How to count duplicate elements in ArrayList?

Question

I need to separate and count how many values in arraylist are the same and print them according to the number of occurrences.

I\'ve got an arraylist called digits :<

Answer 1

Java 8 can handle this problem with 3 lines of code.

    Map<Integer, Integer>  duplicatedCount = new LinkedHashMap<>();
    list.forEach(a -> duplicatedCount.put(a, duplicatedCount.getOrDefault(a, 0) +1));
    duplicatedCount.forEach((k,v) -> System.out.println(v+" times "+k));

Answer 2

Map#merge method can also be used:

 List<Integer> input = Arrays.asList(1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584,
            4181, 6765);
 final Map<Integer, Integer> result = new HashMap<>();
 input.forEach(in -> result.merge(in, 1, Integer::sum));

Answer 3

By using the Stream API for example.

package tests;

import org.junit.Assert;
import org.junit.Test;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Duplicates {

    @Test
    public void duplicates() throws Exception {
        List<Integer> items = Arrays.asList(1, 1, 2, 2, 2, 2);

        Map<Integer, Long> result = items.stream()
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

        Assert.assertEquals(Long.valueOf(2), result.get(1));
        Assert.assertEquals(Long.valueOf(4), result.get(2));
    }
}

Answer 4

use Collections.frequency method to count the duplicates

Answer 5

Well, for that you can try to use Map

Map<Integer, Integer> countMap = new HashMap<>();

  for (Integer item: yourArrayList) {

      if (countMap.containsKey(item))
          countMap.put(item, countMap.get(item) + 1);
      else
          countMap.put(item, 1);
  }

After end of forEach loop you will have a filled map with your items against it count

Answer 6

You can count the number of duplicate elements in a list by adding all the elements of the list and storing it in a hashset, once that is done, all you need to know is get the difference in the size of the hashset and the list.

ArrayList<String> al = new ArrayList<String>();
al.add("Santosh");
al.add("Saket");
al.add("Saket");
al.add("Shyam");
al.add("Santosh");
al.add("Shyam");
al.add("Santosh");
al.add("Santosh");
HashSet<String> hs = new HashSet<String>();
hs.addAll(al);
int totalDuplicates =al.size() - hs.size();
System.out.println(totalDuplicates);

Let me know if this needs more clarification