Missing .jar in Java Build Path with backslaches in path

Question

I am adding new version of TestNG library. So I downloaded new testng-6.13.1.jar to the folder .../tools/testng , and trying to add reference to it the same way I did for previous version. But new jar is added with backslashes in path and is defined as (missing) by the IDE:

Why is it so?

Answer 1

如果jar位于工作空间中的某个位置，请使用Add JARs ...在这种情况下打开，它将使用相对路径，但是如果jar位于工作空间之外，则使用Add External JARs ...选项，它将指向绝对jar文件。

Answer 2

As .jar file was inside of the project, IDE could not see it untill I did refresh in Package Explorer :

So simply right click on the destination folder of library and select Refresh or press F5.