Java Remove Duplicates from an Array?

Question

I am supposed to read in a file containing many different email addresses and print them out using an array. The problem is I need to eliminate duplicate emails.

I was able to get my try / catch working and print out the email addresses. However, I am not sure how to go about removing the duplicates. I do not have an understanding of hashcode's or how to use a Set yet. Any assistance would be appreciated.

Here is what I have so far:

import java.util.Scanner;
import java.io.*;

public class Duplicate {
   public static void main(String[] args) {

      Scanner keyboard = new Scanner(System.in);
      System.out.println("Enter file name: ");
      String fileName = keyboard.nextLine();
      if (fileName.equals("")) {
         System.out.println("Error: User did not specify a file name.");
      } else {
         Scanner inputStream = null;

         try {
            inputStream = new Scanner(new File(fileName));
         } catch (FileNotFoundException e) {
            System.out.println("Error: " + fileName + " does not exist.");
            System.exit(0);
         }

         String[] address = new String[100];

         int i = 0;
         while (inputStream.hasNextLine()) {
            String email = inputStream.nextLine();
            // System.out.println(email);

            address[i] = email;
            System.out.println(address[i]);
            i++;
         }
      }
   }
}

Answer 1

The Simple solution is that use Set of java,

so set remove duplicate value automatically

and in your code you have array than convert array to set directly using code

Set<T> mySet = new HashSet<T>(Arrays.asList(someArray));

Answer 2

Use the ArrayUtil class as you need. I have written some methods other than removing duplicates. This class is implemented without using any Collection framework classes.

public class ArrayUtils {
/**
 * Removes all duplicate elements from an array. 
 * @param arr Array from which duplicate elements are to be removed.
 * @param removeAllDuplicates true if remove all duplicate values, false otherwise 
 * @return Array of unique elements.
 */
public static int[] removeDuplicate(int[] arr, boolean removeAllDuplicates)         {
    int size = arr.length;

    for (int i = 0; i < size;) {
        boolean flag = false;

        for (int j = i + 1; j < size;) {
            if (arr[i] == arr[j]) {
                flag = true;
                shrinkArray(arr, j, size);
                size--;
            } else
                j++;
        }

        if (flag && removeAllDuplicates) {
            shrinkArray(arr, i, size);
            size--;
        } else
            i++;
    }

    int unique[] = new int[size];
    for (int i = 0; i < size; i++)
        unique[i] = arr[i];

    return unique;
}

/**
 * Removes duplicate elements from an array. 
 * @param arr Array from which duplicate elements are to be removed.
 * @return Array of unique elements.
 */
public static int[] removeDuplicate(int[] arr) {
    return removeDuplicate(arr, false);
}


private static void shrinkArray(int[] arr, int pos, int size) {
    for (int i = pos; i < size - 1; i++) {
        arr[i] = arr[i + 1];
    }
}

/**
 * Displays the array.
 * @param arr The array to be displayed.
 */
public static void displayArray(int arr[]) {
    System.out.println("\n\nThe Array Is:-\n");

    for (int i = 0; i < arr.length; i++) {
        System.out.print(arr[i] + "\t");
    }
}

/**
 * Initializes the array with a given value.
 * @param arr The array to be initialized.
 * @param withValue The value with which the array is to be initialized.
 */
public static void initializeArray(int[] arr, int withValue) {
    for (int i = 0; i < arr.length; i++) {
        arr[i] = withValue;
    }
}

/**
 * Checks whether an element is there in the array. 
 * @param arr The array in which the element is to be found.
 * @param element The element that is to be found.
 * @return True if found false otherwise
 */
public static boolean contains(int arr[], int element) {
    for(int i=0; i< arr.length; i++) {
        if(arr[i] == element)
            return true;
    }

    return false;
}

/**
 * Removes a element from an array.
 * @param arr The array from which the element is to removed.
 * @param element The element to be removed
 * @return The size of the array after removing.
 */
public static int removeElement(int[] arr, int element) {
    int size = arr.length;
    for(int i=0; i< arr.length; i++){
        if(arr[i] == element){
            shrinkArray(arr, i, arr.length);
            size--;
        }
    }
    return size;
}

/**
 * Counts unique elements in an array.
 * @param arr The required array.
 * @return Unique element count.
 */
public static int uniqueElementCount(int arr[]) {
    int count = 0;
    int uniqueCount=0;
    int[] consideredElements = new int[arr.length];

    initializeArray(consideredElements, 0);

    for(int i=0;i<arr.length;i++) {
        int element = arr[i];
        for(int j=i+1;j<arr.length; j++){
            if(element != arr[j] && !contains(consideredElements, element)){
                consideredElements[count++] = element;
            }
        }
    }

    for(int i=0;i< consideredElements.length;i++)
        if(consideredElements[i]!=0)
            uniqueCount++;

    return uniqueCount;
}
}

Answer 3

the first thing that comes into my head is to sort the array and then to check if the next element equals the current element. if so, delete the current element.

oh and when you don´t know how many emails are stored in the file, an array is probably not the best way. I´d take some sort of list, so that i don´t have to care how many e-mail addresses are in the file.