The challenge

You are given a string. You must replace any occurrence of the sequence coverage by covfefe, however, if you don’t find the word coverage in the string, you must add covfefe at the end of the string with a leading space.

The solution in Java code

Option 1:

public class Covfefe {
    public static String covfefe(String tweet) {
        return tweet.contains("coverage") ?
                tweet.replaceAll("coverage", "covfefe") :
                tweet+" covfefe";
    }
}

Option 2:

public class Covfefe {
    public static String covfefe(String tweet) {
        String res;
        res = tweet.replace("coverage", "covfefe");
        if(res.equals(tweet)){
          res = res + " covfefe";
        }
        return res;
    }
}

Option 3:

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

public class Covfefe {
    private static String trasform(String word) {
        if (word.equals("coverage")) {
            return "covfefe";
        }
        return word;
    }

    public static String covfefe(String tweet) {
        String[] words = tweet.split(" ");
        if(tweet.contains("coverage")){
            return Arrays.asList(words).stream().map(n -> trasform(n)).collect(Collectors.joining(" "));
        }
        return tweet + " covfefe";
    }
}

Test cases to validate our solution

import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;


public class CovfefeTest {
    @Test
    public void basicTest() {
        assertEquals("covfefe", Covfefe.covfefe("coverage"));
        assertEquals("covfefe covfefe", Covfefe.covfefe("coverage coverage"));
        assertEquals("nothing covfefe", Covfefe.covfefe("nothing"));
        assertEquals( "double space  covfefe" ,Covfefe.covfefe("double space "));
        assertEquals("covfefe covfefe", Covfefe.covfefe("covfefe"));
    }
    
}