I am creating a Swing GUI, I am using a series of `JTextField`. I want to be able to add a custom property to the text field. This is going to be used to complete a calculation when focus is lost, for instance.

```
JTextField1.getCustomePram() * user input

```

I want a text field that will have a predefined value, it is for multiplying £20 \* 5. So the default param will be 20 and the user will put the qty in, and on loss of focus the calculation will be done. 

I am using Netbeans to build the GUI, I have tried extending the `JTextField` but to be honest I am not sure what to do, any advice or if I am going about this completely wrong please let me know.