I'm teaching myself java and I had some basic questions on how well I am writing my code for readability. I feel like sometimes I'm just writing code that works, and I'm not that proud of myself when I have finished. I have attached some code to see below. Simple Hangman game.

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;

public class Hangman implements ActionListener{
    JFrame frame;
    JTextField userInput;
    JLabel textContents;
    String word = "horse";
        int correctGuesses;
    int incorrectGuesses;
    String guessesLeft;
    boolean lose = false;
    StringBuilder wordsGuessedCorrectly;
    JLabel letters;


    Hangman() {
        incorrectGuesses = 0;
        correctGuesses = 0;
        guessesLeft = "You have " + (6 - incorrectGuesses) +" chances left to guess a " + word.length() + " letter word";
        wordsGuessedCorrectly = new StringBuilder();

        frame = new JFrame("A Hangman Game");
        frame.setSize(300,125);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new GridLayout(3, 1));

        userInput = new JTextField();
        userInput.addActionListener(this);
        frame.add(userInput);

        textContents = new JLabel();
        textContents.setText(guessesLeft);
        textContents.setHorizontalAlignment(JLabel.CENTER);
        textContents.setBorder(BorderFactory.createLineBorder(Color.BLUE));
        frame.add(textContents);

        letters = new JLabel("The letters you guess correctly go down here!");
        letters.setHorizontalAlignment(JLabel.CENTER);
        letters.setBorder(BorderFactory.createLineBorder(Color.RED));
        frame.add(letters);

        frame.setVisible(true);


    }

    String seperateLetter(String a, int x, int y) {
        return a.substring(x, y);
    }

    

    void testLetter(String a) {
        if (word.contains(a) && wordsGuessedCorrectly.toString().contains(a) == false && correctGuesses != (word.length() - 1)) {
            correctGuesses++;
            wordsGuessedCorrectly.append(a);
            letters.setText(wordsGuessedCorrectly.toString());
            textContents.setText("Correct Guess!");
        }
        else if (word.contains(a) && wordsGuessedCorrectly.toString().contains(a)) {
            textContents.setText("You've already guessed this letter!");
        }
        else if (word.contains(a) && wordsGuessedCorrectly.toString().contains(a) == false && correctGuesses == (word.length() - 1)) {
            lose = true;
            textContents.setText("You Win!");
            wordsGuessedCorrectly.append(a);
            letters.setText(wordsGuessedCorrectly.toString());
        }
        else if (word.contains(a) == false && incorrectGuesses == 5){
            textContents.setText("You lose!");
            lose = true;
        }
        else {
            incorrectGuesses++;
            textContents.setText("Incorrect Guess! You have " + (6 - incorrectGuesses)  + " left.");
        }
    }

    public void actionPerformed(ActionEvent ae) {
        if (lose == false) {
            testLetter(seperateLetter(userInput.getText(), 0 , 1));
            userInput.setText("");
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Hangman();
            }
        }); 
    }
}

• I'm not asking you to rewrite my code.
• I'm just asking for some pointers on how optimize my code for readability.
• Please comment why you downvote my question, I won't learn to ask better questions if you don't.