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copied!<p>As was pointed out three times so far, this is exactly what <code>DocumentFilter</code>s are made for. Here is a <a href="http://sscce.org/" rel="nofollow">SSCCE</a> that does what you want:</p> <pre><code>import javax.swing.JFrame; import javax.swing.SwingUtilities; import javax.swing.JTextField; import javax.swing.text.AbstractDocument; import javax.swing.text.Document; import javax.swing.text.DocumentFilter; import javax.swing.text.BadLocationException; import javax.swing.text.AttributeSet; public class TestDocumentFilter extends JFrame { JTextField basePriceTF; public TestDocumentFilter() { super("TestDocumentFilter"); basePriceTF = new JTextField(); AbstractDocument basePriceDocument = (AbstractDocument) basePriceTF.getDocument(); basePriceDocument.setDocumentFilter(new PositiveIntegerFilter()); getContentPane().add(basePriceTF); } /** * Resets the document to "0" for input values that do not constitut a non-negative integer. */ private static class PositiveIntegerFilter extends DocumentFilter { @Override public void replace(DocumentFilter.FilterBypass fb, int offset, int length, String inputTextValue, AttributeSet attrs) throws BadLocationException { Document oldDoc = fb.getDocument(); String textValue = oldDoc.getText(0, oldDoc.getLength()) + inputTextValue; Integer basePrice = 0; try { basePrice = Integer.parseInt(textValue); } catch (NumberFormatException e) { basePrice = 0; } if (basePrice < 0) basePrice = 0; fb.replace(0, oldDoc.getLength(), basePrice.toString(), attrs); } } public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { @Override public void run() { JFrame frame = new TestDocumentFilter(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } }); } } </code></pre> <p>You cannot enter "-1"; the field is reset to "0" when "-" is typed. Note the <code>FilterBypass</code> which avoids any recursive call.</p>

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