error: variable might not have been initialized - How to initialize ArrayList?

Question

I have reviewed all the previous questions with a similar title as this one, however I can't find a solution. All the errors are suggesting that I am not initializing the ArrayList.. Am I not initializing the ArrayList as such with, = new ArrayList<Double> ?

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
import java.io.PrintWriter;

public class main
{
    public static void main (String[] args) throws FileNotFoundException
    {
        ArrayList<Double> RPM, bCoeffs, filteredRPM = new ArrayList<Double>();
        Scanner RPMFile = new Scanner(new File("RotationSpeed.txt"));

        while(RPMFile.hasNextLine()){
            String line = RPMFile.nextLine();

            Scanner scanner = new Scanner(line);
            scanner.useDelimiter(",");
            while(scanner.hasNextDouble()){
                RPM.add(scanner.nextDouble());
            }
            scanner.close();
        }
        RPMFile.close();

        int windowSize = 10;
        int filterItterations = 1;

        for (int i = 0; i < windowSize; i++){
                double temp = 1/windowSize;
                bCoeffs.add(temp);
        }

        for (int k = 1; k <= filterItterations; k++){
            if (k == 1){
                for (int n = windowSize; n < RPM.size(); n++){
                    int m = 0;
                    double tempYSum = 0;
                    for (int j = 0; j < windowSize; j++){
                        double tempY = (bCoeffs.get(j))*(RPM.get(n-m));
                        tempYSum += tempY;
                        m++;
                    }
                    filteredRPM.add(tempYSum);
                }
            }else{
                int i = 1;
                for (int n = windowSize; n < filteredRPM.size(); n++){
                    int m = 0;
                    double tempYSum = 0;
                    for (int j = 0; j < windowSize; j++){
                        double tempY = (bCoeffs.get(j))*(filteredRPM.get(n-m));
                        tempYSum += tempY;
                        m++;
                    }
                    filteredRPM.set(i, tempYSum);
                    i++;
                }
            }
        }
    }
}

the errors I am receiving are as follows:

main.java:20: error: variable RPM might not have been initialized
                RPM.add(scanner.nextDouble());
                ^
main.java:31: error: variable bCoeffs might not have been initialized
                                bCoeffs.add(temp);
                                ^
main.java:36: error: variable RPM might not have been initialized
                                for (int n = windowSize; n < RPM.size(); n++){
                                                             ^
main.java:40: error: variable bCoeffs might not have been initialized
                                                double tempY = (bCoeffs.get(j))*(RPM.get(n-m));
                                                                ^
main.java:52: error: variable bCoeffs might not have been initialized
                                                double tempY = (bCoeffs.get(j))*(filteredRPM.get(n-m));
                                                                ^
5 errors

Andy Turner · Accepted Answer · 2018-03-02 16:01:34Z

3

Just assign the variables: you're currently declaring 3 variables, but only assigning a value to the last one.

ArrayList<Double> RPM = new ArrayList<>(), bCoeffs = new ArrayList<>(), filteredRPM = new ArrayList<>();

Note that declaring many variables on the same line is discouraged by some, e.g. it is forbidden by Google's style guide: it is easier to read if you do one per line:

ArrayList<Double> RPM = new ArrayList<>();
ArrayList<Double> bCoeffs = new ArrayList<>();
ArrayList<Double> filteredRPM = new ArrayList<>();

answered Mar 2, 2018 at 16:01

Andy Turner

141k11 gold badges169 silver badges263 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Christopher Byrne Over a year ago

This suggestion solves the problem. Thank you very much

Ben · Accepted Answer · 2018-03-02 16:18:43Z

1

While your first line might look like it initializes all three ArrayLists it basically does the following:

ArrayList<Double> RPM;
ArrayList<Double> bCoeffs;
ArrayList<Double> filteredRPM = new ArrayList<Double>();

If it worked the way you probably intended it though you would create a lot of potential questions that would need clarification. For example: Would all three variables now point to the same memory (in other words to the same ArrayList)? Or do they all point to a different ArrayList.

There is a reason why it is discouraged to initialize several variables in one line as it's quite obfuscating.

The correct way for you to do it would be:

ArrayList<Double> RPM = new ArrayList<>();
ArrayList<Double> bCoeffs = new ArrayList<>();
ArrayList<Double> filteredRPM = new ArrayList<>();

edited Mar 2, 2018 at 16:18

answered Mar 2, 2018 at 16:02

Ben

1,6951 gold badge13 silver badges22 bronze badges

2 Comments

Andy Turner Over a year ago

It is perhaps worth noting that even if it were a "really nice and super optimized" to initialize all variables, it wouldn't be clear if the class instance creation expression (new ArrayList<Double>()) would be evaluated once for each variable, or just once and the same value assigned to all three variables.

Ben Over a year ago

Oh for sure. The "really nice and super optimized" is probably a bad wording anyhow. I will just rephrase.

Collectives™ on Stack Overflow

error: variable might not have been initialized - How to initialize ArrayList?

2 Answers 2

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related