How to imitate cd command in Java

Giving you two path, one is for the original path, and another is the parameter of the cd command.

For Example:

users/documents and stackOverFlow should return users/documents/stackOverFlow

and

users/documents and ../stackOverFlow should return users/stackOverFlow

and

users/documents/test and ../../stackOverFlow should return users/stackOverFlow

How can we implement such a method imitateCDCommand with two parameters str1 and str2 and return the destination path.