The “Split Strings” Challenge using Java

The challenge

Complete the solution so that it splits the string into pairs of two characters. If the string contains an odd number of characters then it should replace the missing second character of the final pair with an underscore (‘_’).

Examples:

StringSplit.solution("abc") // should return {"ab", "c_"}
StringSplit.solution("abcdef") // should return {"ab", "cd", "ef"}

Test cases

import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;
import java.util.Arrays;

public class SampleTest {
    @Test
    public void testEvenString() {
       String s = "abcdef";
       String s1 = "HelloWorld";
       assertEquals("Should handle even string","[ab, cd, ef]", Arrays.toString(StringSplit.solution(s)));
       assertEquals("Should handle even string","[He, ll, oW, or, ld]", Arrays.toString(StringSplit.solution(s1)));
    }
    
    @Test
    public void testOddString() {
       String s = "abcde";
       String s1 = "LovePizza";
       assertEquals("Should handle odd string","[ab, cd, e_]", Arrays.toString(StringSplit.solution(s)));
       assertEquals("Should handle odd string","[Lo, ve, Pi, zz, a_]", Arrays.toString(StringSplit.solution(s1)));
    }
  
  
    @Test
    public void testEmptyString() {
       String s = "";
       assertEquals("Should handle empty string","[]", Arrays.toString(StringSplit.solution(s)));
    }
}

The solution in Java

public class StringSplit {
    public static String[] solution(String s) {
      
        // if it's blank, then return
        if (s.length()==0) return new String[0];
      
        // if not even, then add an underscore to the end
        if (s.length()%2!=0) {
          s+="_";
        }
      
        // determine the space needed for the return array
        int spaceNeeded = s.length()/2;
        
        // create a new array to populate and return
        String[] out = new String[spaceNeeded];
      
        // create an builder index
        int j = 0;
      
        // loop through the input string
        for (int i=0; i<s.length(); i++) {
          
          // create a new string to build
          StringBuilder sb = new StringBuilder();
          // add the current item
          sb.append(s.charAt(i));
          // add the next item
          sb.append(s.charAt(i+1));
          
          // populate our output string as we go
          out[j] = sb.toString();
          
          // increment both counters
          i++;
          j++;
        }
      
        // return the built string
        return out;
    }
}

A simple alternative using regular expressions:

public class StringSplit {
    public static String[] solution(String s) {
        s = (s.length() % 2 == 0)?s:s+"_";
        return s.split("(?<=\\G.{2})");
    }
}

Another short regular expression solution:

class StringSplit {
  static String[] solution(String s) {
    return (s + (s.length() % 2 > 0 ? "_" : "")).split("(?<=\\G.{2})");
  }
}
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