I think my if statement is returning as false and i am not sure why [duplicate]

My code is returning all the letters with a counter of zero. I am not sure why this problem is occurring.

public class LetterFrequencies {

    public static void main(String[] args) {

        String letters = "abcdefghijklmnopqrstuvwxyz";
        String phrase = "This is a phrase!";

        System.out.println("The following shows the letter frequencies for the phrase");

        /* your code here */
        int counter = 0;
        for (int i = 0; i <= 25; i++) {
            for (int j = 0; j <= 16; j++) {
                if (letters.substring(i, i + 1) == phrase.substring(j, j + 1)) {
                    counter++;
                }
            }
            System.out.println(letters.substring(i, i + 1) + counter);
        }
    }
}