如何限制JTextField中的字符数？

Question

如何限制在JTextField中输入的字符数？

假设我想输入最多5个字符。之后，不能输入任何字符。

Answer 1

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

import javax.swing.text.PlainDocument

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）

Answer 2

很好的问题，Swing工具包本身不包含JTextFields的这个功能，这很奇怪。但是，这是我的Udemy.com课程“像孩子一样学习Java”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(); 
    }  
});

这会将猜测游戏文本字段中的字符数限制为3个字符，方法是重写keyTyped事件并检查文本字段是否已有3个字符 - 如果是，则“消耗”关键事件（e ）这样就不会像平时一样进行处理。

Answer 3

如果你想将所有内容都放在一段代码中，那么你可以将tim的答案与JTextField的API上的示例方法混合起来，你会得到这样的结果：

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);
            }
        }       

    }

}

然后没有必要将文档添加到JTextFieldLimit，因为JTextFieldLimit已经具有内部功能。

Answer 4

阅读Implementing a DocumentFilter上Swing教程中的部分，了解更新的解决方案。

此解决方案将适用于任何Document，而不仅仅是PlainDocument。

这是一个比接受的解决方案更新的解决方案。

Answer 5

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();
         }
     }
});

Answer 6

我已使用以下代码段解决了此问题：

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

只需将这段代码放在KeyTyped事件中：

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

其中“ 20”是所需的最大字符数。