How to check for duplicate String in a java String array

Question

I am trying to find duplicates values from the first column 0 in array that's been loaded in a string and split, and then throw an exception when this is found.

Here is my code.

public void loadTrueFalseQuestions() throws Exception {
    try {
        Scanner input = new Scanner(new FileInputStream(TRUE_FALSE_FILE));

        while (input.hasNextLine()) {
            line = input.nextLine();
            String[] split = line.split(",");
            int chapterNumber = Integer.parseInt(split[1]);
            String correctAnswer = (split[3]);
            String questionID = (split[0]);

            if (split.length != TRUE_FALSE_FIELDS) {
                throw new Exception();

            } else if ((!correctAnswer.matches("TRUE")) & (!correctAnswer.matches("FALSE"))) { //throw new Exception();
            } else if (true) {
              
            }

        }
    } catch (FileNotFoundException e) {

        System.out.println("Could not open a file.");
        System.exit(0);
    }

}

This is the CSV file.

TF001,8,Java allows an instance of an abstract class to be instantiated.,FALSE
TF001,8,Downcasting should be used only in situations where it makes sense.,TRUE
TF003,9,The throw operator causes a change in the flow of control.,TRUE
TF004,9,When an exception is thrown the code in the surrounding try block continues executing 
and then the catch block begins execution.,FALSE

I'm not sure how to go about it. I cannot get past the logic. I'm trying an if statement and contains(questionID) string method, but not sure how to combine these two. (if that makes sense).

Appreciate any advice.

The code you've shown checks whether the answer looks like "TRUE" or "FALSE". It does no comparison across lines at any point. What do you want to check for duplicates? A particular column? All columns? — Silvio Mayolo
– Silvio Mayolo, Commented Jul 6, 2021 at 14:07
You are checking the length of split after accessing split's members. You need to do that immediately after calling line.split. — Jeff Holt
– Jeff Holt, Commented Jul 6, 2021 at 14:08
If I understand your question correctly, it sounds like the string matching problem, I suggest taking a look at some string matching algorithms — Long Doan
– Long Doan, Commented Jul 6, 2021 at 14:34
@Silvio Mayolo - I wish to catch duplicates in column 0, and catch the exception. — Zenith2k3
– Zenith2k3, Commented Jul 7, 2021 at 1:12

OldManSeph · Accepted Answer · 2021-07-06 14:11:27Z

2

I use a hash map and put the key field as the key in the hash map. You can check if you already put the key in the hash and if so do something because you already have it.

public void loadTrueFalseQuestions() throws Exception {
try {
    Scanner input = new Scanner(new FileInputStream(TRUE_FALSE_FILE));

    Map<String, String> myHash = new HashMap<>();

    while (input.hasNextLine()) {
        line = input.nextLine();
        String[] split = line.split(",");
        int chapterNumber = Integer.parseInt(split[1]);
        String correctAnswer = (split[3]);
        String questionID = (split[0]);

        if (myHash.containsKey(questionID)) {
            // what to do here?
        } else {
            myHash.put(questionID, line);
        }
    }
} catch (FileNotFoundException e) {

    System.out.println("Could not open a file.");
    System.exit(0);
}

}

edited Jul 6, 2021 at 14:11

answered Jul 6, 2021 at 14:07

OldManSeph

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3 Comments

Ofek Over a year ago

you can use myHash.put(split[0], line) == null

OldManSeph Over a year ago

@Ofek you can, but I never check if the key isn't there. I always need to know if it is there to complete some method.

Ofek Over a year ago

so use else or myHash.put(split[0], line) != null

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