Java只允许输入大写字母的文本框

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 
 * upper case characters. It will work even if text is pasted into from 
 * the clipboard or it is altered via programmatic changes.
 * @author HAN
 *
 */
public class UpperCaseField extends JTextField {
	 
    /**
	 * 
	 */
	private static final long serialVersionUID = 6854878572763032459L;

	public UpperCaseField(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();
            for (int i = 0; i < upper.length; i++) {
                upper[i] = Character.toUpperCase(upper[i]);
            }
            super.insertString(offs, new String(upper), a);
        }
    }
}