Java Program to Find First Non-Repeating Character in a String

Write a Java program to Find first non-repeated character in a String. Suppose, a user has input AABCDEBFCH now the first non-repeated character in this string is D.

Input     - AABCDEBFCH
Output  - D

Java Programs for practice

C, C++ Program to print first non-repeating character of a string

How to Print First Non-repeated Character of a String

i) Method 1 - Naive Approach

Use two for loops and compare a character with each character of a string. The time complexity for this approach is O(n²).

 for(int i = 0; i < len; i++) {
  
     flag = 0; 
  
     for(int j = 0; j < len; j++) {
   
        /* If it's equal and indexes
           is not same */
        if((str[i] == str[j]) && (i != j)) {
             flag = 1;
             break;
        }
     }

ii) Method 2 - Use hash map to create a key and value pair of character and it's count. This approach is much better as compared to first one.

Traverse a string and create a hash table for character and it's count. If there  is no value stored for a character then set it to 1. Else we increment the character value by 1.

Explanation -



Let's implement a hash map to solve this problem.

Java Program to Find First Non-repeating Character in a String

import java.util.*;

public class FirstNonRepeatedCharacter {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        
        System.out.println("Please enter an input string ");
        
        // Take a input

        Scanner in = new Scanner(System.in);
        
        //Assign value into str

        String str = in.nextLine();
        
        // Length of a string

        int len = str.length();
        
        // Using HashMap

        HashMap<Character, Integer> charcount 
                  = new HashMap<Character, Integer>();
        
        Character ch;
        
        /*Traverse a string and create a hash table for 
          character and it's count. 
          If there  is no value stored for a character then set it to 1. 
          Else we increment the character value by 1 */

        for(int i = 0; i < len; i++) {
            
            ch = str.charAt(i);
            
            /* If character is already exists 
               then increment it's count by 1 */

            if(charcount.containsKey(ch)) {
                
                charcount.put(ch, charcount.get(ch)+1);
                
            } else {
                
                // If character is not exist

                charcount.put(ch, 1);
            }
        }
        
        for (int j = 0; j < len; j++) {
            
            ch = str.charAt(j);
            
            if(charcount.get(ch) == 1){
                
                System.out.println("First non-repeated character 
                                    is " + ch);
                break;
                
            }
        }
    }
    
}

Output -

Please enter an input string AABCDEBFCH

First non-repeated character is D

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