Instantiate an ArrayList by reading in lines from a Scanner, where to declare object?

Question

I'd like to populate an array list with lines from an input file, the input file looks like this:

7f00000000000000000000000000000000000000000000000000000000000000027f00000000000000000000000000000000000000000000000000000000000000027f00000000000000000000000000000000000000000000000000000000000000020101
7f00000000000000000000000000000000000000000000000000000000000000037f00000000000000000000000000000000000000000000000000000000000000037f00000000000000000000000000000000000000000000000000000000000000030101
7f00000000000000000000000000000000000000000000000000000000000000047f00000000000000000000000000000000000000000000000000000000000000047f00000000000000000000000000000000000000000000000000000000000000040101
7f00000000000000000000000000000000000000000000000000000000000000057f00000000000000000000000000000000000000000000000000000000000000057f00000000000000000000000000000000000000000000000000000000000000050101
7f00000000000000000000000000000000000000000000000000000000000000067f00000000000000000000000000000000000000000000000000000000000000067f00000000000000000000000000000000000000000000000000000000000000060101

The data object in Java that I'd like to create based on this would have each one of those lines as a new string, and they would live together in a list, so to speak*.

So, in my attempt to read in the lines of the file into different components of this array list, I can't figure out where I need to declare the array list in my main program. My plan is to populate it in a separate method:

import java.io.*;
import java.util.Scanner;
import java.util.List;
import java.util.Array;
import java.util.ArrayList;

class evmTest {

public static void main(String[] args) {


    Array<String> inputLinesObject = new ArrayList<String>();

    // populate from file
    inputLinesObject = readFile("/Users/s.matthew.english/codes.txt", inputLinesObject);


    System.out.println(Array.toString(inputLinesObject));

}

    private static void readFile(String fileName, Array<String> inputLines) {
        try {
            File file = new File(fileName);
            Scanner scanner = new Scanner(file);
            while (scanner.hasNextLine()) {
                // System.out.println(scanner.nextLine());
                inputLines.add(scanner.nextLine());
            }
            scanner.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        return inputLines;
    }

}

Maybe I can initially instantiate it as null, and then pass that null array list to the method to be populated?

* The terms in that last sentence are not totally precise- please forgive me- I'm re-adjusting to the vocabulary of Java, nevertheless I think it should be clear enough what I'm trying to do. If not please let me know and I'll be happy to clarify.

Answer 1

For your test, you simply have to instantiate the ArrayList just above the try

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

public class EvmTest {

    public static void main(final String[] args) {
        // populate from file
        final List<String> inputLinesObject = EvmTest.readFile("/Users/s.matthew.english/codes.txt");

        for (final String line : inputLinesObject) {
            System.out.println(line);
        }
    }

    private static List<String> readFile(final String fileName) {
        final List<String> inputLinesObject = new ArrayList<>();

        try {
            final File file = new File(fileName);
            final Scanner scanner = new Scanner(file);
            while (scanner.hasNextLine()) {
                // System.out.println(scanner.nextLine());
                inputLinesObject.add(scanner.nextLine());
            }
            scanner.close();
        } catch (final FileNotFoundException e) {
            e.printStackTrace();
        }
        return inputLinesObject;
    }
}

That has 2 advantages: - You don't modify an input parameter of your readFile method - You don't have to handle the case where the List is empty or null