You don't need Java for that. If the format 3.990,70 is not dependent from language settings, you can use
parseDecimal('3.241,98', '#.###,##')
otherwise use replaceAll to change the decimal and thousand separators.
From your Design I see you want to convert a decimal to integer
So you can use Change Activity
Object (MyEntity)
Set IntegerAttribute = $MyEntity/DecimalAttribute.toInt()
OR Javascript
toInt($DecimalAttribute)