package com.han;import javax.swing.JTextField;import javax.swing.text.AttributeSet;import javax.swing.text.BadLocationException;import javax.swing.text.Document;import javax.swing.text.PlainDocument;/** * Customized fields can easily be created by extending the model  * and changing the default model provided. For example,  * the following piece of code will create a field that holds only  * digit characters. It will work even if text is pasted into from  * the clipboard or it is altered via programmatic changes. * @author HAN * */public class DigitOnlyField extends JTextField {	   	/**	 * 	 */	private static final long serialVersionUID = 8384787369612949227L;	public DigitOnlyField(int cols) {		// super() 可以被自动调用，但是有参构造方法并不能被自动调用，只能依赖		// super关键字显示地调用父类的构造方法        super(cols);    }    protected Document createDefaultModel() {        return new UpperCaseDocument();    }    static class UpperCaseDocument extends PlainDocument {        /**		 * 		 */		private static final long serialVersionUID = -4170536906715361215L;		public void insertString(int offs, String str, AttributeSet a)            throws BadLocationException {            if (str == null) {                return;            }            char[] upper = str.toCharArray();            String filtered = "";            for (int i = 0; i < upper.length; i++) {            	if (Character.isDigit(Character.codePointAt(upper, i))){            		filtered += upper[i];            	}            }            super.insertString(offs, filtered, a);        }    }}