值更改侦听器到JTextField

将侦听器添加到为您自动创建的基础文档中。

// Listen for changes in the texttextField.getdocument().adddocumentListener(new documentListener() {  public void changedUpdate(documentEvent e) {    warn();  }  public void removeUpdate(documentEvent e) {    warn();  }  public void insertUpdate(documentEvent e) {    warn();  }  public void warn() {     if (Integer.parseInt(textField.getText())<=0){       JOptionPane.showMessageDialog(null,          "Error: Please enter number bigger than 0", "Error Message",          JOptionPane.ERROR_MESSAGE);     }  }});