Consecutive strings in Java

strarr = ["tree", "foling", "trashy", "blue", "abcdef", "uvwxyz"], k = 2

Concatenate the consecutive strings of strarr by 2, we get:

treefoling   (length 10)  concatenation of strarr[0] and strarr[1]
folingtrashy ("      12)  concatenation of strarr[1] and strarr[2]
trashyblue   ("      10)  concatenation of strarr[2] and strarr[3]
blueabcdef   ("      10)  concatenation of strarr[3] and strarr[4]
abcdefuvwxyz ("      12)  concatenation of strarr[4] and strarr[5]

Two strings are the longest: "folingtrashy" and "abcdefuvwxyz".
The first that came is "folingtrashy" so 
longest_consec(strarr, 2) should return "folingtrashy".

In the same way:
longest_consec(["zone", "abigail", "theta", "form", "libe", "zas", "theta", "abigail"], 2) --> "abigailtheta"

class LongestConsec {
    public static String longestConsec(String[] strarr, int k) {
        if (strarr.length == 0 || k > strarr.length || k <= 0)
            return "";

        String longestStr = "";
        for (int index = 0; index < strarr.length - k + 1; index++) {
            StringBuilder sb = new StringBuilder();
            for (int i = index; i < index + k; i++) {
                sb.append(strarr[i]);
            }
            if (sb.toString().length() > longestStr.length()) {
                longestStr = sb.toString();
            }
        }
        return longestStr;
    }
}

import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

class LongestConsec {
    public static String longestConsec(String[] strarr, int k) {
        if (k <= 0) {
            return "";
        }

        return IntStream.rangeClosed(0, strarr.length - k)
                .mapToObj(i -> Arrays.stream(strarr, i, i + k).collect(Collectors.joining()))
                .max(Comparator.comparingInt(String::length))
                .orElse("");
    }
}

class LongestConsec {
    
    public static String longestConsec(String[] strarr, int k) {
        
        if(k < 1 || strarr.length < k || strarr.length == 0){
          return "";
        }
        
        int startIndex = 0;
        int max = strarr[0].length();
        int sum = 0;
        
        for(int i = 0; i < strarr.length; i++){
          sum += strarr[i].length();
          
          if(i > k - 1){
            sum -= strarr[i - k].length();
          }
          
          if(sum > max){
            max = sum;
            startIndex = i - k + 1;
          }
        }
        
        String result = "";
        for (int i = startIndex; i < startIndex + k; i++){
          result += strarr[i];
        }
        return result;
    }
}

import static org.junit.Assert.*;
import org.junit.Test;

public class LongestConsecTest {

    private static void testing(String actual, String expected) {
        assertEquals(expected, actual);
    }
    @Test
    public void test() {
        System.out.println("longestConsec Fixed Tests");
        testing(LongestConsec.longestConsec(new String[] {"zone", "abigail", "theta", "form", "libe", "zas", "theta", "abigail"}, 2), "abigailtheta");
        testing(LongestConsec.longestConsec(new String[] {"ejjjjmmtthh", "zxxuueeg", "aanlljrrrxx", "dqqqaaabbb", "oocccffuucccjjjkkkjyyyeehh"}, 1), "oocccffuucccjjjkkkjyyyeehh");
        testing(LongestConsec.longestConsec(new String[] {}, 3), "");
        testing(LongestConsec.longestConsec(new String[] {"itvayloxrp","wkppqsztdkmvcuwvereiupccauycnjutlv","vweqilsfytihvrzlaodfixoyxvyuyvgpck"}, 2), "wkppqsztdkmvcuwvereiupccauycnjutlvvweqilsfytihvrzlaodfixoyxvyuyvgpck");
        testing(LongestConsec.longestConsec(new String[] {"wlwsasphmxx","owiaxujylentrklctozmymu","wpgozvxxiu"}, 2), "wlwsasphmxxowiaxujylentrklctozmymu");
        testing(LongestConsec.longestConsec(new String[] {"zone", "abigail", "theta", "form", "libe", "zas"}, -2), "");
        testing(LongestConsec.longestConsec(new String[] {"it","wkppv","ixoyx", "3452", "zzzzzzzzzzzz"}, 3), "ixoyx3452zzzzzzzzzzzz");
        testing(LongestConsec.longestConsec(new String[] {"it","wkppv","ixoyx", "3452", "zzzzzzzzzzzz"}, 15), "");
        testing(LongestConsec.longestConsec(new String[] {"it","wkppv","ixoyx", "3452", "zzzzzzzzzzzz"}, 0), "");
    }
}