title: Allow only numbers in JTextfield
body: <p>I'm trying to implement requirements like:</p>
  
  <p>Accept only numbers in JTextield, when a non-digit key pressed, it will not be accepted.</p>
  
  <p>I tried many stuff, even tried to call the backspace event to remove the last character if it's a non-digit. However, not able to remove the value typed in the  textfield. 
  I tried to understand DOCUMENT FILTER but finding it difficult to implement. 
  I will be glad if anyone helps me to resolve the issue.</p>