我找不到“ java.lang.StringIndexOutOfBoundsException”错误的原因

Question

My code takes the first and last words of a user input and turns them into uppercase. However, when the user enters "" or "java"(word without spaces) an error pops up. I could not find the reason for the error.

    Scanner input = new Scanner(System.in);
    String s = input.nextLine();

    String first = "";
    String last = "";

    String end = s;
    int count = 0;

    for (int i = 0;i<s.length()-1;i++) {

        if (s.charAt(i) == ' ') {

            count++;

        }
        if (count == 0) {

            end = s.toUpperCase();
        }

        else if (count > 0) 

        first = s.substring(0,s.indexOf(' '));
        last = s.substring(s.lastIndexOf(' ')+1);

        end = (first.toUpperCase() + " " + s.substring(first.length()+1,s.indexOf(last)) + " " +last.toUpperCase());

    }


    System.out.println(count);

Answer 1

String first = s.substring(0,s.indexOf(' '));
String last = s.substring(s.lastIndexOf(' ')+1);

Your code here is causing the problem. If a space is not present in your String then indexOf(' ') will return -1. You obviously can't substring between 0 and -1. You need to check if ' ' is present in the String before you substring it.