Netbeans GUI editor doesn't support ASCII - Java

So I'm making a basic GUI with the NetBeans IDE (in Java), and I want to make a button with a √ sign in it. It didn't let me copypaste it in, so I tried using its ASCII code - char sqrt = (char) 251. Instead of the square root sign, however, it gave me "û", and I have no idea why. Can someone please explain why this is happening, as well as offer an idea as to how I should go about this?