How to create a unique list of values in Java?

Question

I am trying to create a list, which only consists of unique values.

String[] arr = {"5", "5", "7", "6", "7", "8", "0"};
    List<String> uniqueList = new ArrayList<String>(new HashSet<String>( Arrays.asList(arr) ));
    System.out.println( uniqueList );

What I expect as an output is: 6,8,0. So, if duplicates exist, I want to delete both of them. The HashSet only removes the duplicates, so that each value only occurs once. However, I want to remove both the numbers, so that I end up with a list, which only has the numbers that occur once in the original list.

Jacob G. · Accepted Answer · 2018-11-10 01:48:39Z

3

One solution would be to build a frequency Map and only retain the keys whose value equals 1:

String[] arr = {"5", "5", "7", "6", "7", "8", "0"};

Arrays.stream(arr)
      .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
      .entrySet()
      .stream()
      .filter(e -> e.getValue() == 1)
      .map(Map.Entry::getKey)
      .collect(Collectors.toList()));

One possible value of this List is:

[0, 6, 8]

answered Nov 10, 2018 at 1:48

Jacob G.

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4 Comments

flakes Over a year ago

Ah, you beat me to it by like 5 seconds!

Jacob G. Over a year ago

@flakes I can't tell you how many times that's happened to me!

user8231110 Over a year ago

what if i want to print the output? Can i use an ArrayList instead of an array?

Jacob G. Over a year ago

@user8231110 The above code returns a List. If you want an ArrayList specifically, then you'll need to change the Collector to Collectors.toCollection(ArrayList::new).

GBlodgett · Accepted Answer · 2018-11-10 02:05:50Z

2

Another possiblilty with Stream's:

List<String> arr1 = Arrays.asList(arr).stream()
                   .filter(i -> Collections.frequency(Arrays.asList(arr), i)  < 2)
                   .collect(Collectors.toList());
arr1.forEach(System.out::println);

Which will create a filter out all the elements that occur more than once using Collections::frequency. Which returns the List:

[6, 8, 0]

edited Nov 10, 2018 at 2:05

answered Nov 10, 2018 at 1:54

GBlodgett

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1 Comment

flakes Over a year ago

O(n^2) time complexity. Should opt for a different solution. Many duplicates can result in many redundant calls to Collections.frequency

Udaya Shankara Gandhi Thalabat · Accepted Answer · 2018-11-10 02:42:53Z

-1

One other possible solution is collecting the list data into set and then get back to list again.

String[] arr = {"5", "5", "7", "6", "7", "8", "0"};

List<String> stringList = Arrays.stream(arr).collect(Collectors.toSet()).stream().collect(Collectors.toList());

for (String s : stringList) {
     System.out.println(s);
}

answered Nov 10, 2018 at 2:42

Udaya Shankara Gandhi Thalabat

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1 Comment

Hovercraft Full Of Eels Over a year ago

The OP already shows this sort of thing in the original post. This will remove duplicates but that's not what the OP is trying to do. They are trying to remove completely any and all items that have more than one entry, a greater trimming than just duplicates.

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