Change File Extension On Path Name In JTextField

I have a GUI that has three jTextFiled's. The first on gets filled in with a file that I choose and shows the path to that file with the file name

i.e. - c:
s34bil.exe
un estrun.i

I want to change the file extension from .i to .r in one jTextField and then .i to .o in another jTextField.

I am slowly learning Java. I have this coded in VB, but not sure how to do it in java.

Here is the VB script :

Dim thefile As String = txtInput.Text
Dim fn1 As String = My.Computer.FileSystem.GetName(thefile)
Dim fn2 As String = fn1.Replace(".i", "")

[Code] ....