计算字符串中空格和句点数的递归方法

Question

I'm having a bit of trouble trying to create a recursive method that counts the number of periods and spaces in a string. I can do this pretty easily using iteration, but I'm still fairly new to the concept of recursion. Here's my code so far, could anyone tell me where I'm going wrong?

public static int periodsAndSpaces(String s){ //option 3
    if(s.length()<0){ //base case
        return 0;
    }

    else if(s.charAt(0) == ' ' || s.charAt(0) == '.'){ //general case
        return periodsAndSpaces(s.substring(1)) + 1;
    }
    return 0;
}

Answer 1

package com.test.demo;

public class Counter {

    public static void main(String[] args) {
        System.out.println(new Counter().countPeriodsAndSpaces(" test. . .a"));
    }

    int countPeriodsAndSpaces(String rs) {
        if (rs == null || rs.isEmpty()) 
            return 0;
        
        char c = rs.charAt(0);
        if (c == ' ' || c == '.') 
            return 1 + countPeriodsAndSpaces(rs.substring(1)); 
        else 
            return countPeriodsAndSpaces(rs.substring(1)); 
    }
    
}

// Output 6

Answer 2

New method with everyone's suggestions:

public static int periodsAndSpaces(String s){ //option 3
    if(s.length()==0){ //base case
        return 0;
    }

    else if(s.charAt(0) == ' ' || s.charAt(0) == '.'){ 
        return periodsAndSpaces(s.substring(1)) + 1;
    }
    else{
        return periodsAndSpaces(s.substring(1));
    }
}

Method stops running when there are no more characters left in the string, and I've added another case for adding onto the sum of periods and spaces when the current char doesn't have either of these.