No "Invalid Input" after input "Alas! it failed#' In Pangram Word

[YashDoesCode]

[CodesByPrafful]

package ISC_XII_Computer_Science;

import java.util.Scanner;

public class PangramWord {
String sentence = "";

public void accept() {
    Scanner scanner = new Scanner(System.in);
    System.out.println("Enter a sentence (end with '.', '!', or '?'):");

    while (true) {
        String input = scanner.nextLine();
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == '.' || input.charAt(i) == '!' || input.charAt(i) == '?') {
                sentence += input;
                if (input.charAt(input.length() - 1) == '.' || input.charAt(input.length() - 1) == '!' || input.charAt(input.length() - 1) == '?') {
                    System.out.println("Sentence: " + sentence.trim());
                    return;
                } else {
                    System.out.println("INVALID INPUT");
                    sentence = "";
                    return;
                }
            }
        }
        sentence += input + " ";
    }
}

public void isPangram() {
    boolean[] alphabet = new boolean[26];
    String Long = "", Short = "";
    boolean hasLetters = false;
    int wordStart = 0, len = 0;

    for (int i = 0; i < sentence.length(); i++) {
        char store = sentence.charAt(i);

        if (store == ' ' || store == '.' || store == '!' || store == '?') {
            if (len > 0) {
                String word = sentence.substring(wordStart, wordStart + len);

                if (Long.length() < word.length()) {
                    Long = word;
                }
                if (Short.length() == 0 || Short.length() > word.length()) {
                    Short = word;
                }

                for (int k = 0; k < word.length(); k++) {
                    char ch = Character.toLowerCase(word.charAt(k));
                    if (ch >= 'a' && ch <= 'z') {
                        alphabet[ch - 'a'] = true;
                        hasLetters = true;
                    }
                }
                len = 0;
            }
        } else {
            if (len == 0) {
                wordStart = i;
            }
            len++;
        }
    }

    boolean isPangram = true;
    for (boolean letter : alphabet) {
        if (!letter) {
            isPangram = false;
        }
    }

    if (hasLetters) {
        if (isPangram) {
            System.out.println("IT IS A PANGRAM LONGEST WORD: " + Long + " SHORTEST WORD: " + Short);
        } else {
            System.out.println("IT IS NOT A PANGRAM LONGEST WORD: " + Long + " SHORTEST WORD: " + Short);
        }
    } else {
        System.out.println("INVALID INPUT");
    }
}

public static void main(String[] args) {
    PangramWord obj = new PangramWord();
    obj.accept();
    obj.isPangram();
}

}

[YashDoesCode]

That Works for me
Thanks !