Counting unique values in an array (Java)

Question

It's been asked before, I know, but I really don't know why my code isn't working. I'm trying to figure out how many unique values are in an undefined array. (i.e. {0, 0, 1, 1, 1, 5, 5, 6, 6, 7} should return a value of 5. This is what I have so far:

public static int numUnique(double[] list) {
    int counter = 0;
    
    for(int i = 0; i < list.length; i++) {
        for(int j = i + 1; j < list.length; j++) {
            if(list[i] != list[j])
                counter ++;
        }
    }
    
    
    return counter;
}

Answer 1

Assuming that the array is sorted, you should only be checking if each element is not equal to the element immediately after it. The counter should also be initialized to 1 since if all elements are the same, the number of unique elements is 1. We also need to add a check for a null or empty array, for which the result is 0. (If it isn't sorted, you could use Arrays.sort to sort it, but that is not the fastest method.)

public static int numUnique(double[] list) {
    if(list == null || list.length == 0) return 0;
    int counter = 1;
    
   for(int i = 1; i < list.length; i++)
       if(list[i - 1] != list[i]) ++counter;
    
    
    return counter;
}

Alternate methods include using Stream#distinct or a Set, which do not rely on order^*.

System.out.println(java.util.Arrays.stream(array).distinct().count());

or

System.out.println(java.util.stream.IntStream.of(array).distinct().count());

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^{* LinkedHashSet implementation provides you with predictable iteration order.}