Find unique character in a string ? What is wrong in logic . I'm unable to find error

I am trying to find characters that occur exactly once in a string, where the string contains only letters.

Here's my code:

public static void main(String[] args)
{
	int count=0;
	Scanner sc= new Scanner(System.in);
	System.out.println("Enter the string: ");
	String s=sc.nextLine();
	char ch1=s.charAt(0);
	if(s.matches("[A-Za-z]+"))
	{
		for(int i=0;i<s.length();i++)
		{
			System.out.println(s.length());
			System.out.println(ch1);
			for(int j=1;i<s.length();j++)
			{
				if(s.charAt(i)==s.charAt(j))
				{
					count++;
				}				
			}
			if(count == 1)
			{
				System.out.println(s.charAt(i));
			}
				
		}
	}
	else
		System.out.println("Invalid");	
}

I'm getting a StringIndexOutOfBoundsException.

What am I doing wrong?

You are forgetting to set your count variable back to 0 in the outer loop after checking s[i]th character.

for(int i = 0; i < s.length(); i++)
{
    System.out.println(s.length());
    System.out.println(ch1);

    count = 0; //missing line

    for(int j = 1; j < s.length(); j++)
    {
        if(s.charAt(i) == s.charAt(j))
        {
            count++;
        }               
    }
    if(count == 1)
    {
        System.out.println(s.charAt(i));
    }
        
}

Without that, it would store the value of count from previously checked character and so would not start from zero (which is what we have to do to check frequency for each character).

Try and initialize counter inside the first loop (Or since its not being used outside the loop, shift the declaration). The way you are initializing it (once) for the entire program will keep on increasing the value even when you move to the next character. Also, since you are testing using 1, change the inner loop initialization to 0 and the condition to j (its currently i)

import java.util.*;
import java.lang.*;

public class UniqueChar
{
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the string: ");
        String s = sc.nextLine().toLowerCase(); //good idea to change casing
        char ch1 = s.charAt(0);

        if (s.matches("[A-Za-z]+"))
        {
            for (int i = 0; i < s.length(); i++)
            {
                int count = 0; // initialize here
                System.out.println(s.length());
                System.out.println(ch1);

               for (int j = 0; j < s.length(); j++) // changed initialization to 0 and condition to j
                {
                    if (s.charAt(i) == s.charAt(j))
                    {
                        count++;
                    }               
                }
                if (count == 1)
                {
                    System.out.println(s.charAt(i));
                }
                    
            }
        } else {
            System.out.println("Invalid");  
        }
    }
}

Also, its a good idea to change the input to a uniform casing. The current setup will treat A and a as different, unless thats the requirement (in which case you should ignore the toLowerCase).

Apart from setting count = 0 before the beginning of each inner loop (or even better - moving the declaration inside the loop) and correcting your typo from i < s.length() to j < s.length() in the inner loop, here's a suggestion:

If you're expecting huge strings, you could implement a O(n) solution by iterating the string just once and counting how many times each character occurred and finally checking which of them occurred once.

This task could be implemented using Java Stream API:

• calculate frequencies of each character in the input string, collect them into map
• filter map entries where frequency is 1 (unique character)

Example:

public static void printUnique(String str) {
    str.chars()                            // get IntStream of characters
	   .mapToObj(x -> (char) x)            // convert to stream of Character
	   .collect(Collectors.groupingBy(     // count frequencies using LinkedHashMap to prevent order of insertion
		   x -> x, LinkedHashMap::new, Collectors.counting())) 
	   .entrySet().stream()                // get stream of entries
	   .filter(e -> e.getValue() == 1)     // detect uniques
	   .map(Map.Entry::getKey)             // remap Map.Entry<Character, Long> to character
	   .forEach(System.out::print);        // print in a row
    System.out.println();
}

Test:

printUnique("Hello World");
printUnique("quickbrownfoxjumpsoverthelazydog");

Output:

He Wrd
qickbwnfxjmpsvthlazydg