How to resolve Expression must be a list type: String issue in Apex?

I have written Apex code to return a list of alphanumeric characters from a string in reverse order. However, I am encountering the error “Expression must be a list type: String”. I need assistance in resolving this error and ensuring my function reverseAlphaNumericCharacters correctly reverses the alphanumeric characters in the provided string.

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  public class assessment {
             public static List<String> reverseAlphaNumericCharacters(String s) {
                List<String> reversedChars = new List<String>();
                for (Integer i = s.length() - 1; i >= 0; i--) {
                    if (isAlphaNumeric(s[i])) {
                        reversedChars.add(s[i]);
                    }
                }
                return reversedChars;
            }

            private static Boolean isAlphaNumeric(String c) {
                return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z') || ('0' <= c && c <= '9');
            }

            }

Solution 1:

The issue arises because, in Apex, a string isn’t treated like an array of characters, and direct indexing like s[i] isn’t supported. Instead, you should use the substring method to get individual characters. Here’s how you can modify your reverseAlphaNumericCharacters method:

public class Assessment {
    public static List<String> reverseAlphaNumericCharacters(String s) {
        List<String> reversedChars = new List<String>();
        for (Integer i = s.length() - 1; i >= 0; i--) {
            String singleChar = s.substring(i, i + 1);
            if (isAlphaNumeric(singleChar)) {
                reversedChars.add(singleChar);
            }
        }
        return reversedChars;
    }

    private static Boolean isAlphaNumeric(String c) {
        // Assuming c is always a string of length 1
        return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z') || ('0' <= c && c <= '9');
    }
}

In this revised version:

Instead of s[i], s.substring(i, i + 1) is used to get a string containing the single character at position i.

The isAlphaNumeric function now correctly handles a string input, assuming it’s always a single character in length.

Solution 2:

The issue with your code arises from trying to use the square bracket notation (s[i]) to access characters in a string, which is not valid in Apex. Strings need to be accessed using the charAt() method. Also, charAt() returns a char, not a String, so you’ll need to convert the char to a String before adding it to the List<String>.

Here’s the corrected version of your code:

public class Assessment {
    public static List<String> reverseAlphaNumericCharacters(String s) {
        List<String> reversedChars = new List<String>();
        for (Integer i = s.length() - 1; i >= 0; i--) {
            String currentChar = String.valueOf(s.charAt(i)); // Convert char to String
            if (isAlphaNumeric(currentChar)) {
                reversedChars.add(currentChar);
            }
        }
        return reversedChars;
    }

    private static Boolean isAlphaNumeric(String c) {
        return ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z') || ('0' <= c && c <= '9');
    }
}

Changes made:

1. Used s.charAt(i) to get the character at index i from the string s.
2. Converted the character to a String using String.valueOf() before using it in the isAlphaNumeric function and adding it to the reversedChars list.

This should resolve the “Expression must be a list type: String” error you were encountering.

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