cannot find symbol - javax.swing.JTextField.getDocument().setDocumentFilter(javax.swing.text.DocumentFilter)

Question

I'm using BlueJ, and I have two classes.

public class mainMenu extends JFrame
{
    ArrayList<String> one=new ArrayList<String>();
    ArrayList<String> zero=new ArrayList<String>();
    public static void main(String[] args){
        mainMenu m=new mainMenu();
    }

    public mainMenu(){
        JPanel p=new JPanel();
        JTextField idOne=new JTextField(4);
        DocumentFilter fOne=new LengthFilter();
        idOne.getDocument().setDocumentFilter(fOne);
    }
}

and

class LengthFilter extends DocumentFilter{
    private int max=4;

    public void insertString(DocumentFilter.FilterBypass fb, int offset,String text, AttributeSet attr) throws BadLocationException {  
        if (fb.getDocument().getLength() + text.length() <= max){
            fb.insertString(offset, text, attr);  
        }else {
            Toolkit.getDefaultToolkit().beep();  
        }
    }

    public void replace(DocumentFilter.FilterBypass fb, int offset, int length,  
    String text, AttributeSet attr) throws BadLocationException {  
        if (fb.getDocument().getLength() + text.length() - length <= max){
            fb.replace(offset, length, text, attr); 
        }else {
            Toolkit.getDefaultToolkit().beep();
        }
    }  
}

When I try to compile the first class, it comes up with a compiler error:

cannot find symbol - method setDocumentFilter(javax.swing.text.DocumentFilter)

Why and how do I fix this?

Answer 1

setDocumentFilter is undefined for the Document class. You need

DocumentFilter filter = new LengthFilter();
((AbstractDocument)idOneTextField.getDocument()).setDocumentFilter(filter);