Java JTextField设置输入限制

您好，我正在尝试使用setDocument方法限制用户可以在文本字段中输入的字符数。但不知何故，它并没有限制输入字符的数量。这是密码

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


public class JTextFieldCharLimit extends PlainDocument {


    private int limit;

    public JTextFieldCharLimit(int limit)
    {
        super();
        this.limit = limit;
    }
    public void inserString(int offset, String str, AttributeSet set) throws BadLocationException
    {
        if(str == null)
        {
            return;
        } else if((getLength() + str.length()) <= limit)
        {
            str = str.toUpperCase();
            super.insertString(offset, str, set);
        }
    }




}

---

使用

文档过滤器

。有关更多详细信息，请参阅和

public class SizeFilter extends DocumentFilter {

    private int maxCharacters;    

    public SizeFilter(int maxChars) {
        maxCharacters = maxChars;
    }

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

        if ((fb.getDocument().getLength() + str.length()) <= maxCharacters)
            super.insertString(fb, offs, str, a);
        else
            Toolkit.getDefaultToolkit().beep();
    }

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

        if ((fb.getDocument().getLength() + str.length()
                - length) <= maxCharacters)
            super.replace(fb, offs, length, str, a);
        else
            Toolkit.getDefaultToolkit().beep();
    }
}

---

playerName.addJTextFieldToPanel（面板，文本输入）中的内容我有一个方法可以在单独的类中向面板添加文本字段。playerName是该类的对象。
public class SizeFilter extends DocumentFilter {

    private int maxCharacters;    

    public SizeFilter(int maxChars) {
        maxCharacters = maxChars;
    }

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

        if ((fb.getDocument().getLength() + str.length()) <= maxCharacters)
            super.insertString(fb, offs, str, a);
        else
            Toolkit.getDefaultToolkit().beep();
    }

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

        if ((fb.getDocument().getLength() + str.length()
                - length) <= maxCharacters)
            super.replace(fb, offs, length, str, a);
        else
            Toolkit.getDefaultToolkit().beep();
    }
}

((AbstractDocument)textEntry.getDocument()).setDocumentFilter(new SizeFilter(5));