如何限制 JTextField 中输入的字符数？
假设我想输入最多 5 个字符。之后就不能再输入任何字符了。
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);
}
}
}
然后
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));
}
}
（谷歌的第一个结果）
如果您想将所有内容都放入一段代码中，那么您可以将 Tim 的答案与在 JTextField 的 API 上找到的示例方法混合，您将得到如下所示的内容：
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);
}
}
}
}
那么就不需要向 JTextFieldLimit 添加 Document，因为 JTextFieldLimit 已经具有内部功能。
很好的问题，奇怪的是 Swing 工具包本身不包含 JTextFields 的此功能。但是，这是我的 Udemy.com 课程“像孩子一样学习 Java”中的一个很好的答案：
txtGuess = new JTextField();
txtGuess.addKeyListener(new KeyAdapter() {
public void keyTyped(KeyEvent e) {
if (txtGuess.getText().length() >= 3 ) // limit textfield to 3 characters
e.consume();
}
});
通过覆盖 keyTyped 事件并检查文本字段是否已经有 3 个字符，这将猜谜游戏文本字段中的字符数限制为 3 个字符 - 如果是这样，您将“消耗”按键事件 (e)，因此它没有像平常一样被处理。
试试这个：
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();
}
}
});
我已经使用以下代码段解决了这个问题：
private void jTextField1KeyTyped(java.awt.event.KeyEvent evt) {
boolean max = jTextField1.getText().length() > 4;
if ( max ){
evt.consume();
}
}
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();
}
}
}
}
只需将此代码放入 KeyTyped 事件中即可：
if ((jtextField.getText() + evt.getKeyChar()).length() > 20) {
evt.consume();
}
其中“20”是您想要的最大字符数。
哇！非常奇特的答案，这个简单的答案对我有用（将 1 更改为您想要允许的任意字符数）- christophm
txtField01.addKeyListener(new KeyAdapter() {
@Override
public void keyReleased(KeyEvent evt) {
if (txtField01.getText().length() > 1)
txtField01.setText(txtField01.getText().substring(0,1));
}
});