限制JTextField的字符数

Question

I want to limit the number of characters to be inserted in a JTextField to 10 character. I use a graphical interface under netbeans.thanks.

Answer 1

To prevent the user from entering more the 10 charaters in a textfield you can use a document.

Create a simple document by extending PlainDocument and to change the behavior override

public void insertString(...) throws BadLocationException

This is an example

public class MaxLengthTextDocument extends PlainDocument {
    //Store maximum characters permitted
    private int maxChars;

    @Override
    public void insertString(int offs, String str, AttributeSet a)
            throws BadLocationException {
        // the length of string that will be created is getLength() + str.length()
        if(str != null && (getLength() + str.length() < maxChars)){
            super.insertString(offs, str, a);
        }
    }

}

After this, only insert your implementation in JTextField , this way:

...
MaxLengthTextDocument maxLength = new MaxLengthTextDocument();
maxLength.setMaxChars(10); 

jTextField.setDocument(maxLength);
...

And that's it!

Answer 2

Best thing is you will get running demo ;)

class JTextFieldLimit extends PlainDocument {
  private int limit;
  JTextFieldLimit(int limit) {
    super();
    this.limit = limit;
  }

  JTextFieldLimit(int limit, boolean upper) {
    super();
    this.limit = limit;
  }

  public void insertString(int offset, String str, AttributeSet attr) throws BadLocationException {
    if (str == null)
      return;

    if ((getLength() + str.length()) <= limit) {
      super.insertString(offset, str, attr);
    }
  }
}

public class Main extends JFrame {
  JTextField textfield1;

  JLabel label1;

  public static void main(String[]args){
      new Main().init();
  }
  public void init() {
    setLayout(new FlowLayout());
    label1 = new JLabel("max 10 chars");
    textfield1 = new JTextField(15);
    add(label1);
    add(textfield1);
    textfield1.setDocument(new JTextFieldLimit(10));

    setSize(300,300);
    setVisible(true);
  }
}

Answer 3

Read the section from the Swing tutorial on Implementing a DocumentFilter for a more current solution.

This solution will work an any Document, not just a PlainDocument.