Q:

is there any convert reference between java (for android) and C#?

is there any convert reference between java (for android) and C# ?
for example: 
in C#: messagebox.show(sum.tostring()); ==>  in java ???

A:

There's no way to convert C# code to Java, but they're really similar.  If you have a strong handle on C#, I find it very easy to ramp up with Java for Android.  In your example, there are two things going on, so look at them individually.
Converting between value types and strings is very similar.  Just as in C#, all the primitive types have corresponding wrapper classes (int has Integer, etc).  These all have a parseXXX method and a toString method, and the String class has overloads of the valueOf method for all of the primitive types.
for example:
int someInt = Integer.parseInt(someString);
String someString = someInt.toString();
String someString = String.valueOf(someInt);

As far as showing a dialog in one line of code, that doesn't exist in Android (assuming you're talking about Android specifically because of the tag).
If you just want to see information, you can use Toast.  If you need to actually display a dialog that the user interacts with, you need to learn how to display dialogs in Android.
http://developer.android.com/guide/topics/ui/dialogs.html
http://developer.android.com/guide/topics/ui/notifiers/toasts.html
http://developer.android.com/reference/java/lang/String.html
http://developer.android.com/reference/java/lang/package-summary.html

