Why a backslash displays as "W"+strikethrough in Netbeans?

It's a currency symbol for won.

Check your editor's font settings, in particular the "Script" field (if there is one).

Michael Kaplan's post When is a backslash not a backslash? says:

The character in question is U+005c, the REVERSE SOLIDUS, also known as the backslash or '\'. It is the path separator for Windows, which is encoded at 0x5c across all of the ANSI code pages.

Since path separators are a pretty important requirement, the title of this post may seem a little scary -- how could it not be a backslash, a reverse solidus?

Well, on Japanese code page 932, 0x5c is the YEN SIGN, and on Korean code page 949, 0x5c is the WON SIGN.

Which is not to say that 0x5c does not act as a path separator -- it still does. And which is also not to say that the Unicode code points for the Yen and the Won (U+00a5 and U+20a9) do act as path separators -- because they do not.