Hi All,

Greetings of the day!

Today we will see how we can write a program in Java to find the first repeating character of a string, so let's start.

First, we will declare & initialize a String and then we will convert the String to a character array as shown below.
   String testString="interview";

   char[] charArray = testString.toCharArray();
Now we will use HashSet to find the first repeating character from the String, so the basic logic is we will loop through each element of a character array and check if HashSet contains that character. If HashSet already contains that character it's a first repeating character and we will break our loop, else we will add a character to HashSet as shown below.
  
package com.demo.programs;

import java.util.HashSet;
import java.util.Set;

public class FindFirstRepeatingCharacter {

    public static void main(String[] args) {
        String testString="interview";

        char[] charArray = testString.toCharArray();

        Set characterSet=new HashSet<>();

        for(char c:charArray){
            if(characterSet.contains(c)){
                System.out.println("First repeating character is "
                +c);
                break;
            }else{
                characterSet.add(c);
            }
        }
    }
}
And O/P will be 
First repeating character is i

Process finished with exit code 0