs" list in the "main" method, yet when I create a new variable which would not possibly link back it still does weird thing.

I know a way I could fix this by creating some choppy code and repeating variables, but that seems really wasteful and I really want to know the source of this unintended behavior so I can fix it.

I am probably making a stupid mistake here, but if I am not then could someone please explain to me just what the heck is going on? Haha.

Answer 1

Objects in Java (such as Arrays) are not passed as copies. Your function receives a reference to the argument passed in, so when you call coefficients.remove(0) you are removing the first element of the array you passed from main() (the coeffs array).

Answer 2

Aha! Here is the answer OP.

When you return results.get(results.size()-1) == 0; that .size()-1 is shrinking your array. This is because when you look at your code you are passing the Array List coeffs directly into that method and changing it b/c like @par said you are passing references and not copies.

Edit: Here is the simplest way to modify your code.

public static void main(String[] args) {
        List<Integer> coeffs = new ArrayList<>();
        coeffs.add(1);
        coeffs.add(-6);
        coeffs.add(11);
        coeffs.add(-6);
        List<Integer> coeffCopy = new ArrayList(coeffs);
        System.out.println(coeffs); // Returns [1, -6, 11, -6]
        System.out.println(doesPqWork(coeffCopy, 1)); // Returns 'true' as intended
        System.out.println(coeffCopy); // Returns [-6, 11, -6]
        System.out.println(doesPqWork(coeffCopy, -2)); // Returns 'false' not as intended
        System.out.println(coeffs); // Returns [11, -6]
        System.out.println(doesPqWork(coeffCopy, 3)); // Returns 'false' not as intended
        System.out.println(coeffs); // Returns [-6]
    }

    private static boolean doesPqWork(List<Integer> coefficients, double PQ) {

        List<Double> results = new ArrayList<>();
        double lastResult = coefficients.get(0);
        coefficients.remove(0);
        for (int coeff : coefficients) // 2, 1
        {
            double multiplied = PQ * lastResult;
            lastResult = coeff + multiplied;
            results.add(lastResult);
        }
        return results.get(results.size() - 1) == 0;
    }

Answer 3

It is because you're calling coefficients.remove(0).

Java is pass by value, however; it passes the reference to the object as the value. See Is Java "pass-by-reference" or "pass-by-value"?

So in your case, you're passing your coeffs ArrayList and actually are manipulating it within doesPqWork().

Just creating a new reference won't fix it either. I.e. ArrayList<Integer> tmp = coeffs; just creates another reference, not a new "copy" of it.

A possible solution is to send a "copy" of your coeffs object to doesPqWork(), e.g. doesPqWork(new ArrayList<Integer>(coeffs), 1));

This creates a new object with the same values as coeffs.

How do I clone a generic List in Java? may help as well.

Answer 4

You need to copy the array:

    private static boolean doesPqWork(List<Integer> coefficients, double PQ) {


    // here you need to copy the coefficients. for example:
    List<Integer> copyOfCoeffs = new ArrayList<Integer>(coefficients);

    List<Double> results = new ArrayList<>();
    double lastResult = copyOfcoeffs.get(0);
    copyOfcoeffs.remove(0);
    for(int coeff : coefficients) // 2, 1
    {
        double multiplied = PQ * lastResult;
        lastResult = coeff + multiplied;
        results.add(lastResult);
    }
    return results.get(results.size() - 1) == 0;
}