How do I reverse a string in java by creating a new temporary string?

Question

What's the error in the following code? I am not able to identify it shows the index 0 out of bounds exception.

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

            StringBuilder str = new StringBuilder("Shubham");
            StringBuilder str2= new StringBuilder();

            int j=0;

            for(int i = str.length()-1; i>=0 ; i--)
            {
                str2.setCharAt(j++,str.charAt(i));
            }

            System.out.print(str2);

        }

}

Answer 1

As you are already iterating from end to start you can just append the characters to the fresh and empty string builder. setChar() is only intended to replace existing characters.

public class StringBuilders {
    public static void main(String[] args) {
        String str = "Shubham";
        StringBuilder str2 = new StringBuilder();

        for (int i = str.length() - 1; i >= 0; i--) {
            str2.append(str.charAt(i));
        }

        System.out.println(str2.toString());
    }
}

gives

$ java StringBuilders.java
mahbuhS

Answer 2

You haven't declared a value to the str2 string.

The for loop is trying to find a character at index j=0 in str2 to set value i of str2, which it cannot find.

Hence the error.

For this to work, you need declare a value of str2 to a length >= str1. i.e., if str1 = "0123456", str2 should be at least 0123456 or more.

Instead of using StringBuilder to set the char, you can use String to just append to it.

        String str1 = new String("Shubham");
        String str2 = new String();
        
        int iter = str1.length() -1;
        
        for(int i=iter; i>=0; i--) {
            str2 += str1.charAt(i);
        }
        
        System.out.println(str2)