First Non-Repeating Character in Java: Solution, Explanation & Practice

Find the first character that does not repeat

Problem summary

Write a program that finds the first character that appears only once in a string.

Starter code

public class Main {
    public static void main(String[] args) {
        String text = "programming"; // Test case 1
        
        // Find first non-repeating character
        // Print: First unique: <char> OR No unique character
    }
}

Expected output and test cases

p is first unique
```
First unique: p
```
First unique in aabbc
```
First unique: a
```
All repeat
```
No unique character
```

Hints

Count occurrence of each character first
Then find first with count 1
Can use array of size 26 for counts

Validated solution

Reveal Java solution

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {
    static Character firstUnique(String text) {
        Map<Character, Integer> counts = new LinkedHashMap<>();
        for (char ch : text.toCharArray()) counts.put(ch, counts.getOrDefault(ch, 0) + 1);
        for (char ch : text.toCharArray()) if (counts.get(ch) == 1) return ch;
        return null;
    }
    public static void main(String[] args) {
        Character unique = firstUnique("aabbp");
        System.out.println(unique == null ? "No unique character" : "First unique: " + unique);
    }
}

How to approach the problem

First count every character, then scan in original order to find the first count of one. Separating counting from selection avoids prematurely choosing a character that later repeats.

Approach

Build a frequency map.
Keep the original order for the second pass.
Return no result when every count exceeds one.

Time and space complexity

Time: O(n). Space: O(k), where k is the number of distinct characters.

Edge cases to test

An empty String has no unique character.
Case-sensitive and case-insensitive behavior are different contracts.

Common mistakes

Returning the first newly seen character before the full count is known.
Using HashMap iteration order as if it represented text order.

Follow-up challenge

Find the first non-repeating Unicode code point rather than char.

Related Strings exercises

Practice all Strings exercises · Run this idea in the Java compiler