如何限制 JTextField 中的字符数？

Question

How to limit the number of characters entered in a JTextField?

Suppose I want to enter say 5 characters max. After that no characters can be entered into it.

Answer 1

http://www.rgagnon.com/javadetails/java-0198.html

import javax.swing.text.PlainDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;

public class JTextFieldLimit extends PlainDocument {
  private int limit;

  JTextFieldLimit(int limit) {
   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);
    }
  }
}

Then

import java.awt.*;
import javax.swing.*;

 public class DemoJTextFieldWithLimit extends JApplet{
   JTextField textfield1;
   JLabel label1;

   public void init() {
     getContentPane().setLayout(new FlowLayout());
     //
     label1 = new JLabel("max 10 chars");
     textfield1 = new JTextField(15);
     getContentPane().add(label1);
     getContentPane().add(textfield1);
     textfield1.setDocument
        (new JTextFieldLimit(10));
     }
}

(first result from google)

Answer 2

If you wanna have everything into one only piece of code, then you can mix tim's answer with the example's approach found on the API for JTextField , and you'll get something like this:

public class JTextFieldLimit extends JTextField {
    private int limit;

    public JTextFieldLimit(int limit) {
        super();
        this.limit = limit;
    }

    @Override
    protected Document createDefaultModel() {
        return new LimitDocument();
    }

    private class LimitDocument extends PlainDocument {

        @Override
        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);
            }
        }       

    }

}

Then there is no need to add a Document to the JTextFieldLimit due to JTextFieldLimit already have the functionality inside.

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.

This is a more current solution than the one accepted.

Answer 4

Great question, and it's odd that the Swing toolkit doesn't include this functionality natively for JTextFields. But, here's a great answer from my Udemy.com course "Learn Java Like a Kid":

txtGuess = new JTextField();
txtGuess.addKeyListener(new KeyAdapter() {
    public void keyTyped(KeyEvent e) { 
        if (txtGuess.getText().length() >= 3 ) // limit textfield to 3 characters
            e.consume(); 
    }  
});

This limits the number of characters in a guessing game text field to 3 characters, by overriding the keyTyped event and checking to see if the textfield already has 3 characters - if so, you're "consuming" the key event (e) so that it doesn't get processed like normal.

Answer 5

Just Try This :

textfield.addKeyListener(new java.awt.event.KeyAdapter() {
    public void keyTyped(java.awt.event.KeyEvent evt) {
        if(textfield.getText().length()>=5&&!(evt.getKeyChar()==KeyEvent.VK_DELETE||evt.getKeyChar()==KeyEvent.VK_BACK_SPACE)) {
            getToolkit().beep();
            evt.consume();
         }
     }
});

Answer 6

I have solved this problem by using the following code segment:

private void jTextField1KeyTyped(java.awt.event.KeyEvent evt) {
    boolean max = jTextField1.getText().length() > 4;
    if ( max ){
        evt.consume();
    }        
}

Answer 7

import java.awt.KeyboardFocusManager;
import javax.swing.InputVerifier;
import javax.swing.JTextField;
import javax.swing.text.AbstractDocument;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;
import javax.swing.text.DocumentFilter.FilterBypass;

/**
 *
 * @author Igor
 */
public class CustomLengthTextField extends JTextField {

    protected boolean upper = false;
    protected int maxlength = 0;

    public CustomLengthTextField() {
        this(-1);
    }

    public CustomLengthTextField(int length, boolean upper) {
        this(length, upper, null);
    }

    public CustomLengthTextField(int length, InputVerifier inpVer) {
        this(length, false, inpVer);
    }

    /**
     *
     * @param length - maksimalan length
     * @param upper - turn it to upercase
     * @param inpVer - InputVerifier
     */
    public CustomLengthTextField(int length, boolean upper, InputVerifier inpVer) {
        super();
        this.maxlength = length;
        this.upper = upper;
        if (length > 0) {
            AbstractDocument doc = (AbstractDocument) getDocument();
            doc.setDocumentFilter(new DocumentSizeFilter());
        }
        setInputVerifier(inpVer);
    }

    public CustomLengthTextField(int length) {
        this(length, false);
    }

    public void setMaxLength(int length) {
        this.maxlength = length;
    }

    class DocumentSizeFilter extends DocumentFilter {

        public void insertString(FilterBypass fb, int offs, String str, AttributeSet a)
                throws BadLocationException {

            //This rejects the entire insertion if it would make
            //the contents too long. Another option would be
            //to truncate the inserted string so the contents
            //would be exactly maxCharacters in length.
            if ((fb.getDocument().getLength() + str.length()) <= maxlength) {
                super.insertString(fb, offs, str, a);
            }
        }

        public void replace(FilterBypass fb, int offs,
                int length,
                String str, AttributeSet a)
                throws BadLocationException {

            if (upper) {
                str = str.toUpperCase();
            }

            //This rejects the entire replacement if it would make
            //the contents too long. Another option would be
            //to truncate the replacement string so the contents
            //would be exactly maxCharacters in length.
            int charLength = fb.getDocument().getLength() + str.length() - length;

            if (charLength <= maxlength) {
                super.replace(fb, offs, length, str, a);
                if (charLength == maxlength) {
                    focusNextComponent();
                }
            } else {
                focusNextComponent();
            }
        }

        private void focusNextComponent() {
            if (CustomLengthTextField.this == KeyboardFocusManager.getCurrentKeyboardFocusManager().getFocusOwner()) {
                KeyboardFocusManager.getCurrentKeyboardFocusManager().focusNextComponent();
            }
        }
    }
}

Answer 8

Just put this code in KeyTyped event:

    if ((jtextField.getText() + evt.getKeyChar()).length() > 20) {
        evt.consume();
    }

Where "20" is the maximum number of characters that you want.