There are many ways to find duplicate elements in an array in Java. Here, we will cover the most used ones.
Find duplicate elements
We can find duplicates by iterating using two loops, inner and outer. We also need to make sure we are not comparing element with itself. Depending on the number of elements in the array, this way of comparing elements can have a very high time complexity (O (n ^ 2)).
Example
class FindDuplicateElements {
public static void main(String[] args) {
int[] array = new int[]{2, 4, 7, 2, 11, 5, 7, 14, 22, 11, 49, 58, 14, 101, 1, 3, 205, 49, 101, 12};
for (int i = 0; i < array.length; i++) { // outer loop
for (int j = i + 1; j < array.length; j++) { // inner loop
if (i != j && array[i] == array[j]) {
System.out.println("Duplicate element found:" + array[i]);
}
}
}
}
}
We can avoid that using the list. Let’s try it.
class FindDuplicateElements {
public static void main(String[] args) {
int[] array = new int[]{2, 4, 7, 2, 11, 17, 2, 19, 7, 22, 7, 49};
List<Integer> duplicates = new ArrayList<>();
for (int i = 0; i < array.length; i++) {
for (int j = i + 1; j < array.length; j++) {
if (i != j && array[i] == array[j] && !duplicates.contains(array[i])) {
duplicates.add(array[i]);
}
}
}
System.out.println("Duplicate elements: ");
System.out.println(duplicates);
}
}
Using a Set
Set in Java is a Collection that can’t contain duplicates. In this example, we will iterate over array of strings and add each element to a set. If the element is already present in the set, it will be ignored.
class FindDuplicateElements {
public static void main(String[] args) {
String[] array = new String[]{"Megan", "Tom", "Mellisa", "Tom", "John", "Megan"};
Set<String> set = new HashSet<>();
for (String element : array) {
if (!set.add(element)) {
System.out.println("Duplicate element found: " + element);
}
}
}
}
Find duplicate elements in an Array using Streams
In this example, we will use Streams API introduced in Java 8 to check for the frequency of the elements, and if the element is found more than once, it will be added to a set.
class FindDuplicateElements {
public static void main(String[] args) {
String[] array = new String[]{"London", "Paris", "Amsterdam", "London", "New York", "Paris"};
Set<String> set = Arrays.stream(array).filter(i -> Collections.frequency(Arrays.asList(array), i) > 1).collect(Collectors.toSet());
System.out.println(set);
}
}
