Can't make button visible in Java Swing

Question

I have a button called 'Play Again!' that I want to hide and then show when the user wins or loses and wants to play again.

This is my function:

public void checkGuess() {
    String guessText = txtGuess.getText();
    txtGuess.setText("");// Empties the contents of the text field.
    String message = "";
    try {
        int guess = Integer.parseInt(guessText);
        if (numberOfTries == 0) {
            message = "You Lost! A new game has begun and you have 8 guesses remaining.";
        }
        else if (guess < theNumber) {
            message = guess + " is too low. Try again. You have " + numberOfTries + " tries left!";

        }
        else if (guess > theNumber) {
            message = guess + " is too high. Try again. You have " + numberOfTries + " tries left!";
        }
        else {
            message = guess + " is correct. Let's play again!";
        }
    } catch (Exception e) {
        message = "Enter a whole number between 1 and 100.";
    } finally {
        lblOutput.setText(message);
        txtGuess.requestFocus();
        txtGuess.selectAll();
        
    }
    decrementNumberOfTries();
}

If I show the button when it's created the button is visible. This is only a demo I want to hide it when the program starts, by adding the lines:

    JButton btnPlayAgain = new JButton("Play Again");
    btnPlayAgain.setVisible(true);

Ordinarily it would start out as hidden:

    JButton btnPlayAgain = new JButton("Play Again");
    btnPlayAgain.setVisible(false);

This is how the window should look when the button is hidden:

If I set the button visible where I want it in my function, the button does not become visible and the other messages (such as "You lost...") don't appear on the screen at all :

public class GuessingGame extends JFrame {
    private static final long serialVersionUID = 1L;
    private JLabel lblOutput;
    private int theNumber;
    private JTextField txtGuess;
    private int numberOfTries = 8;
    private JButton btnPlayAgain;
    
    public void checkGuess() {
        String guessText = txtGuess.getText();
        txtGuess.setText("");// Empties the contents of the text field.
        String message = "";
        try {
            int guess = Integer.parseInt(guessText);
            if (numberOfTries == 0) {
                message = "You Lost! A new game has begun and you have 8 guesses remaining.";
                btnPlayAgain.setVisible(false);
                
            }
            else if (guess < theNumber) {
                message = guess + " is too low. Try again. You have " + numberOfTries + " tries left!";

            }
            else if (guess > theNumber) {
                message = guess + " is too high. Try again. You have " + numberOfTries + " tries left!";
            }
            else {
                message = guess + " is correct. Let's play again!";
                btnPlayAgain.setVisible(false);
            }
        } catch (Exception e) {
            message = "Enter a whole number between 1 and 100.";
        } finally {
            lblOutput.setText(message);
            txtGuess.requestFocus();
            txtGuess.selectAll();
            
        }
        decrementNumberOfTries();
    }

I'm declaring the button at the top of my class:

private JButton btnPlayAgain;

These are my decermentNumberOfTries() and newGame() methods:

public void newGame() {
    numberOfTries = 8;
    theNumber = (int) (Math.random() * 100 + 1);
}

public void decrementNumberOfTries() {
    --numberOfTries;        
}

And this is the function that creates the window:

public GuessingGame() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setTitle("Tim's Hi-Lo Guessing Game");
    getContentPane().setLayout(null);

    JLabel lblTitle = new JLabel("Tim's Hi-Lo Guessing Game");
    lblTitle.setFont(new Font("Tahoma", Font.BOLD, 15));
    lblTitle.setHorizontalAlignment(SwingConstants.CENTER);
    lblTitle.setBounds(-10, 15, 436, 32);
    getContentPane().add(lblTitle);

    JLabel lblGuessANumber = new JLabel("Guess a number between 1 and 100:");
    lblGuessANumber.setBackground(new Color(240, 240, 240));
    lblGuessANumber.setHorizontalAlignment(SwingConstants.RIGHT);
    lblGuessANumber.setBounds(83, 57, 215, 13);
    getContentPane().add(lblGuessANumber);

    txtGuess = new JTextField();
    txtGuess.addActionListener((ActionEvent e) -> {
        checkGuess();
    });
    txtGuess.setHorizontalAlignment(SwingConstants.RIGHT);
    txtGuess.setBounds(239, 85, 27, 19);
    getContentPane().add(txtGuess);
    txtGuess.setColumns(10);

    JButton btnGuess = new JButton("Guess!");
    btnGuess.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            checkGuess();
        }
    });
    btnGuess.setBounds(146, 84, 85, 21);
    getContentPane().add(btnGuess);

    lblOutput = new JLabel("Enter a number above and click Guess!");
    lblOutput.setHorizontalAlignment(SwingConstants.CENTER);
    lblOutput.setBounds(-40, 152, 500, 13);
    getContentPane().add(lblOutput);
    
    JButton btnPlayAgain = new JButton("Play Again");
    btnPlayAgain.setVisible(false);
    btnPlayAgain.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            newGame();
        }
    });
    btnPlayAgain.setBounds(146, 115, 132, 21);
    getContentPane().add(btnPlayAgain); 
    
}

My questions are, why does the button not become visible when I set visible to true?

If I try to set visible in an if statement like this:

        if (numberOfTries == 0) {
            message = "You Lost! A new game has begun and you have 8 guesses remaining.";
            btnPlayAgain.setVisible(false);
            
        }

Why does the message not appear?

How can I set the button to visible correctly?

This is all my code, in case that helps: full code

Answer 1

The problem is a NullPointerException when you try to put visible the button.

if (numberOfTries == 0) {
    message = "You Lost! A new game has begun and you have 8 guesses remaining.";
    btnPlayAgain.setVisible(false); //This is null
    
}

To fix this problem you need to modify the declaration of the btnPlayAgain. In your code in the Main was declared like this:

JButton btnPlayAgain = new JButton("Play Again");

Try to change to this:

btnPlayAgain = new JButton("Play Again");