Using try-catch to catch exception when user input == null but it won't work

Question

The try-catch is supposed to catch if the user doesn't input anything on both or just one of them but right now it does nothing. It crashes if you input a sentence but not a Character but not vice versa. I'm quite new to Java but here is what I have:

import javax.swing.*;

public class Main {

public static void main(String[] args) {
    countSentence();
}

static void countSentence() {
    String input;
    char ch;
    int count = 0;
    int countLetters = 0;

    
    input = JOptionPane.showInputDialog(null, "Write a sentence");

    ch = JOptionPane.showInputDialog(null, "Write a character").charAt(0);

    if (!input.equals("") || ch != ' ') {
        try {
            for (int i = 0; i < input.length(); i++) {
                if (input.charAt(i) != ' ')
                    count++;
            }


            for (int i = 0; i < input.length(); i++) {
                if (input.charAt(i) == ch)
                    countLetters++;
            }

            JOptionPane.showMessageDialog(null,
                    "The sentence has " + count + " characters \n" + "The character " + ch
                            " occurs " + countLetters + " times. " +
                            "First time at index place " + input.indexOf(ch) +
                            "\n Last time at index place " + input.lastIndexOf(ch));
        }
        catch (Exception e) {
            JOptionPane.showMessageDialog(null, "You need to input both a sentence and a character!");
        }
    }

}

}

Answer 1

Here your a referencing the char at the first position

ch = JOptionPane.showInputDialog(null, "Write a character").charAt(0);

But if the user does not enter something the code fails with an index out of range Exception.

I would do a rewrite like this

static void countSentence() {
String input;
String ch;
int count = 0;
int countLetters = 0;



input = JOptionPane.showInputDialog(null, "Write a sentence");

ch = JOptionPane.showInputDialog(null, "Write a character");

if (input.equals("") || ch.equals("")) {
    JOptionPane.showMessageDialog(null, "You need to input both a sentence and a character!");
} else {        
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) != ' ')
                count++;
        }

        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == ch.charAt(0))
                countLetters++;
        }

        JOptionPane.showMessageDialog(null,
                "The sentence has " + count + " characters \n" + "The character " + ch +
                        " occurs " + countLetters + " times. " +
                        "First time at index place " + input.indexOf(ch) +
                        "\n Last time at index place " + input.lastIndexOf(ch));
    }        
} }

Do not use try-catch for Flow Control. See: https://wiki.c2.com/?DontUseExceptionsForFlowControl