The “Split Strings” Challenge using Java

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

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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)));
    }
}

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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;
    }
}

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

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