java - JFormattedTextField's is add characters on focus

Question

I have a JFormattedTextField being initialized with the code

JFormattedTextField f = new JFormattedTextField(createFormatter());
f.setValue(null);
f.setColumns(1);
f.setEditable(true);
f.setCaretPosition(0);

The textField is 1 column wide and in the createFormatter method I have

MaskFormatter formatter = null;
    try {
        formatter = new MaskFormatter("#");
        formatter.setValidCharacters("123456789");
    } catch (java.text.ParseException exc) {
        System.err.println("formatter is bad: " + exc.getMessage());
        System.exit(-1);
    }
    return (formatter);

As you can see I just want to numbers 1-9 in the text field. This works fine when tabbing to the text field but when I actually click on the text field to type I get this weird thing happening with the cursor.

Here the cursor is flashing blank... And the cursor flashing black.

As you can see the there is a little black dot in the top left corner and the cursor moves away from the left side. I can highlight the area to the left and cannot add any more characters to this text field (not even numbers 1-9). This makes me believe that when I focus on the text field with the cursor a character is being added. I do not know what this character is and I do not know how to fix this.

Does anyone know how this can be fixed?

Here is a sscce

public class FormattedTextField {

public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(100, 75);
    JPanel content = new JPanel(new FlowLayout());
    frame.setContentPane(content);

    JFormattedTextField f1 = new JFormattedTextField(createFormatter());
    f1.setValue(null);
    f1.setColumns(1);

    JFormattedTextField f2 = new JFormattedTextField(createFormatter());
    f2.setValue(null);
    f2.setColumns(1);

    content.add(f1);
    content.add(f2);

    frame.setVisible(true);
}

public static MaskFormatter createFormatter() {
    MaskFormatter formatter = null;
    try {
        formatter = new MaskFormatter("#");
        formatter.setValidCharacters("123456789");
    } catch (java.text.ParseException exc) {
        System.err.println("formatter is bad: " + exc.getMessage());
        System.exit(-1);
    }
    return (formatter);
}

}

In this example There is not a little black dot in the top left corner but when the text field is focused on with the mouse, a blank character is still added.

Answer 1

On my Mac with JDK7, I only get the annoying caret behavior with your code. I did not see the dot.

Switching from a MaskFormatter to a Format seems to solve this issue. My personal experience with JFormattedTextFields was always in combination with a Format which seems to work (after some little tweaks, see this post).

Anyway, here an adjusted version of your SSCCE which uses a Format

import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.text.MaskFormatter;
import java.awt.FlowLayout;
import java.text.AttributedCharacterIterator;
import java.text.FieldPosition;
import java.text.Format;
import java.text.NumberFormat;
import java.text.ParsePosition;

public class FormattedTextFieldDemo {
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(100, 75);
    JPanel content = new JPanel(new FlowLayout());
    frame.setContentPane(content);

    JFormattedTextField f1 = new JFormattedTextField(createFormat());
    f1.setValue(null);
    f1.setColumns(1);

    JFormattedTextField f2 = new JFormattedTextField(createFormat());
    f2.setValue(null);
    f2.setColumns(1);

    content.add(f1);
    content.add(f2);

    JFormattedTextField f3 = new JFormattedTextField(createFormatter());
    f3.setValue(null);
    f3.setColumns( 1 );

    JFormattedTextField f4 = new JFormattedTextField(createFormatter());
    f4.setValue(null);
    f4.setColumns(1);

    content.add(f3);
    content.add(f4);

    frame.setVisible(true);
  }

  private static MaskFormatter createFormatter() {
    MaskFormatter formatter = null;
    try {
      formatter = new MaskFormatter("#");
      formatter.setValidCharacters("123456789");
    } catch (java.text.ParseException exc) {
      System.err.println("formatter is bad: " + exc.getMessage());
      System.exit(-1);
    }
    return (formatter);
  }

  private static Format createFormat(){
    final NumberFormat format = NumberFormat.getInstance();
    format.setParseIntegerOnly( true );
    return new Format() {
      @Override
      public StringBuffer format( Object obj, StringBuffer toAppendTo, FieldPosition pos ) {
        return format.format( obj, toAppendTo, pos );
      }

      @Override
      public AttributedCharacterIterator formatToCharacterIterator( Object obj ) {
        return format.formatToCharacterIterator( obj );
      }


      @Override
      public Object parseObject( String source, ParsePosition pos ) {
        int initialIndex = pos.getIndex();
        Object result = format.parseObject( source, pos );
        if ( result != null && pos.getIndex() > initialIndex + 1 ) {
          int errorIndex = initialIndex + 1;
          pos.setIndex( initialIndex );
          pos.setErrorIndex( errorIndex );
          return null;
        }
        return result;
      }
    };
  }
}