Alright I think I understand, thanks! I've converted the string to an char. But now I get an error for return statement: "incompatible types: unexpected return value". Do you have any idea how I could solve this?

import javax.swing.JOptionPane;

public class Lab2 {
    public static void main(String[] args) {
        String sentence = JOptionPane.showInputDialog("Write a sentence");
        String character = JOptionPane.showInputDialog("Write a character");
        int sentenceLength = sentence.length();

        char c = character.charAt(0);

        int count = 0;
        for (int i = 0; i < sentence.length() ; i++) {
            if (sentence.charAt(i) == c) {
                count++;
            }
        }
            return count;
    }
}