查找不重复的数字

Question

I'm trying to find non-repeating numbers in array and print those numbers to new array. It's kinda working but prints one number twice for some reason and I don't understand why. The output should be 4 and 7 but it prints 4 4 7.

import java.util.*;
import java.util.ArrayList;
import java.util.Arrays;
public class Frequence {
    public static void main(String[] args) {
        int[] array = new int[]
                {
                        1, 2, 3, 2, 3, 3, 5, 4, 7, 1, 5
                };
        List<Integer> arr = new ArrayList<>();
        for (int value : array) {
            int count = 0;
            for (int i : array)
                if (value == i)
                    count++;
            if (count == 1) {
                arr.add(value);
                Integer[] arr2 = new Integer[arr.size()];
                arr2 = arr.toArray(arr2);
                for (Integer k : arr2)
                    System.out.println(k);
            }
        }
    }
}

Answer 1

Since you are using a List<> you might consider using a Set<> . Since set values don't contain duplicates and their values are hashed they lend themselves to this type of operation.

The following works with a single loop:

• first add the value to the nonDups set.
• now add it to the found set. If this value is already in that set it was located earlier and returns false. Thus it must be a duplicate so remove it from nonDups .
• continue until all values are processed.

Set<Integer> nonDups = new HashSet<>();
Set<Integer> found = new HashSet<>();
for (int k : array) {
    nonDups.add(k);
    if (!found.add(k)) { // returns false if already present
        nonDups.remove(k);
    }
}

System.out.println(nonDups);

Prints

[4,7]

Answer 2

Your print loop prints the whole arr each time unique element is found so it should be outside the for (int value : array) loop:

for (int value : array) {
    // ...
    if (count == 1) {
        arr.add(value);
    }
}

for (Integer k : arr) {
    System.out.println(k);
}

---

UPD: The simplest approach to find unique element of an array would be using the Set collection. The whole task can be solved with a one-liner: Set<Integer> uniques = new HashSet(Arrays.asList(array));

Or if you are OK with unmodifiable collection and use Java 9+: Set<Integer> uniques = Set.of(array);

Answer 3

I simplified your code and the problem is you are printing inside the first loop:

int[] array = new int[] { 1, 2, 3, 2, 3, 3, 5, 4, 7, 1, 5 };
List<Integer> arr = new ArrayList<>();

for (int i = 0; i < array.length; i++) {
    int count = 0;
    for (int j = 0; j < array.length; j++) {
        if(array[i] == array[j]) {
            count++;
        }
    }
    if(count == 1) {
        arr.add(array[i]);
    }
}
System.out.println(arr.toString());