How to find a unique character in a string using java?

In this article, we will learn how to find a unique character in a string in Java. A unique character is a character that occurs only once in the string, while all others can occur multiple times.

We can solve this problem using the following ways:

• Using Brute Force method.
• Using HashMap.

Using brute force method

To find a unique character in a given string, we will use a brute force approach. In the brute force approach, we will use two loops to compare each character to the rest of the string.

Let's look at the steps:

• Initiate a for loop with i = 0 and run it until i is less than the string length.
• Use charAt() method to get the character at the current index.
• Take a isUnique boolean variable and set it true.
• Within the first loop, start another nested for loop with j = 0, and it goes till j is less than the string length.
• Check if the character at index i matches the character at index j and ensure i is not equal to j.
• If it does match immediately set isUnique to false and break the inner loop.
• Once you are done with traversing the whole inner loop, check if isUnique is still true, and if so, print the character.

Example

Following is the Java program to find a unique character in a string. You are given a string that has a duplicate character except one character, we need to find that.

public class FindUniqueChar{
   public static void main(String args[]){
       String s = "abbbccdd";
       for(int i = 0; i < s.length(); i++){
           char ch = s.charAt(i);
           boolean isUnique = true;
         for(int j = 0; j < s.length(); j++){
            if(i != j && ch == s.charAt(j)){
               isUnique = false;
               break;
            }
         }
         if(isUnique){
            System.out.println("The unique character is [" + ch + "]");
         }
      }
   }
}

Output

Following is the output of the above code :

The unique character is [a]

Using HashMap

We can also use HashMap, which is part of Java Collections Framework. We will use a HashMap to store the frequency of each character in the string, and will check if the character has a single frequency. In this way, we will get a unique character in a string.

Steps to find a unique character in a string using a HashMap.

• Initiate a HashMap, and store the frequency of characters.
• Loop through the HashMap, check if any value is equal to 1. If so, print the respective key.

Example

Following is the Java program to find a unique character in a string using a HashMap.

import java.util.*;
public class UniqueChar{
   public static void main(String args[]){
      String s = "bbssdcd";
      HashMap<Character, Integer> map = new HashMap<>();
      for(char ch :s.toCharArray()){
         if(!map.containsKey(ch)){
            map.put(ch,1);
         }else{
            map.put(ch, map.get(ch)+1);
         }
      }
      for(Map.Entry<Character, Integer> i : map.entrySet()){
        if(i.getValue() == 1){
            System.out.println(i.getKey());
        }
      }
   }
}

Example

Following is the output:

c