You are concatenating the result of previous computations. Also, your final output have trailing spaces. Seems like you've solved it though, closing!

Who knows? We're not mindreaders.

Not a valid kata issue. See https://docs.codewars.com/training/troubleshooting for tips.

Documentation suggests that package slices is only available since version 1.21, but only 1.20 is available at the moment. So, don't import unsupported packages.

You're not returning anything, that's why. Check out https://docs.codewars.com/training/training-example

Your code not working is not a kata issue. Please check https://docs.codewars.com/training/troubleshooting for some tips.

If you want to post code, use code formatting: https://docs.codewars.com/references/markdown/#displaying-code

The issue lies with your if-check. Can't get into more detail otherwise you wouldn't be able to see this comment if it's marked as a spoiler. But, essentially, you're not applying the case-insensitivity on the original string. You're doing it on the wrong parts. Think about how and why your solution would mess up with strings containing uppercase chars like HOOP or hOOp. Are you searching for a version of that char that doesn't exist? That's about as many hints I can give without spoiling.

expected:<emocleW[]> but was:<emocleW[ ]>
I don't have ideas what's this problem is about?

public class SpinWords {

public String spinWords(String sentence) {
    String[] str = sentence.split(" ");
    String result = "";
    for(int i = 0; i<str.length;i++){
        if(str[i].length()>=5){
            result+=new StringBuilder(str[i]).reverse().toString()+" ";
        }
        else{
            result+=str[i]+" ";
        }
    }
    return result.trim()+" ";
}

}