java.lang.StringIndexOutOfBoundsException: String index out of range: 4

Question

I have been trying to write a Java program which converts the first letter of every word of a string into a capital letter. Right now it looks like this:

package strings;
import java.util.Scanner;

public class small_cap {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the sentence");
        String st = sc.next();
        String str = " " + st;

        int j = 0; char chr = ' ';

        for (int i = 0; i < str.length(); i++){
            j = i + 1;
            chr = str.charAt(j);
            if (chr == ' '){
                char a = Character.toUpperCase(str.charAt(j));
                str = str.replace(str.charAt(j), a);
            }
            else{
                char a = Character.toLowerCase(str.charAt(j));
                str = str.replace(str.charAt(j), a);
            }
        }
        System.out.println(str);
    }
}

Unfortunately I keep on getting the error:

java.lang.StringIndexOutOfBoundsException: String index out of range: 4
    at java.lang.String.charAt(String.java:658)
    at small_cap.main(small_cap.java:19)

I don't really see any fault in the code. Can someone please point out where I am going wrong?

Answer 1

    for (int i = 0; i < str.length(); i++){
        j = i + 1;

When i reaches the last valid index length - 1 , j will be equal to length , which is out of bounds.

I don't really see the point of the j variable to begin with - did you mean to use i somewhere else inside the loop as well, or should you just make your loop start from 1? Or did you perhaps mean to check the previous character by doing j = i - 1; (in that case make sure you don't read before index 0)

Answer 2

1. You were using Scanner.next() rather then Scanner.nextLine() which will read the whole sentence rather then single word.
2. You are adding extra space in String. Don't know the big reason behind that. It can be done without it.
3. Let say the Input is : "abc def" which will become " abc def" after adding extra space. Length of string will become : 7
4. Now for loop will iterate from 0 to 6. But on i = 6 it will try to alter the element on 7th position (since you are doing j=i+1) which will cause string index out of range error.

5. You are using String.Replace which will replace all matching characters irrespective of their position.

   import java.util.Scanner;

    public class small_cap { public static void main(String[] args) { Scanner sc = new Scanner(System.in); System.out.println("Enter the sentence"); String st = sc.nextLine(); String str = " " + st; int j = 0; char chr = ' '; for (int i = 0; i < str.length()-1; i++){ j = i+1; chr = str.charAt(i); if (chr == ' '){ char a = Character.toUpperCase(str.charAt(j)); str = str.substring(0,j)+a+str.substring(j+1); } else{ char a = Character.toLowerCase(str.charAt(j)); str = str.substring(0,j)+a+str.substring(j+1); } } System.out.println(str); } 

   }