我无法使用removeDuplicate方法工作？

Question

My program reads an input file, which is a txt file that contains duplicates of first and last names. I am unsure why the removeDuplicate method isn't removing the duplicates and instead giving me an error. What am I doing wrong?

public static void main(String[] args) throws IOException {

        ArrayList<String> names = new ArrayList<String>();

        String fName;
        String lName;

        System.out.println("What is the input file?");

        Scanner kb = new Scanner(System.in);
        String fileName = kb.next();

        File list = new File(fileName);

        Scanner in = new Scanner(list);

        System.out.println("What is the output file?");

        String outFileName = kb.next();

        PrintWriter outFile = new PrintWriter(outFileName);

        while (in.hasNext()) {

            fName = in.next();

            lName = in.next();

            names.add(fName + " " + lName);
            removeDuplicates(names);
            display(names);


            outFile.println(fName + " " + lName);

        }
        outFile.close();

    }
}

Here's the methods outside of my Public main

public class StudentList {

    public static void display(ArrayList<String> n) {
        // step through all positions of the ArrayList n and display the values
        // at each positoin
        for (int i = 0; i < n.size(); i = i + 1) {
            System.out.println(n.get(i));
        }
    }


    public static int find(ArrayList<String> names, int i) {

        String s = names.get(i);
        for (i = 0; i < names.size(); i = i + 1) {
            for (int j = i + 1; j < names.size(); j = j + 1) {
                if (s.equals(names.get(j))) {

                    return j;

                }
            }

        }
        return -1;
    }


    public static void removeDuplicates(ArrayList<String> names) {

        for (int i = 0; i < names.size(); i = i + 1) {
            while (find(names, i) > 0) {
                names.remove(find(names, i));
            }
        }

    }

Answer 1

In order to simplify your code and not need to programmatically remove any duplicate you could use a HashSet , LinkedHashSet or TreeSet instead of an ArrayList .

Basically either:

• Set<String> names = new HashSet<String>(); // unordered, doesn't keep duplicates
• Set<String> names = new LinkedHashSet<String>(); // keeps insertion order, doesn't keep duplicates
• Set<String> names = new TreeSet<String>(); // ordered by lexicographic order, doesn't keep duplicates

You could then dispose of both find and removeDuplicates .

Note that in either case duplicates would be case-sensitive - but that's what your code does at the moment.

Answer 2

Everyone's comments about using a Set are correct. This is the data structure you should use. However, the problem with your code is in your find() method. You pass in an int i and set String s = names.get(i) you then peform a nested for loop but never change your string s.

Try this:

public static int find(ArrayList<String> names) {


    for (i = 0; i < names.size(); i = i + 1) {
        String s = names.get(i);
        for (int j = i + 1; j < names.size(); j = j + 1) {
            if (s.equals(names.get(j))) {

                return j;

            }
        }

    }
    return -1;
}

Notice that you set s equal to the ith element inside your for loop. You no longer need the parameter I in your method. However, this may change your code. If you want to simply try to find everytime something occurs you would want this:

public ArrayList<Integer> find(String name, ArrayList<String> names) {
  ArrayList<Integer> duplicateIndices = new ArrayList<Integer>();
  for (int i = 0; i < names.size(); i++) {
    if (names.get(i).equals(name)) {
      duplicateIndices.add(new Integer(i));
    }
  }
  return duplicatIndices;
}