I used Java for tool dev for the same reason one would use C# or VB.net. These languages are pushed for "Rapid Application Development" and if you know how to use them, they do exactly as it says on the tin.
C++ is supported on platforms you won't run your tools on and has performance advantages that do not apply to tools (or any amateur level games I've ever seen).
.NET offers Winforms which makes it easy to develop GUIs. It has managed memory which is a development time saver. These are things you need to make your tools.
Quote:how to make a simple mapeditor? will i be drawing a dx buffer inside a window(now whole window)?
I used the basic Java Graphics API to draw into a Component. Buttons and all these things are components. You can make most of your GUI in Winforms, then use a lower level graphics API (whatever it is in .NET) to paint your level visualization. You could use DX or whatever is quickest and easiest to do.