Difference between equals() and startsWith() in java

Question

Im practicing recursion for my own amusement using the codingBat exercises. Im doing this exercise:

Given a string, compute recursively the number of times lowercase "hi" appears in the string, however do not count "hi" that have an 'x' immedately before them.

countHi2("ahixhi") → 1
countHi2("ahibhi") → 2
countHi2("xhixhi") → 0

I tried to do this code but it keep throwing out of bounds exception:

public int countHi2(String str){    
    if(str.length()<2){
        return 0;
    }
    else if(str.substring(0,3).equals("xhi")){
        return countHi2(str.substring(3));
    }
    else if(str.substring(0,2).equals("hi")){
        return 1+countHi2(str.substring(2));
    }
    else{
        return countHi2(str.substring(1));
    }
}

I changed the substring() and equals to startsWith()

else if(str.startsWith("xhi")){
            return countHi2(str.substring(3));

And now works perfectly, can someone please point out why my first code wasnt correct? Is there a difference between startsWith() and equals()?

Answer 1

First you make sure the string has at least 2 characters in it, then you test if the first three characters are xhi . String.substr throws an exception if the string is not long enough.

String.startsWith doesn't have this problem, it doesn't throw an exception when you check whether a 2 character string starts with 3 characters - it just returns false .