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

String letter = JOptionPane.showInputDialog(null, "Write a letter");



while (true) {



    if (letter.equals("Stop"))

        System.exit(0);    

    //to calculate number of specific character

    else {

        int countLetter = 0;

        int L = letter.length();

        for (int i = 0; i < L; i++) {

            if ((letter.charAt(i) = .....))     

                countLetter++;

        }

    }

}

Is it possible to replace the dots to make the program count how many times the given letter occures in the sentence written in the first string?

Since Java 8, there is an elegant solution to this.

int count = letter.chars().filter(ch -> ch == 'e').count();

This will return the number of occurences of letter 'e'.

if your String letter contains a one character use this letter.charAt(0) and then replace dots with this. Also remember to use == instead of = here. = means you are just asigning and == uses to compare two values.

If you have to use a for loop and want to stick to the old fashioned way, try this:

    String sentence = "This is a really basic sentence, just for example purpose.";

    char letter = 'a';



    int occurrenceOfChar = 0;



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

        if (sentence.charAt(i) == letter) {

            occurrenceOfChar++;

        }

    }



    System.out.println("The letter '" + letter

            + "' occurs " + occurrenceOfChar

            + " times in the sentence ""

            + sentence + """);

The sentence and the letter are just examples, you have to read the user input.

You can use Guava Lib to perform this operation faster without iterating string.

CharMatcher.is('e').countIn("Write a letter");

Will return 3