String Compression in Java: Solution, Explanation & Practice

Compress string using counts of repeated characters

Problem summary

Write a program that compresses a string by replacing consecutive repeated characters with the character followed by count.

Starter code

public class Main {
    public static void main(String[] args) {
        String text = "aaabbbccc"; // Test case 1
        
        // Compress: consecutive chars -> char + count
        // Print: Compressed: <result>
    }
}

Expected output and test cases

Compress aaabbbccc
```
Compressed: a3b3c3
```
Compress aabcccc
```
Compressed: a2b1c4
```
No compression needed
```
Compressed: a1b1c1
```

Hints

Track current character and count
When character changes, append to result
Don't forget the last group

Validated solution

Reveal Java solution

public class Main {
    static String compress(String text) {
        if (text.isEmpty()) return "";
        StringBuilder result = new StringBuilder();
        int run = 1;
        for (int i = 1; i <= text.length(); i++) {
            if (i < text.length() && text.charAt(i) == text.charAt(i - 1)) run++;
            else { result.append(text.charAt(i - 1)).append(run); run = 1; }
        }
        return result.toString();
    }
    public static void main(String[] args) {
        System.out.println("Compressed: " + compress("aaabbbccc"));
    }
}

How to approach the problem

Run-length encoding needs a count for the current group. The loop deliberately reaches length so the final group is flushed by the same branch that flushes earlier groups.

Approach

Handle an empty String before reading character zero.
Grow a run while neighboring characters match.
Append the previous character and count when a run ends.

Time and space complexity

Time: O(n). Space: O(n).

Edge cases to test

One character compresses to that character plus 1.
A compressed representation can be longer than the original.

Common mistakes

Forgetting to append the final run.
Resetting the count before appending it.

Follow-up challenge

Return the original text when compression does not save space.

Related Strings exercises

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