How to find duplicate elements in array using for each loop

Question

I am trying to print duplicate elements in one d array using for each loop. But my output was an unexpected one. Could anyone please assist?

package Login;
public class DupsArray {
 static int[] a = {1,2,3,3};
 public static void main(String[] args) {
 int length = a.length;
 for(int i=0;i<=length-1;i++) {
 for(int j : a) {
 for(j=1;j<=length-1;j++) {
 if(a[i]==(a[j]) ) {
 System.out.println("Found duplicate");
 } else {
 System.out.println("No duplicates found");
 }
 }
 }
 }
}

The results show as follows:

The expected results to be print duplicate found.

Answer 1

Try using the below logic which compares every element with all other element in the array, if any duplicate is found,it stops the execution to continue futher

for(int i = 0; i < a.length;i++) {
 for (int j = i + 1 ; j < a.length; j++) {
 if (a[i] == a[j]) {
 System.out.println("Found duplicate");
 return;
 }
 }
}
System.out.println("No duplicate Found");

• Butiri Dan: This will only print the first duplicate item
• Gowtham: yes. But in case we need to find all the duplicate rather than returning we can do our desired operation there.
• Thanos M: Traversing the array 2 timesis bad if we need to find duplicates in big arrays. This will take a lot of time to complete for bigger arrays.

Answer 2

We can do something like this

Integer[] arr = {1, 2, 3, 3, 5, 5, 7, 8, 7};
Set<Integer> set = new HashSet<Integer>();
for (Integer i : arr) { 
 if (set.add(i) == false) 
 {
 System.out.println(i);
 } 
}

• Shashank Gupta: yes true, but I got confused why you used 2 different hash sets.
• Thanos M: In the second set i keep the duplicates for later usage(if needed)

Answer 3

try this and update as per your requirement

public class Main{
 public static void main(String[] args) {
 int[] ar = new int[] {1, 1, 3, 3, 4, 5, 7, 8};
 int size = ar.length;
 int error = 0;
 for(int i = 0; i < size; i++){
 for(int j = i+1; j < size; j++){
 if(ar[i] == ar[j]){
 if(i != j){
 error = error + 1;
 System.out.println("dulicate element " + j);
 }
 }
 }
 }
 System.out.println("total number of duplicate element " + error);
 }
}

Answer 4

You can use Sets like that :

 Integer[] a = {1, 2, 3, 3, 5, 5, 7, 8, 7};
 Set<Integer> duplicatesSet = new HashSet<>();
 Set<Integer> helperSet = new HashSet<>();
 for (Integer element : a) {
 if (!helperSet.add(element)) { // NOTE*
 System.out.println("Duplicate found : " + element);
 duplicatesSet.add(element);
 } 
 }

Then you can do whatever you like with the duplicates set

for(Integer el : duplicatesSet){
 System.out.println(el);
}

Note*

According to javadocs :

boolean add(E e);

Adds the specified element to this set if it is not already present

return true if this set did not already contain the specified element

This way we can know if the element is already in the set, if it is we can add it to our duplicates.