The Java Developers Almanac 1.4

Home > List of Packages > javax.swing.text [49 examples] > JTextComponent [11 examples]

e975. Limiting the Capacity of a JTextComponent

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()) <= maxSize) {
                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 <= maxSize) {
                fb.replace(offset, length, str, attrs);
            } else {
                throw new BadLocationException("New characters exceeds max size of document", offset);
            }
        }
    }