Finding and printing duplicates in a string Arraylist [duplicate]

Question

Would anyone know the most efficient way to look for duplicates in a String ArrayList and print out the duplicates?

For example I have an ArrayList containing the following:

ArrayList<String> carLIST = new ArrayList<String>();
carLIST = {"Car1", "Car2", "Car3", "Car1", "Car2", "Car2"};

Basically anything in the list more than once I'm looking to find the duplicates (which I think I've done below) and also return a System.out.println(); to show the following:

Car1 : count=2

Car2 : count=3

Map<String,Integer> repeatationMap = new HashMap<String,Integer>();
for(String str : carLIST) {
    if (repeatationMap.containsKey(str) {
        repeatationMap.put(str,repeatationMap.get(str) +1);
    }
    else {
         epeatationMap.put(str, 1);
    }

//  if (repeatationMap.get(str) > 1) {
//       System.out.println(repeatationMap.entrySet());
//  }
}

The code commented out is what I thought it would be to print out the duplicates but I'm seriously wrong! Have no idea how to print out the duplicate cars in the list and show its count.

The commented line prints all the entries of the map if the current string is met for at least the second time. It's not what you want. You want, after the loop, to print the entries that have a value > 1. — JB Nizet
– JB Nizet, Commented Mar 3, 2016 at 17:22

Mureinik · Accepted Answer · 2016-03-03 17:32:24Z

3

Once you've done populating the map, you could iterate it and print only the entries with keys greater than 1:

for (Map.Entry<String, Integer> e : repeatationMap.entrySet()) {
    if (e.getValue() > 1) {
        System.out.println (e.getKey());
    }
}

Note, BTW, that Java 8 allows you to do the entire counting and reduction flow in a single statement in a relatively elegant fashion:

List<String> duplicateCars =
    carLIST.stream()
           .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
           .entrySet()
           .stream()
           .filter(e -> e.getValue() > 1)
           .map(Map.Entry::getKey)
           .collect(Collectors.toList());

edited Mar 3, 2016 at 17:32

answered Mar 3, 2016 at 17:22

Mureinik

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Comments

Sason Ohanian · Accepted Answer · 2016-03-03 17:24:53Z

0

The commented out section will print something unreadable object description. Instead use:

System.out.println(repeatationMap.get(str));

Also, to avoid printing the same string several time create a boolean set to track which strings have already been printed. In this way you avoid looping through the map again.

If you want to show number of duplicates, you will be forced to make a second loop anyways.

answered Mar 3, 2016 at 17:24

Sason Ohanian

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