This Swing tip shows how to limit JTextField input to a maximum length.

 import javax.swing.text.*;

public class JTextFieldLimit extends PlainDocument {
    private int limit;
    // optional uppercase conversion
    private boolean toUppercase = false;
    
    JTextFieldLimit(int limit) {
        super();
        this.limit = limit;
    }
    
    JTextFieldLimit(int limit, boolean upper) {
        super();
        this.limit = limit;
        toUppercase = upper;
    }
    
    public void insertString
            (int offset, String  str, AttributeSet attr)
            throws BadLocationException {
        if (str == null) return;
        
        if ((getLength() + str.length()) <= limit) {
            if (toUppercase) str = str.toUpperCase();
            super.insertString(offset, str, attr);
        }
    }
}

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

public class tswing 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));
    }
}