这里面包含了一百多个JAVA源文件

				Prior to J2SE 1.4, limiting the capacity of a text component involved overriding the insertString() method of the component's model. Here's an example: 
				    JTextComponent textComp = new JTextField();
				    textComp.setDocument(new FixedSizePlainDocument(10));
				    
				    class FixedSizePlainDocument extends PlainDocument {
				        int maxSize;
				    
				        public FixedSizePlainDocument(int limit) {
				            maxSize = limit;
				        }
				    
				        public void insertString(int offs, String str, AttributeSet a) throws BadLocationException {
				            if ((getLength() + str.length()) 				                super.insertString(offs, str, a);
				            } else {
				                throw new BadLocationException("Insertion exceeds max size of document", offs);
				            }
				        }
				    }
				
				J2SE 1.4 allows the ability to filter all editing operations on a text component. Here's an example that uses the new filtering capability to limit the capacity of the text component: 
				    JTextComponent textComponent = new JTextField();
				    AbstractDocument doc = (AbstractDocument)textComponent.getDocument();
				    doc.setDocumentFilter(new FixedSizeFilter(10));
				    
				    class FixedSizeFilter extends DocumentFilter {
				        int maxSize;
				    
				        // limit is the maximum number of characters allowed.
				        public FixedSizeFilter(int limit) {
				            maxSize = limit;
				        }
				    
				        // This method is called when characters are inserted into the document
				        public void insertString(DocumentFilter.FilterBypass fb, int offset, String str,
				                AttributeSet attr) throws BadLocationException {
				            replace(fb, offset, 0, str, attr);
				        }
				    
				        // This method is called when characters in the document are replace with other characters
				        public void replace(DocumentFilter.FilterBypass fb, int offset, int length,
				                String str, AttributeSet attrs) throws BadLocationException {
				            int newLength = fb.getDocument().getLength()-length+str.length();
				            if (newLength 				                fb.replace(offset, length, str, attrs);
				            } else {
				                throw new BadLocationException("New characters exceeds max size of document", offset);
				            }
				        }
				    }