AttributeSet is null, unsure how to fix this

Question

So my problem is that for some reason my AttributeSet is null and I'm not sure how to fix this.

public class MyDocumentFilter extends DocumentFilter {

//TODO FIX THIS, as IS NULL FOR SOME REASON

@Override
public void replace(FilterBypass fb, int i, int o, String s, AttributeSet set) throws BadLocationException{
    for(int n = s.length(); n > 0; n--){
        char c = s.charAt(n - 1);
        System.out.println(set);
        if(Character.isAlphabetic(c) || c == ' '){
            super.replace(fb, i, o, String.valueOf(c), set);
        }else{
            JOptionPane.showMessageDialog(null, "Error: Type field must not contain numbers and should be longer than 1 character");
        }
    }
}

@Override
public void remove(FilterBypass fb, int i, int o) throws  BadLocationException{
    super.remove(fb, i, o);
}

public void insertString(FilterBypass fb, int i, String s, AttributeSet set) throws BadLocationException{
    super.insertString(fb, i, s, set);
}

this is my code, which I had based off of another answer on stackoverflow.(jTextField accept only alphabet and white space)

I could have sworn this worked properly the other day, and when I gave my program test data today it started giving me NullPointerExceptions. I'm completely lost as to why my AttributeSet is null, can someone help?

Edit1: Okay now I'm not completely sure that it's my AttributeSet that's null. I'm also not sure if it's the .replace() method that's the source of the NPE. There is another class where I initialize all my swing components and if I delete out typeField.setText(null) (typeField is the name of the jTextArea), the program doesn't throw NPE anymore. Now I'm even more baffled.

Answer 1

You are not printing the char it should be System.out.println(c); not System.out.println(set);

Edit: if you call a method of a null object it will throw a NullPointerException. Check if it is null before calling a method.

Edit 1: This test case doesn't throw a NullPointerException. Something else could be causing the problem.

import javax.swing.*;
import javax.swing.text.AbstractDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;
import java.awt.event.ActionEvent;

class Test {
public static String input = null;

private Test() {
    initComponents();
}

public static void main(String[] args) {
    SwingUtilities.invokeLater(Test::new);
}

private void initComponents() {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JTextField jtf = new JTextField();
    ((AbstractDocument) jtf.getDocument()).setDocumentFilter(new MyDocumentFilter());

    frame.add(jtf);

    jtf.addActionListener(new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            input = jtf.getText();
            System.out.println(input);
            }
        });

        frame.pack();
        frame.setVisible(true);
    }
}

class MyDocumentFilter extends DocumentFilter {
    @Override
    public void replace(FilterBypass fb, int i, int o, String s, AttributeSet set) throws BadLocationException{
        for (int n = s.length(); n > 0; n--) {
            char c = s.charAt(n - 1);
            if (Character.isAlphabetic(c) || c == ' ') {
                super.replace(fb, i, o, String.valueOf(c), set);
            } else {
                JOptionPane.showMessageDialog(null, "Error: Type field must not contain numbers and should be longer than 1 character");
            }
        }
    }

@Override
public void remove(FilterBypass fb, int i, int o) throws  BadLocationException {
    super.remove(fb, i, o);
}

public void insertString(FilterBypass fb, int i, String s, AttributeSet set) throws BadLocationException {
    super.insertString(fb, i, s, set);
}
}

Edit 2: Hacky solution to clear the text field

jtf.addActionListener(new AbstractAction() {
        @Override
        public void actionPerformed(ActionEvent e) {
            try {
                jtf.setText(" "); 

                //Simulated backspace to delete space
                Robot robot = new Robot();
                int backspace = KeyEvent.VK_BACK_SPACE;
                robot.keyPress(backspace);
                robot.keyRelease(backspace);
            } catch (AWTException ex) {
                ex.printStackTrace();
            }
        }
    });

Answer 2

It's probably because you print set instead of char c. You should look into the error stack trace to see what line exception is thrown and try to figure out what object in that line cause the exception.

Answer 3

The set that is being passed to this method is most likely null.